Van Den

: Co-Firstauthor

Population genetics of African Schistosoma species

Keywords: Schistosoma, Population structure, Genetic diversity, Phylogeography, Hybrids

Blood flukes within the genus Schistosoma (schistosomes) are responsible for the major disease, schistosomiasis, in tropical and sub-tropical areas. This disease is predominantly present on the African continent with more than 85% of the human cases. Schistosomes are also parasites of veterinary importance infecting livestock and wildlife. Schistosoma population genetic structure and diversity are important characteristics that may reflect variations in selection pressures such as those induced by host (mammalian and snail) environments, habitat change, migration and also treatment / control interventions, all of which also shape speciation and evolution of the whole Schistosoma genus. Investigations intoschistosome population genetic structure, diversity and evolution has been an area of important debate and research.

Supported by advancesin molecular techniques with capabilitiesfor multi-locus genetic analyses for single larvae schistosome geneticinvestigations have greatly progressed in the last decade. This paper aims to review the genetic studies of both animal and human infecting schistosome. Population genetic structures are reviewed at different spatial scales: local, regional or continental (i.e. phylogeography). Within species genetic diversities are discussed compared and the compounding factors discussed, including the effect of mass drug administration. Finally, the ability for intra-species hybridisation questions species integrities and poses many questions in relation to the natural epidemiology of co-endemic species. Here we review molecularly confirmed hybridisation events (in relation to human disease) and discuss the possible impact for ongoing and future control and elimination.

Introduction

Schistosomes are parasitic trematodes of mammals that are transmitted by freshwater intermediate molluscan hosts. A total of 23described Schistosoma species belong tothe Schistosoma genus [START_REF] Boissier | Schistosoma spp[END_REF] (Figure 1). Among trematodes, this genus is particularly well studied because six species are of medical importance. Schistosomes cause schistosomiasis or bilharziasis, a chronic neglected tropical disease (NTD). It is the most prevalent NTD in sub-Saharan Africa (Colley et al., 2014;Lai et al., 2015;McManus et al., 2018) and a major public health problem in several tropical and subtropical countries. The number of schistosomiasis cases in the world is considered to be greater than 250 million, mostly in sub Saharan Africa and the DALYs index ("Disability-Adjusted Life Years") was estimated at 1.4 (Hotez et al., 2014).

Schistosoma species have a two host life-cycle including a mammalian definitive host and a snail intermediate host. A unique feature of schistosomes is that they have separate sexes (i.e. gonochoric) whereas trematodes are typically hermaphroditic (Beltran andBoissier, 2008, 2010;[START_REF] Mone | Sexual biology of schistosomes[END_REF]. Within the mammalian hosts, highly sexually dimorophic male and female adult worms pair in the veins of either the peri-vesical vascular system (S. haematobium) or in the mesenteric system(most other species). This sexual reproduction ensures genetic mixing and the emergence of novel combinations of genes in each offspring.

This allows schistosome populations to potentially adapt to selection pressures, both natural and anthropogenic, with natural selection leading to highly successful and diverse populations.

The mammalian host spectrum varies greatly according to the species concerned. For instance,some species only infect humans (e.g. S. guineensis), others have a narrow host spectrum (e.g. S. mansoni infects humans, rodents and non-human primates) while S. japonicum is able to infect many mammalian species from variousgroups including Cetartiodactyla, primates, Rodentia, Carnivora, Insectivora or Lagomorpha [START_REF] Pitchford | A check list of definitive hosts exhibiting evidence of the genus Schistosoma Weinland, 1858 acquired naturally in Africa and the Middle East[END_REF], and is considered to be highly zoonotic. Multi host versus restricted host specificities appears to be evolutionary, with the multi host species being more ancestral. Host specificity will also affect population genetics, increasing diversity where multiple hosts are involved versus reduced diversity where schistosomes are more specific.

Eggs from adult worms are releasedvia the urine (S. haematobium) or the faeces (other species) and hatch in freshwater systems, releasing the highly motile larva: the miracidium. This larva actively searches for a specific/compatible planorbid host. As with all trematodes the intermediate host spectrum is often narrowwith clear species-specific compatibilities, which governs geographical species distributions. Within the snail,asexual reproduction occurs with the clonal production of the free swimming larvae: the cercariaewhich once released from the snail actively search for their mammal definitive hosts, penetrating the skin and infecting their host. This clonal replication is highly productive with a single miracidia resulting in many thousands of cercariae produced over many weeks. The tight compatibilities between individual snails and their schistosomes leads to high selection pressures and reduced population variation, however the prolific cercarial production ensures ongoing transmission. It is clear that schistosomes have evolved unique life-history traits to maintain high-levels of transmission whilst maintaining populations diversity through alternate asexual and sexual life-cycle stages.

Molecular epidemiological studiesprovide valuable insightsinto the genetic variation of Schistosoma species and populations in time and space. This may reflect variations inselection pressures such as host (mammalian and snail) environments, habitat change, emergence in new areas and also treatment / control interventions, all of which will also shape speciation and evolution. Several population genetic studies have shown their importance in the identification of reservoir hosts, the potentiality/likelihood of drug resistance development and spread, the understanding of the recruitment pattern in each host and the genetic structure and gene flow in the context of disease control. This review aims to review several of these studies, highlights how dynamic both human and animal Schistosoma species are in terms of their ability to adapt and evolve. Certainly, evidence shows that Schistosoma species adapt quickly to new environments and challenges to ensure their survival, a trait that should not be underestimated in global efforts for control, elimination and eventual eradication.

Nested Russian doll genetic structuration

For Schistosoma spp., different levels of nested populations can be defined as i) all the parasites(worms, sporocysts, excreted miracidia or emitted cercariae) from an individual host (i.e. infrapopulation) ii) all the parasites from a communityiii) all the villages from a region and iv) all the regionsof thearea of repartition of a given species. Sinceschistosometransmission is restricted to freshwaterbodies, the 'community' scale could be equivalent to a small number of transmission sites. The extent of gene flow between these populations will determine the degree of population structure at each level. Here we define three levels of spatial scales. The first level,usually referredto as the'local scale',includes the study of subpopulations between hosts or spatial populations distanced from each otherby 100's of metres to 10's of kilometres (Table 1). The second level, usually referred to as the 'regional scale', includes analysesof parasite subpopulations far from each other by 10's to 100's of kilometres (Table 1). The differentiation between these two first scales can be blurry. The third level, usually referred to as 'phylogeography', analyses parasites subpopulations far from each other by more than 100's of kilometres (i.e. between and within countries) and give insights on the evolutionary history of the species.

A. Local scale

For schistosomes or trematodes in general, an infrapopulation (i.e. all individuals from a same host individual) cannot be considered a single population because it depends on a continuous recruitment of new individuals during re-infection and notthrough intra-host natality. As for any population genetic study on schistosomes, the miracidial populations are used as a proxy of genetic diversity of parental populations locked inside the mammal host.

Moreover, the fact that the parasite asexually reproduces inside the snail, creating clonal populations, means that different definitive hosts may be infected by genetically identical schistosomes. Theoretically, the structuration of the infrapopulation and the differentiation between infrapopulations is a balance between factors having tendencies to homogenize the population, and factors having tendencies to differentiate the parasite infrapopulations (Théron et al. 2004). At a local scale, it seems that the majority of these factors are expected to favour the homogenization of the population; (i) snails are not highly mobile within and between sites supporting infections of different definitive hosts with the same clonal schistosomes, (ii) definitive hosts are infected through a repeated light exposure mechanism (i.e. trickle infection process), (iii) mammalian hosts are mobile and long lived and (iv) genetic exchange due to sexual reproduction of adult worms, with the ability to change partners. Opposite to this (i) the rapid turn-over of infected snails and (iii) the acquired immunity of the definitive host, could lead to infrapopulation discriminations. Finally, an expected consequence for parasites in general is a deviation from Hardy-Weinberg equilibrium at the global scale but not within each host. This could be explained by a Wahlund effect with a non-random mating of parasites between hosts and random mating within each host.

The first empirical study aiming to analyse the infrapopulation genetic structure of schistosomes concern adult Schistosoma mansoni worms recovered from naturally infected Rattus rattus on the island of Guadeloupe [START_REF] Sire | Parasite (Schistosoma mansoni) and host (Biomphalaria glabrata) genetic diversity: population structure in a fragmented landscape[END_REF]. At that time no method was available for the molecular analysis of individual larval stages (samples were subject to passage through laboratory rodents) and the genetic marker used was non-allelic (e.g. Random Polymorphic DNA marker). The main finding was that S. mansoniexhibited high genetic diversity and this diversity occurs on average more within than between hosts [START_REF] Sire | Parasite (Schistosoma mansoni) and host (Biomphalaria glabrata) genetic diversity: population structure in a fragmented landscape[END_REF]. Despite this, a significant degree of differentiation was also observed between the schistosome infrapopulations [START_REF] Sire | Parasite (Schistosoma mansoni) and host (Biomphalaria glabrata) genetic diversity: population structure in a fragmented landscape[END_REF]. The first study that investigatedS. mansoni from humans also used adult wormsresulting fromsuccessive passage through experimental snail and rodent hosts using miracidia excreted by Brazilian patients [START_REF] Thiele | Genetic diversity and population structuring of Schistosoma mansoni in a Brazilian village[END_REF]. This passage system presents obvious sampling bias, bottlenecks and/or selection pressures (Gower et al., 2007).

Despite this, the study showed both an important genetic diversity within the host and that the genetic variation within the infrapopulations accounted for the majority of the variance observed [START_REF] Thiele | Genetic diversity and population structuring of Schistosoma mansoni in a Brazilian village[END_REF]. Later, the development of methods to collect, store and to amplify multiple (>10) microsatellite markers from a single larval stage has greatly advanced our knowledgeof schistosome population genetics (Gower et al., 2007). The microsatellite analysis of individual miracidia collected from infected children in Kenya showed significant Fst values both between patients (ranging from 0.44% to 3.98% ) and between schools (ranging from 0.16% to 3.92%) [START_REF] Agola | Genetic diversity and population structure of Schistosoma mansoni within human infrapopulations in Mwea, central Kenya assessed by microsatellite markers[END_REF]. At this scale (<7 km between villages), the genetic structure wasmore similar between patients than between sites suggesting high levels of gene flow between infrapopulations. Similar results were obtainedfrom Uganda and Senegal, from analysis of both mitochondrial cytochrome c oxidase (cox1)or nuclear microsatellite DNA regions, respectively [START_REF] Betson | New insights into the molecular epidemiology and population genetics of Schistosoma mansoni in Ugandan pre-school children and mothers[END_REF][START_REF] Van Den Broeck | Inbreeding within human Schistosoma mansoni: do hostspecific factors shape the genetic composition of parasite populations?[END_REF]. In one village located on the shore of Lake Albert (Uganda), no genetic differentiation was observed between infrapopulations and very weak differences (accounting for only 3.5% of variation) were observed within another village on the shore of Lake Victoria (Uganda) [START_REF] Betson | New insights into the molecular epidemiology and population genetics of Schistosoma mansoni in Ugandan pre-school children and mothers[END_REF].

The authors explained this difference by the fact that the shore of Lake Victoria is more convoluted than that of Lake Albert, providing micro-environments for transmission. People living in the village associated with Lake Victoriaappeared to acquire their infections from different micro-foci, hosting different parasite infrapopulation [START_REF] Betson | New insights into the molecular epidemiology and population genetics of Schistosoma mansoni in Ugandan pre-school children and mothers[END_REF]. In Senegal, hierarchical analyses of genetic structure revealed no spatial-temporal structure in S. mansoni from three communities located on the Western shores of Lake Guiers( [START_REF] Van Den Broeck | Inbreeding within human Schistosoma mansoni: do hostspecific factors shape the genetic composition of parasite populations?[END_REF]. Although significant, parasite genetic differentiation was found to be low between the three communities located 4 to 22 km from eachother (FST = 0.003), and between infrapopulations within each community (FST = 0.004) [START_REF] Van Den Broeck | Inbreeding within human Schistosoma mansoni: do hostspecific factors shape the genetic composition of parasite populations?[END_REF]. Genetic differentiation between infrapopulations from all three communities ranged between -0.027 and 0.067 (Table 1), and the large majority of FST-values were not significant after Bonferroni correction, suggesting relatively high connectivity among the three communities [START_REF] Van Den Broeck | Inbreeding within human Schistosoma mansoni: do hostspecific factors shape the genetic composition of parasite populations?[END_REF].Most studies on population structure have focused on S. mansoniwhile only two studies have focused on S. haematobiumat a local scale [START_REF] Boon | No barrier breakdown between human and cattle schistosome species in the Senegal River Basin in the face of hybridisation[END_REF]Gower et al., 2011). These studies are in accordance with results already observed for S. mansoni. Gower et al (2011)show low genetic differentiation between two schools separated by 7 km (Fst=0.004) and among children within the schools (Fst=0.001) and these variations accounted for only 0.35% and 0.1%, respectively. The 99.5% remaining variation wasattributable to the variation within each child. [START_REF] Boon | No barrier breakdown between human and cattle schistosome species in the Senegal River Basin in the face of hybridisation[END_REF] analysed the genetic structure of S.haematobium10 villages in the North of Senegal. Weak genetic structuration was observed from 7 villages clustering in a 45 km range. A tendency for isolation by distance was noticed for two other villages further awayfrom this cluster. To conclude all these studies,related to human hosts, suggest that few barriers to gene flow between populations exist at the local scale. Finally, similar results have been obtained in the single study on Schistosoma bovis [START_REF] Djuikwo-Teukeng | Population genetic structure of Schistosoma bovis in Cameroon[END_REF]. This study showed that most variation isobserved within individual animals (98.09% and 88.83% for microsatellites and cox1 markers, respectively) rather than between animals.

B. Regional scale

At a regional scale the population structure mainly depends on the movement of human or animal hosts between different suitable transmission sites. At this scale a strong genetic population structure is expected. It is expected that at this scale little gene flow exists and that the genetic variation is more determined by the effective population size,rather than by gene flow between sites. This restriction in gene flow is reinforced by the fact that schistosomiasis is primarily transmitted by children who do not move muchbetween regionscompared to adults.

The snail-miracidium compatibility could also restrict the gene flow between different sites [START_REF] Mitta | The Compatibility Between Biomphalaria glabrata Snails and Schistosoma mansoni: An Increasingly Complex Puzzle[END_REF], due to fine scale snail-schistosome strain compatibilities.

The first empirical study on S. mansoni at the regional scale, analysed worms recovered from infected laboratory mice after exposure to cercariae from naturally infected molluscs [START_REF] Agola | Microsatellite typing reveals strong genetic structure of Schistosoma mansoni from localities in Kenya[END_REF]. This approach will reduce the number of genotypes represented within the given population and in turn artificially increase the differentiation between the populations. The maximum Fst was 0.273 for two villages distancedby 175 km. Using the same sampling approach (i.e. genotyping adult worms after passage through a laboratory host), similar strong genetic structure was been observed in Uganda between lake Victoria and lake Albert [START_REF] Stothard | Molecular epidemiology of Schistosoma mansoni in Uganda: DNA barcoding reveals substantial genetic diversity within Lake Albert and Lake Victoria populations[END_REF]. Thislatter studies used a DNA barcoding approach and showed two distinct lineages corresponding to the two lakes [START_REF] Stothard | Molecular epidemiology of Schistosoma mansoni in Uganda: DNA barcoding reveals substantial genetic diversity within Lake Albert and Lake Victoria populations[END_REF]. Using miracidia collected from infected patients both low andhigh Fst values were also obtained in Ethiopia(See Table1) [START_REF] Aemero | Genetic diversity, multiplicity of infection and population structure of Schistosoma mansoni isolates from human hosts in Ethiopia[END_REF]. In 2015, Van den Broeck and colleagues reconstructed the disease outbreak of intestinal schistosomiasis in Northwest Senegal ( [START_REF] Van Den Broeck | Reconstructing Colonization Dynamics of the Human Parasite Schistosoma mansoni following Anthropogenic Environmental Changes in Northwest Senegal[END_REF].

Parasites were sampled over a period of 14 years in seven villages located 4 to 65 km from eachother. They found that most parasites belonged to the same genetic population, with relatively low genetic differentiation between most villages, suggesting relatively high gene flow in the region (Table 4 in [START_REF] Van Den Broeck | Reconstructing Colonization Dynamics of the Human Parasite Schistosoma mansoni following Anthropogenic Environmental Changes in Northwest Senegal[END_REF]). An interesting observation was that S. mansoni parasites from Mbodjene in the Lampsar region were significantly differentiated from most other samples in the vicinity of Lake Guiers and the Senegal River. Likewise, the B. pfeifferi population sampled close to this locality was genetically different from the other two populations near the Lampsar River and the populations around Lake Guiers [START_REF] Campbell | Low genetic diversity in a snail intermediate host (Biomphalaria pfeifferi Krass, 1848) and schistosomiasis transmission in the Senegal River Basin[END_REF]. This correspondence between host and parasite geographic structure suggests that the genetic composition of the intermediate snail hosts could be an important factor determining establishment success of S. mansoni in a certain region [START_REF] Van Den Broeck | Reconstructing Colonization Dynamics of the Human Parasite Schistosoma mansoni following Anthropogenic Environmental Changes in Northwest Senegal[END_REF]. At the molecular level, this could comply with the hypothesis of a matching phenotype model where the interactions between parasite antigens and host immune receptors during the early stages of the infection determine the success or failure of the infection [START_REF] Mitta | Compatibility polymorphism in snail/schistosome interactions: From field to theory to molecular mechanisms[END_REF]. Van den Broeck and colleagues (2015) also estimated genetic differentiation between the eight villages in North-West Senegal, one village in South-East Senegal and one village in Western Mali. At this scale, almost all pairwise FST estimates were significant after Bonferroni correction (p < 0.001). FST estimates between samples from the same year (2007) ranged between 0.032 and 0.066 when compared between sites from North-West Senegal and Western Mali (located 1200 km from eachother), and between 0.014 and 0.069 between sites from North-West Senegal and South-East Senegal (located 600 km from eachother) (Table 1). One noticeable exception of this general pattern was observed forS. bovis [START_REF] Djuikwo-Teukeng | Population genetic structure of Schistosoma bovis in Cameroon[END_REF]. Indeed, at very large distances (175-672 km between sampled sites) no evidence for inter-or intrapopulation structuring was observed. Because the parasites were recovered from naturally infected cows, there was no possible bias due toselection within experimental snail or rodent hosts. The absence of genetic structure for S. bovis at a country scale (i.e. Cameroon) was explained by a large effective population size and/or movement of the definitive hosts within the country [START_REF] Djuikwo-Teukeng | Population genetic structure of Schistosoma bovis in Cameroon[END_REF]. More studies on genetic diversity and population genetic structuring of animal schistosomes are needed to infer the role of the definitive hosts on parasite gene flow. Within all the studies reviewed the interpopulation similarity indices showed that the populations do not follow a pattern of isolation by distance (Table 1) but instead they exhibit an island model of gene flow. This could be due to several factors such as restricted gene flow between sites, sib-transmission or due to local adaptations.

C. Phylogeography

A total of 13 Schistosoma species are described in Africa, four of which are associated with humans: S. mansoni, S. haematobium, S. guineensis and S. intercalatum [START_REF] Webster | A revision of the interrelationships of Schistosoma including the recently described Schistosoma guineensis[END_REF] with the others being pathogens primarily of other animals. Schistosoma mansoni and S. haematobium are the two most prevalent species in Africa with important impacts on human health and de facto they have received considerable research interest. Conversely, our knowledge on the distribution and the evolutionary history of the other human-associated Schistosoma species (S. intercalatum and S. guineensis) is very limited (e.g. [START_REF] Webster | A revision of the interrelationships of Schistosoma including the recently described Schistosoma guineensis[END_REF]Tchuem Tchuenté et al., 2003) and there are currently no genetic population studies on these species. Animal Schistosoma species also receive very little attention and their phylogeography is still under documented. Yet, some animal Schistosoma species such as S.

bovis have been recently studied, mainly at a local scale, mainly because they have been shown to interbreed with human associated species raising the possibility for the emergence of zoonoses (Leger and Webster, 2016;[START_REF] Catalano | Rodents as Natural Hosts of Zoonotic Schistosoma Species and Hybrids: An Epidemiological and Evolutionary Perspective From West Africa[END_REF]; see sections below). However, we still lack a global vision on the distribution and the phylogeography of most Schistosoma species at the continental scale. Below we review our knowledge on the phylogeography of the two best studied species, S. mansoni and S. haematobiumand also the evolutionary history of S.

bovis.

Schistosoma mansoni is by far the best studied species at variousgeographical scales, from local to global (even intercontinental) scales. Globally, S. mansoni displays high levels of genetic diversity at both the mitochondrial and nuclear level, which at a global scale is geographically well structured [START_REF] Van Den Broeck | Reconstructing Colonization Dynamics of the Human Parasite Schistosoma mansoni following Anthropogenic Environmental Changes in Northwest Senegal[END_REF][START_REF] Faust | Regular treatments of praziquantel do not impact on the genetic make-up of Schistosoma mansoni in Northern Senegal[END_REF]Morgan et al., 2005, Webster et al., 2013); but see Crellen et al., 2016). Conversely, although S. mansoni can infect several molluscs including, among others, Biomphalaria pfeifferi, B. sudanica and B. choanonphala and a range of vertebrate definitive hosts; humans, rodents and non-human primates, no genetic signature is associated with host use (Lawton et al., 2011;Webster et al., 2013). Lawton and collaborators argue that this lack of structure by host results from continuous gene flow between parasites using different hosts hence arguing the need to considera 'One Health' approach for S. mansoni control. Second, populations from Eastern Africa are far more diverse than Western populations [START_REF] Faust | Regular treatments of praziquantel do not impact on the genetic make-up of Schistosoma mansoni in Northern Senegal[END_REF]Morgan et al., 2005;Webster et al., 2013). This pattern has led several authors to hypothesise that S. mansoni has emerged in the former region and migrated toward Western Africa (Lawton et al., 2011;Morgan et al., 2005;Webster et al., 2013). More recent genomic evidence suggests that S. mansoni emerged in Eastern Africa around 126,5 KYA and that populations were confined to this area over a long period until around 7 KYA (Crellen et al., 2016). Populations have then expanded and further divergence events occurred giving rise to populations now present in Cameroon sixthousand years ago (KYA) and more recently in Senegal 1.5 KYA (Crellen et al., 2016). If confirmed, this evolutionary scenario might at least partly explain global geographical patterns of genetic structure and the gradual decrease of genetic diversity from Eastern to Western Africa. One exception concerning the globally observed geographical structure of S. mansoni populations concern those established in Coastal Kenya and Zambia (Webster et al., 2013). These two closely related populations genetically cluster together and display high genetic divergence compared to other West-African populations (Webster et al., 2013). Based on this former study and based on further genomic evidence that Coastal Kenya is genetically distinct from other East African populations, Crellen et al. ( 2016) argue that Zambia and Coastal Kenya share a common evolutionary history probably due to important human movements between these two countries during the 19 th century. These results suggest that some cryptic diverging lineages of S. mansoni that emerged under the influence of human history could occur in other regions of Africa and that the overall genetic diversity of this species might therefore still be underestimated.

Molecular studies on S. haematobium are scarce at the global scale and mainly based on the analysis of mt DNA regions, few nuclear DNA regions (e.g. ITS) and more recently microsatellites [START_REF] Webster | Development of novel multiplex microsatellite polymerase chain reactions to enable high-throughput population genetic studies of Schistosoma haematobium[END_REF]. Although useful to identify potential hybrids, the rDNA ITS region has been found to be monomorphic and cannot be used for reconstructing genetic relationships between populations at any scale [START_REF] Webster | Genetic diversity within Schistosoma haematobium: DNA barcoding reveals two distinct groups[END_REF]. Despite these few studies and the poor resolution of the routinely used genetic markers, some common patterns emerged regarding the genetic diversity, genetic structure and more recently the genome-wide evolutionary history of S. haematobium populations. The first major characteristic of S. haematobium is that this species displays very low genetic diversity compared to S. mansoni [START_REF] Faust | Regular treatments of praziquantel do not impact on the genetic make-up of Schistosoma mansoni in Northern Senegal[END_REF][START_REF] Webster | Genetic diversity within Schistosoma haematobium: DNA barcoding reveals two distinct groups[END_REF] but see Glenn et al. (2013). Despite such low genetic diversity, two major clades have been identified based on their mitochondrial haplotypes including populations from Eastern Africa on one hand and Western Africa on the other hand [START_REF] Webster | Genetic diversity within Schistosoma haematobium: DNA barcoding reveals two distinct groups[END_REF]. The Eastern clade also includes populations established in some Indian Ocean islands (e.g. Madagascar, Mauritius). Interestingly, one mitotype (H1) is overrepresented at a global scale [START_REF] Webster | Genetic diversity within Schistosoma haematobium: DNA barcoding reveals two distinct groups[END_REF]. This pattern has been interpreted as the result of a recent and important population bottleneck before recent expansion of this parasite species across Africa [START_REF] Webster | Genetic diversity within Schistosoma haematobium: DNA barcoding reveals two distinct groups[END_REF]. However, contrary to S. mansoni, [START_REF] Webster | Genetic diversity within Schistosoma haematobium: DNA barcoding reveals two distinct groups[END_REF] did not find the differences in genetic diversity between Eastern and Western populations. Gower et al.

(2013) found important genetic differentiation among populations according to an Isolation-bydistance pattern although such patterns have been called into question (Glenn et al., 2013).

Results from the latter study might at least partly be explained by the sampling of a highly diverging population in South Africa which increases genetic differentiation indexes computed between geographically close populations.

Although no genetic data exists at a global continental scale for the other Schistosoma species, some authors attempted to draw general phylogeographic patterns of some species based on their geographical distribution, the evolutionary history and the compatibility to their respective host snails. For instance, based on the comparison of the geographical distribution of S. bovis and that of their potential snail host species, combined with experimental assessments of compatibility between different S. bovis populations and different snail species, authors found important diversity among S. bovis populations and clear differences between populations from southern Mediterranean zones and southern Saharan zones (another distinct population being identified in southern Europe) [START_REF] Mone | The distribution of Schistosoma bovis Sonsino, 1876 in relation to intermediate host mollusc-parasite relationships[END_REF]. Based on these results the authors proposed a three-step evolutionary scenario for S. bovis. First this species emerged from (Western) South Sahara and extended its distribution to the South, East and West of Africa.

Second and more recently this species reached the Mediterranean zone together with an important reduction of host spectrum. Finally, S. bovis colonised the Iberian Peninsula most likely through dispersion events over the Arabian Peninsula first and next around the Mediterranean basin. So far, no genetic data exists to confirm this scenario. We expect that with the recent interest for hybridization events between S. bovis with other Schistosoma species, together with the development of genomic resources for this species in particular, we will soon better understand the evolutionary history of this broadly distributed parasite species across Africa (and southern Europe).

Factors affecting schistosome genetic diversity

Parasite genetic diversity is expected to be balanced by the clonality within the mollusc host and by the fact that snails are usually infected by only a few genotypes [START_REF] Theron | Molecular ecology of Schistosoma mansoni transmission inferred from the genetic composition of larval and adult infrapopulations within intermediate and definitive hosts[END_REF].

Opposite to this, the definitive host mobility, the rapid turnover of infected snails, the snailparasite compatibility, the long lifespan of the definitive host and the genotype dependent host re-infection [START_REF] Beltran | Vertebrate host protective immunity drives genetic diversity and antigenic polymorphism in Schistosoma mansoni[END_REF], should all increase the parasite genetic diversity within the definitive host. In this context, some authors have proposed that the definitive host is a 'genetic mixing bowl' for the parasite [START_REF] Curtis | Schistosome population genetic structure: when clumping worms is not just splitting hairs[END_REF]. The 'genetic mixing bowl' hypothesis was validated by several studies. The classical indexes used to infer population diversity such as heterozygosity or allelic richness are always very high for S. mansoni or S. haematobium infrapopulations [START_REF] Agola | Genetic diversity and population structure of Schistosoma mansoni within human infrapopulations in Mwea, central Kenya assessed by microsatellite markers[END_REF]Gower et al., 2011;[START_REF] Sire | Parasite (Schistosoma mansoni) and host (Biomphalaria glabrata) genetic diversity: population structure in a fragmented landscape[END_REF][START_REF] Steinauer | Genetic structure of Schistosoma mansoni in western Kenya: the effects of geography and host sharing[END_REF][START_REF] Thiele | Genetic diversity and population structuring of Schistosoma mansoni in a Brazilian village[END_REF][START_REF] Van Den Broeck | Inbreeding within human Schistosoma mansoni: do hostspecific factors shape the genetic composition of parasite populations?[END_REF]. Using relatedness indices Steinaueur et al. (2009) showed that 94.44% of the schistosomes coming from a single patient was unrelated [START_REF] Steinauer | Genetic structure of Schistosoma mansoni in western Kenya: the effects of geography and host sharing[END_REF]. Similarly, [START_REF] Aemero | Genetic diversity, multiplicity of infection and population structure of Schistosoma mansoni isolates from human hosts in Ethiopia[END_REF] using sibship analysis showed that the estimated number of unique S. mansoni worm pairs within a single human host is between 66% and 92% of the entire worm infrapopulation [START_REF] Aemero | Genetic diversity, multiplicity of infection and population structure of Schistosoma mansoni isolates from human hosts in Ethiopia[END_REF]. This parasite genetic diversity could also be modulated by intrinsic factors such as sex of the parasite or the species concerned, or by extrinsic factors such as the sex of the host, the age of the host, the infection intensity or the drug treatment.

A. A sex-specific population genetic structure

Sexual dimorphism is a remarkable trait in schistosomes [START_REF] Mone | Sexual biology of schistosomes[END_REF]. The muscular male holds the thin female within its gynaecophoric canal. Considering this sexual dimorphism it has been shown that the male elicits a stronger immune response compared to females [START_REF] Boissier | Preliminary study on sexrelated inflammatory reactions in mice infected with Schistosoma mansoni[END_REF]. After repeated light exposure on experimentally infected mice it has been shown that male parasites induced a genotype-dependant negative effect, via the host immune system, on subsequent male infections [START_REF] Dumont | Influence of pattern of exposure, parasite genetic diversity and sex on the degree of protection against reinfection with Schistosoma mansoni[END_REF]. Females alone seem unable to induce this protective effect. The same authors have demonstrated that the protective effect is genotype dependent and correlates with genetic dissimilarity between the male schistosomes (i.e. the smaller the genetic distance between the initial and secondary infecting schistosomes, the lower the infectivity rate of secondary infecting males) [START_REF] Beltran | Vertebrate host protective immunity drives genetic diversity and antigenic polymorphism in Schistosoma mansoni[END_REF].

Comparative 2D proteomic analysis on different male clones revealed differences in antigens known to be involved in the induction of the immune system [START_REF] Beltran | Vertebrate host protective immunity drives genetic diversity and antigenic polymorphism in Schistosoma mansoni[END_REF]. This gender difference in host-parasite interaction may be at the origin of a sex-specific population structure found in naturally infected rats [START_REF] Prugnolle | Sex-specific genetic structure in Schistosoma mansoni: evolutionary and epidemiological implications[END_REF]. These last authors have shown that the female parasites were more genetically similar than the males in infected rats [START_REF] Prugnolle | Sex-specific genetic structure in Schistosoma mansoni: evolutionary and epidemiological implications[END_REF]. This pattern could be the result of the gender differences previously demonstrated in experimental approaches and the successive trickle infection process that will in term lead to more dissimilar males compared to females in a single host. However, this sex-specific pattern has never been investigated for parasites recovered from humans.

B. Differences of genetic diversity between species

Important differences in genetic diversity indices have been observed according to the schistosome species. Several studies have shown that S. mansoni is much more diverse than S. haematobium [START_REF] Faust | Regular treatments of praziquantel do not impact on the genetic make-up of Schistosoma mansoni in Northern Senegal[END_REF][START_REF] Sady | New insights into the genetic diversity of Schistosoma mansoni and S. haematobium in Yemen[END_REF]Webster et al., 2013a;[START_REF] Webster | Genetic diversity within Schistosoma haematobium: DNA barcoding reveals two distinct groups[END_REF]. At the African continental scale, the haplotype diversity (h) and the nucleotide diversity (Π) of the cox1 gene were 0.94 ± 0.0067 and 0.02553 (Webster et al., 2013a) and 0.36 ± 0.014 and 0.00434 [START_REF] Webster | Genetic diversity within Schistosoma haematobium: DNA barcoding reveals two distinct groups[END_REF] for S. mansoni and S. haematobium, respectively. The haplotype diversity is also higher for S. bovis (0.961 ± 0.012), even at a small population size than for S. haematobium [START_REF] Djuikwo-Teukeng | Population genetic structure of Schistosoma bovis in Cameroon[END_REF]. The higher haplotype diversity of S. bovis compared to S. haematobium were confirmed by comparing microsatellite markers common to these two sister species [START_REF] Djuikwo-Teukeng | Population genetic structure of Schistosoma bovis in Cameroon[END_REF]. However, if S. bovis is similar to S. mansoni concerning its haplotype diversity it differs concerning its nucleotide diversity (Π = 0.00602), which is more similar to S. haematobium. As a consequence, the Tajima's D* index did not differ from zero for S. mansoni or S. haematobium [START_REF] Sady | New insights into the genetic diversity of Schistosoma mansoni and S. haematobium in Yemen[END_REF] but it is significantly negative for S. bovis, which could constitute a signature of population demographic expansion [START_REF] Djuikwo-Teukeng | Population genetic structure of Schistosoma bovis in Cameroon[END_REF]. This last study hypothesises that this difference in both nuclear and mitochondrial diversity could be the consequence of the absence of praziquantel treatment of infected animals compared to humans, imposing different bottlenecks/and selective pressures within these hosts accordingly.

C. The effect of the host age on parasite genetic diversity

The trickle infection model is expected to lead to more diverse parasite populations in older patients because they should be exposed to a wider variety of parasites(mixing bowl hypothesis). This expectation has been confirmed for S. mansoni populations in Senegal (Van den [START_REF] Van Den Broeck | Inbreeding within human Schistosoma mansoni: do hostspecific factors shape the genetic composition of parasite populations?[END_REF]. The study showedsignificant correlations between inbreeding (negative correlation) or heterozygosity (positive correlation) and the age of the patients: the children being more infected by related parasites than adults. To date, this study is the only one showing this pattern. The explanation could be that the authors have sampled a variety of ages (between 4 and 50 years) and not only children, as in the majority of otherstudies [START_REF] Agola | Genetic diversity and population structure of Schistosoma mansoni within human infrapopulations in Mwea, central Kenya assessed by microsatellite markers[END_REF]Gower et al., 2011). A more recent study failed to find a difference in parasite genetic diversity between mothers and young children [START_REF] Betson | New insights into the molecular epidemiology and population genetics of Schistosoma mansoni in Ugandan pre-school children and mothers[END_REF]. These authors proposed that the trickle dynamics is rapidly saturated and that two years old children could host the same parasite genetic diversity as their mothers [START_REF] Betson | New insights into the molecular epidemiology and population genetics of Schistosoma mansoni in Ugandan pre-school children and mothers[END_REF]. The differentfindings in these two studies could also relate to the difference in the genetic markersused. The first study used a more polymorphic and biparentally inherited genetic marker (microsatellite) compared to the second one that looked at haplotypic diversity of the maternal mitochondrial marker. It could also relate to the exposure and mobility of the hosts, the mothers [START_REF] Betson | New insights into the molecular epidemiology and population genetics of Schistosoma mansoni in Ugandan pre-school children and mothers[END_REF]) might be less mobile than males [START_REF] Van Den Broeck | Inbreeding within human Schistosoma mansoni: do hostspecific factors shape the genetic composition of parasite populations?[END_REF].

D. The effect of host sex on parasite genetic diversity

Because of behavioural or immunological differences, the sex of the host could also influence the parasite infrapopulation. For instance, it is commonly observed that boys are more heavily infected than girls due to difference in sexual roles in work, play and ultimately water contact [START_REF] Angora | Prevalence and Risk Factors for Schistosomiasis among Schoolchildren in two Settings of Cote d'Ivoire[END_REF][START_REF] Kebede | Schistosoma mansoni infection in human and nonhuman primates in selected areas of Oromia Regional State, Ethiopia[END_REF][START_REF] Elwali | Susceptibility to periportal (Symmers) fibrosis in human Schistosoma mansoni infections: evidence that intensity and duration of infection, gender, and inherited factors are critical in disease progression[END_REF]. This gender bias is also observed in travellerinfections [START_REF] Nicolls | Characteristics of schistosomiasis in travelers reported to the GeoSentinel Surveillance Network 1997-2008[END_REF]. It has also been observed in mouse experimental models that females were more sensitive to schistosome infection than males due to physiological differences between sexes [START_REF] Boissier | Preliminary study on sexrelated inflammatory reactions in mice infected with Schistosoma mansoni[END_REF][START_REF] Nakazawa | Schistosoma mansoni: susceptibility differences between male and female mice can be mediated by testosterone during early infection[END_REF]. These host gender differences in either behaviour and/or physiology are expected to shape the parasite genetic diversity. In naturalS. mansoniinfected rats, no difference was observed between male and females in the genetic composition of the parasite infrapopulation [START_REF] Prugnolle | Sex-specific genetic structure in Schistosoma mansoni: evolutionary and epidemiological implications[END_REF].

The same tendency has been observed forS. mansoniandS. haematobiumfrom infected humans (Gower et al., 2011;[START_REF] Thiele | Genetic diversity and population structuring of Schistosoma mansoni in a Brazilian village[END_REF][START_REF] Van Den Broeck | Inbreeding within human Schistosoma mansoni: do hostspecific factors shape the genetic composition of parasite populations?[END_REF]. However, two recent studies showed that males have a higher schistosome genetic diversity than females [START_REF] Faust | Regular treatments of praziquantel do not impact on the genetic make-up of Schistosoma mansoni in Northern Senegal[END_REF][START_REF] Kebede | Genetic evidence for the role of non-human primates as reservoir hosts for human schistosomiasis[END_REF]. This could reflect the differences in behaviour with young males having higher mobility and greater water contact compared to young females.

One interesting outcome that differsbetween these two studies is that parasite genetic diversity increases only with male or female age [START_REF] Kebede | Genetic evidence for the role of non-human primates as reservoir hosts for human schistosomiasis[END_REF] or [START_REF] Faust | Regular treatments of praziquantel do not impact on the genetic make-up of Schistosoma mansoni in Northern Senegal[END_REF], respectively. This confirms that human behaviour can shape the parasite genetic composition but also,that acquired immunity does not seem to limit this diversity because no decline in genetic diversity was observed [START_REF] Faust | Regular treatments of praziquantel do not impact on the genetic make-up of Schistosoma mansoni in Northern Senegal[END_REF].

E. Infection intensity and schistosome genetic diversity

Schistosomiasis infection intensity is estimated by the number of eggs per gram of faeces for S. mansoni or per 10 mls of urine for S. haematobium. This intensity is dependent on several factors including the sex, age or the occupation of the host, the history of treatment, the transmission foci, the virulence of the parasite etc….Intuitively, we could expect a positive link between infection intensity and parasite genetic diversity. Using mitochondrial cox1 analysis, no correlation between S. mansoniinfection intensity and either haplotype or nucleotide diversity was observed [START_REF] Betson | New insights into the molecular epidemiology and population genetics of Schistosoma mansoni in Ugandan pre-school children and mothers[END_REF]. Similar results wereobtained forS.

haematobium (Gower et al., 2011). Based on microsatellite markers the authors showed that the allelic richness is the same whatever the intensity of infection, from low to very high (Gower et al., 2011). In this same study, the link between sibship analysis (measured as the number of unique parental genotypes) and infection intensity did not give consistent results. First, when comparing two villages, the higher the intensity of infection the less diverse the schistosome infrapopulations appear to be. Second, no difference in infection intensity was observed between girls and boys but the number of unique parental genotypes was lower in girls than in boys. Third, counter-intuitively children with light infections had more diverse parasites than those with high infections. Finally, after controlling for several demographic parameters (village, gender andage) and co-infection status with S. haematobium, no link was observed between S. mansoniinfection intensity and heterozygosity or allelic richness in Senegalese patients( [START_REF] Van Den Broeck | Inbreeding within human Schistosoma mansoni: do hostspecific factors shape the genetic composition of parasite populations?[END_REF].

F. Effect of praziquantel treatment

Praziquantel (PZQ) is the drug of choice to treat schistosomiasis because of the absence of toxicity (Cioli and Pica-Mattoccia, 2003;[START_REF] Thetiot-Laurent | Schistosomiasis chemotherapy[END_REF], low cost (Fenwick, 2006)and the effectiveness against all Schistosoma species( [START_REF] Kumar | Use of praziquantel against schistosomiasis: a review of current status[END_REF]. In 2002, the "Schistosomiasis Control Initiative" was founded with the aim for broad-scale elimination of schistosomiasis by regular (mass)treatment of school-aged children. This could exert an enormous selection pressure on parasite infrapopulations, which might, amongst others, increase the risk of drug resistance development [START_REF] Doenhoff | Resistance of Schistosoma mansoni to praziquantel: is there a problem?[END_REF]Fenwick and Webster, 2006), although there is currently no conclusive evidence for PZQ-induced adaptive evolution.

Here, we consider the neutral evolution of Schistosoma populations in response to Mass Drug Administration (MDA) treatment programs. To this end, we explore the genetic concepts and tools that have enabled investigations into the possible impact of PZQ on the population genetic structure of schistosomes, and demographic changes over time to verify whether current programs actually decrease parasite transmission (and could ultimately lead to their elimination).

A theoretical framework to assess the impact of PZQ treatment on schistosome population genetics was outlined in (Huyse et al., 2013b). Within this framework it is assumed that praziquantel randomly kills worms in the treated individuals, leaving a random subset of worms from the infrapopulation after treatment. This will decrease the allelic richness and gene diversity of the treated infrapopulation, while observed heterozygosity is less sensitive to changes in the effective population size. Low effective population size will furthermore lead to a random loss and fixation of alleles (genetic drift) and thus an increase in genetic differentiation between the treated infrapopulations, and between the pre-and post-treatment infrapopulations.

In addition, pre-and post-treatment infrapopulations could become genetically different due to re-infection of parasites from genetically differentiated source populations. In conclusion, the following population statistics are important to assess the impact of PZQ treatment on schistosome populations: allelic richness (AR), gene diversity (D), inbreeding coefficient (FIS) and genetic differentiation of infrapopulations (FST). The inbreeding coefficient is included as it is also sensitive to changes in effective population size (see Huyse et al., 2013b).

Norton and colleagues were the first to investigate the impact of treatment on the population biology of schistosomes [START_REF] Norton | Genetic consequences of mass human chemotherapy for Schistosoma mansoni: population structure pre-and post-praziquantel treatment in Tanzania[END_REF]. Using seven microsatellite markers, they compared the genetic diversity and differentiation of S. mansoni populations in Tanzanian school aged children before (in April 2005) and twelve months after (in April 2006) the first MDA. The authors found a small but significant difference in AR and D as estimated per infrapopulation in 2005 and 2006, suggesting that parasite infrapopulations may have experienced genetic bottlenecks in response to PZQ treatment. However, this conclusion should be nuanced as the authors also reported a decline in AR in a control group of children that had not been previously treated as they were not yet of school age during baseline sampling [START_REF] Norton | Genetic consequences of mass human chemotherapy for Schistosoma mansoni: population structure pre-and post-praziquantel treatment in Tanzania[END_REF], suggesting that factors other than preventive chemotherapy may have caused the decline in infrapopulation genetic diversity. This hypothesis is supported by the study of (Gower et al., 2017) that revisited the two Tanzanian schools in 2010 and genotyped miracidia using the same microsatellite markers as [START_REF] Norton | Genetic consequences of mass human chemotherapy for Schistosoma mansoni: population structure pre-and post-praziquantel treatment in Tanzania[END_REF]. Children had received two (in 2005 and 2007) or four (in 2005, 2006, 2007 and 2009) rounds of treatment depending on the school. The authors observed a significant increase in infrapopulation AR and D in 2010 compared to baseline estimates in April 2005. The initial decline in 2006 [START_REF] Norton | Genetic consequences of mass human chemotherapy for Schistosoma mansoni: population structure pre-and post-praziquantel treatment in Tanzania[END_REF] and then increase in 2010(Gower et al., 2017) may reflect temporal changes in parasite transmission dynamics, rather than being a direct consequence of PZQ treatment. Gower and colleagues (2017) also found a significant decline in worm burdens within an individual host as estimated with parentage analyses. Although the authors did not correct their estimates for family structure or the number of sampled miracidia [START_REF] Steinauer | Non-invasive sampling of schistosomes from humans requires correcting for family structure[END_REF], their observations could suggest that PZQ treatment reduced infrapopulation sizes, but not to an extent that it resulted in concomitant genetic bottlenecks of the entire parasite population. Both [START_REF] Norton | Genetic consequences of mass human chemotherapy for Schistosoma mansoni: population structure pre-and post-praziquantel treatment in Tanzania[END_REF]Gower et al., (2017) found higher FST between years (within schools) compared to between schools (within years), an observation that may suggest high re-infection levels in the Tanzanian villages.

These reports of a long-term decrease in genetic diversity [START_REF] Norton | Genetic consequences of mass human chemotherapy for Schistosoma mansoni: population structure pre-and post-praziquantel treatment in Tanzania[END_REF] and number of adult worms per infrapopulation (Gower et al., 2017) in response to PZQ treatment could not be reproduced by other studies. In Senegal, 12 Senegalese children were followed over two years and samples were collected prior to treatment: once in April 2007 before the first double treatment (i.e. two PZQ treatments 3 weeks apart), once six months after the first double treatment and two years after baseline surveys with a total of five treatments (Huyse et al., 2013b). Although miracidial genotypes (based on nine microsatellite markers) could not be generated for all children at all timepoints, the authors found no change in AR, D, FIS or FST.

More-over, K-means clustering revealed 11 distinct genetic clusters, most of which were present in all children at all timepoints, suggesting that the genetic composition of parasite infrapopulations remained largely stable for two years despite a relatively large number of PZQ treatments (see Figure 1 in [START_REF] Faust | Regular treatments of praziquantel do not impact on the genetic make-up of Schistosoma mansoni in Northern Senegal[END_REF]. This observation, together with the sustained high egg counts and high genetic diversity after several treatments is indicative of a surviving parasite population, that is tolerant to the administered PZQ dose. In Kenya, [START_REF] Lelo | No apparent reduction in schistosome burden or genetic diversity following four years of school-based mass drug administration in mwea, central kenya, a heavy transmission area[END_REF] followed parasite genetic diversity for four years (2008)(2009)(2010)(2011)(2012) in 15 children that received four treatments during a MDA program, whereby samples were taken once a year prior to treatment [START_REF] Lelo | No apparent reduction in schistosome burden or genetic diversity following four years of school-based mass drug administration in mwea, central kenya, a heavy transmission area[END_REF]. One of the major strengths of this study is that it is the only African-based study that followed a relatively large cohort of children for four years (but see also [START_REF] Blanton | Schistosoma mansoni population structure and persistence after praziquantel treatment in two villages of Bahia, Brazil[END_REF] for an example in Brazil), allowing comparable genetic statistics of the same infrapopulations before and after PZQ treatments (see Table 2 in [START_REF] Lelo | No apparent reduction in schistosome burden or genetic diversity following four years of school-based mass drug administration in mwea, central kenya, a heavy transmission area[END_REF]). In addition, the authors performed a relatively deep miracidial sampling per child (average of 82miracidia per child), allowing robust parentage analyses and reducing the amount of bias introduced due to family structure [START_REF] Steinauer | Non-invasive sampling of schistosomes from humans requires correcting for family structure[END_REF]. Contrary to expectations, [START_REF] Lelo | No apparent reduction in schistosome burden or genetic diversity following four years of school-based mass drug administration in mwea, central kenya, a heavy transmission area[END_REF] found a small but significant increase in AR andD after four years of MDA, but the number of full sibling families and effective number of breeders per infrapopulation did not change over time.

In Uganda, Betson et al. [START_REF] Betson | New insights into the molecular epidemiology and population genetics of Schistosoma mansoni in Ugandan pre-school children and mothers[END_REF] used mt cox1 data to compare the genetic diversity of S. mansoni before and 6 months after PZQ treatment of cohorts of pre-school aged children and mothers. Note that all participants were treated at baseline with PZQ, albendazole and artemether-lumefantrine, and that samples were taken from the same individual or from the same family. The authors found no decline in haplotype or nucleotide diversity. Finally, also in Uganda, Faust and colleagues (2019) performed one of the most extensively designed sampling studies [START_REF] Faust | Regular treatments of praziquantel do not impact on the genetic make-up of Schistosoma mansoni in Northern Senegal[END_REF]. Children aged 6-12 years from three primary schools were sampled at 11 timepoints over two years, including one week, four weeks and six or 12 months after PZQ treatment. Most children received a total of four treatments. The major strength of this study is that the sampling scheme allowed examination of the parasite genetic diversity over short follow-ups after treatment (but see also [START_REF] Blanton | Schistosoma mansoni population structure and persistence after praziquantel treatment in two villages of Bahia, Brazil[END_REF] for an example in Brazil).

One of the main insights from this study is that AR only decreased in samples taken four weeks after treatment, suggesting that PZQ induced a genetic bottleneck, but this then recovered six months after treatment [START_REF] Faust | Regular treatments of praziquantel do not impact on the genetic make-up of Schistosoma mansoni in Northern Senegal[END_REF]. The authors argued that high rates of gene flow in combination with refugia in snails and untreated individuals ensure rapid recovery of parasite genetic diversity. Sibship analysis furthermore demonstrated that adult worms can survive treatment but high gene flow and refugia prevent fixation of resistant/tolerant genotypes [START_REF] Faust | Regular treatments of praziquantel do not impact on the genetic make-up of Schistosoma mansoni in Northern Senegal[END_REF].All these observations together made the authors conclude that MDA alone is not sufficient for schistosomiasis elimination [START_REF] Faust | Regular treatments of praziquantel do not impact on the genetic make-up of Schistosoma mansoni in Northern Senegal[END_REF]. Also, [START_REF] Lelo | No apparent reduction in schistosome burden or genetic diversity following four years of school-based mass drug administration in mwea, central kenya, a heavy transmission area[END_REF] concluded that MDA programs targetingschool-aged children only may have limited impact on parasite transmission at the community level. This was also the main finding of a simulation study that used an island model to investigate the impact of MDA on long-term parasite population dynamics [START_REF] Van Den Broeck | A Darwinian outlook on schistosomiasis elimination[END_REF]. The study revealed that repeated treatments induced strong and lasting declines in parasite infrapopulation sizes, resulting in concomitant genetic bottlenecks. However, parasite genetic diversity recovered quickly in a few generations due to re-infection, and there was little or no impact of treatment on the genetic diversity of the component population when treatment coverage was 95% or lower [START_REF] Van Den Broeck | A Darwinian outlook on schistosomiasis elimination[END_REF]. The authors concluded that community-wide distribution of praziquantel is essential to induce long-term effects on parasite population dynamics [START_REF] Van Den Broeck | A Darwinian outlook on schistosomiasis elimination[END_REF].

However, as also concluded in [START_REF] Barbosa | Repeated praziquantel treatments remodel the genetic and spatial landscape of schistosomiasis risk and transmission[END_REF], more studies are needed to cover the enormous heterogeneity in transmission contexts in Africa before any firm conclusions can be made [START_REF] Barbosa | Repeated praziquantel treatments remodel the genetic and spatial landscape of schistosomiasis risk and transmission[END_REF]. But as it stands now, comparative analyses are complicated due to differences in sampling design, molecular markers and type of population genetic analysis used in the studies. Adopting standardized procedures is therefore key. Future studies may also need to distinguish infrapopulation from component population genetics. At the infrapopulation level, parentage analyses using genome-scale polymorphism data (e.g. [START_REF] Shortt | Population genomic analyses of schistosome parasites highlight critical challenges facing end-game elimination efforts[END_REF]) of large miracidial samples per child may become a powerful tool to assess the strength of the genetic bottleneck and dissect re-infection from persistent infections following treatment. At the component population level, genomic data of a few miracidial samples from a large number of hosts will provide major insights into the long-term regional effects of treatment programs, ultimately allowing the identification of the main factors/interventions that could help schistosomiasis elimination.

Schistosome genetic variation and disease outcome

Several experiments have been performed to study the influence of the parasite and/or hostsgenetic background on the host/parasite association. The work of André Theron's group is an example of how the genetic background of both the parasite and its mollusc host is important for successful infection (e.g. a phenomenon called "compatibility") [START_REF] Mitta | The Compatibility Between Biomphalaria glabrata Snails and Schistosoma mansoni: An Increasingly Complex Puzzle[END_REF][START_REF] Theron | Multi-parasite host susceptibility and multi-host parasite infectivity: a new approach of the Biomphalaria glabrata/Schistosoma mansoni compatibility polymorphism[END_REF]. Interestingly, experimental comparative studies have shown adaptative trade-offsaccording to the parasite strain [START_REF] Davies | Trade-offs in the evolution of virulence in an indirectly transmitted macroparasite[END_REF][START_REF] Clec'h | Striking differences in virulence, transmission and sporocyst growth dynamics between two schistosome populations[END_REF].

However, the influence of parasite genetic variations on virulence in term of human disease outcome is certainly an understudied aspect. Brouwer et al (2013) was first to find genetic differences, measured by randomly amplified polymorphic (RAPD) DNA regions, associated with pathology observed withinS. haematobium infected children [START_REF] Brouwer | Urinary tract pathology attributed to Schistosoma haematobium: does parasite genetics play a role?[END_REF]. No link was observed between heterozygosity and the severity of infection,however they found a positive association between genetically related clusters and pathology, measured as bladder or kidney pathology and infection intensity [START_REF] Brouwer | Urinary tract pathology attributed to Schistosoma haematobium: does parasite genetics play a role?[END_REF].In comparison this link was not observedin a study that investigatedS. haematobiuminfections in Sudanese patients (Gasmelseed et al., 2014). However, this study used only one RAPD marker showing only 3 distinct genotypes clearly lacking thepower to concludethere is any link between genetic diversity and pathology. There is only one study that has used microsatellite markers to investigate any links between genetic diversity, prevalence and intensity of infection [START_REF] Aemero | Genetic diversity, multiplicity of infection and population structure of Schistosoma mansoni isolates from human hosts in Ethiopia[END_REF]. However, with only three studied sites no statistical tests could be validated. Only one study has clearly shown an association between genetic variation, in terms of microsatellite allelic variation, and human disease phenotype for S. mansoni [START_REF] Boon | Barcoding hybrids: heterogeneous distribution of Schistosoma haematobium x Schistosoma bovis hybrids across the Senegal River Basin[END_REF]. The study showed a positive association between a specific microsatellite allele and infection intensity, while a negative association for another allele of the same locus was also associated [START_REF] Boon | Barcoding hybrids: heterogeneous distribution of Schistosoma haematobium x Schistosoma bovis hybrids across the Senegal River Basin[END_REF]. Twenty one percent of the variation at this locus was explained by infection intensity, host age and bladder morbidity [START_REF] Boon | Barcoding hybrids: heterogeneous distribution of Schistosoma haematobium x Schistosoma bovis hybrids across the Senegal River Basin[END_REF]. This microsatellite allele is located near the cGMP-dependent protein kinase gene that is expressed in reproductive organs of adult schistosome worms and appears to be linked to egg production [START_REF] Leutner | Characterization of the cGMP-dependent protein kinase SmcGK1 of Schistosoma mansoni[END_REF][START_REF] Leutner | Combinatory microarray and SuperSAGE analyses identify pairing-dependently transcribed genes in Schistosoma mansoni males, including follistatin[END_REF].

Hybridization

A. Schistosoma inter-species interactions and hybridization

It is clear that Schistosoma species are diverse and have evolved several genetically well-defined species that have distinct phenotypic traits. In Africa both biological (host specificities/preferences, relatedness and anatomical site of infection, inter-species dominance/competition) and ecological barriers (geographical distribution, intermediate snail host habitats, transmission site ecology) have certainly driven speciation within the genus and also prevent inter-species interactions and hybridisation. However, it is well known that given the opportunity species can interact and also inter-breed, facilitated by the dioecious sexual stage in the definitive host, and this has been clearly demonstrated through multiple experimental hybridization studies involving different Schistosoma species (reviewed in [START_REF] Leger | Hybridizations within the Genus Schistosoma: implications for evolution, epidemiology and control[END_REF]. This means that the presence or absence of reproductive isolating mechanisms cannot be used as a criterion for defining the different species. The success of inter-species interactions and the production of viable hybrid offspring is highly dependent of the species and also geographical strains involved, and there are also several biological features observed within different combinations. These include non-reciprocal crosses, species competition (particularly male worm competition) and dominance, parthenogenetic reproduction, hybrid vigour and also hybrid instability. These features are not uniform, are highly dynamic and probably play a crucial role in preventing prolific admixing in nature, particularly for species that co-infect the same definitive host. Relatedness also plays a key role in preventing genetic exchange and hybrid speciation (Figure 1, Table 2), with interactions between more distantly related species resulting in parthenogenetic reproduction or no viable hybrid offspring, compared to viable hybridisation between more closely related species. It does appear that under natural conditions interspecies genetic exchanges are limited by the existence of geographic separation and/or the existence of pre-or postzygotic reproductive isolating barriers. However, there is increasing evidence for natural hybridization between species possibly influenced by increased environmental changes, both manmade and natural, together with the movement of parasites within their definitive hosts into new areas enabling interspecies interactions. Additionally, our improved ability to characterise schistosome populations and identify hybrids, with the utility of molecular and genomic analyses, may also be uncovering hybridization and genetic introgression that have been occurring as part of the natural epidemiology of these species, but until recently remained undetected. The existence of genetic exchanges between species of schistosomes and the formation of natural hybrids is indeed a topical subject under investigation in several African foci. The relationship of changes in disease transmission and human/animal health with inter-species interactions, together with the potential for novel reservoirs of infection and zoonotic transmission are clearly important in terms of disease control and elimination. In the next sections we review and discuss the genetic evidence for some inter-species interactions between African Schistosoma species in their natural settings with a focus on those involving human species.

B. Molecular identification of hybrids

Modern genetic methods allow for a more accurate assessment of infections and also allow for the identification and deciphering of inter-species hybridization. Certain DNA regions are now routinely used for species and hybrid identification. These include a mitochondrial (mtDNA) DNA region (routinely cox1, although other gene regions have been used) to identify the maternal line. Species specific SNP's within nuclear ribosomal DNA regions (nrDNA) (routinelythe ITS1 and ITS2 rDNA regions are used however other markers are needed for certain species combinations)are used to provide the nuclear genetic profile of the species involved [START_REF] Huyse | Bidirectional introgressive hybridization between a cattle and human schistosome species[END_REF][START_REF] Pennance | Interactions between Schistosoma haematobium group species and their Bulinus spp. intermediate hosts along the Niger River Valley[END_REF][START_REF] Webster | Schistosome Interactions within the Schistosoma haematobium Group, Malawi[END_REF][START_REF] Webster | Genetic diversity within Schistosoma haematobium: DNA barcoding reveals two distinct groups[END_REF](Figure 1). The occurrence of hybridisation eventsare often identified by the unexpected mitochondrial signatures presented by samples that do not relate to the infected hoststhat are being sampled e.g. S. bovis mt signatures for samples retrieved from human hosts. This then leads to more scrutiny of the nuclear profile of the samples with clear discordance between mt and nuclear signatures incriminating hybrid forms (Figure 1). It is acknowledged that this basic genetic profiling does not enable the identification of the hybrid generation or parental backcrosses however [START_REF] Platt | Ancient Hybridization and Adaptive Introgression of an Invadolysin Gene in Schistosome Parasites[END_REF], itinfersthat inter-species genetic exchange has occurred successfully with the generation of hybrid offspring. Mixed nuclear profiles (copies from both parental species) do suggest earlier generation hybrids, and possibly F1 generations however, it still remains unknown as to the generational speed of genetic homogenization, and loss of signal from one of the parental species preventing accurate hybrid identification [START_REF] Leger | Prevalence and distribution of schistosomiasis in human, livestock, and snail populations in northern Senegal: a One Health epidemiological study of a multi-host system[END_REF]. This may also mean that hybrids are often missed by this form of identification and that they may be more prevalent than the data suggests. Whole genome studies are providing a more recent and valuable opportunity for investigating and understanding these hybridization events, and their outcomes, and should form an integral component of natural hybridization investigations going forward [START_REF] Kincaid-Smith | Whole genome sequencing and morphological analysis of the humaninfecting schistosome emerging in Europe reveals a complex admixture between Schistosoma haematobium and Schistosoma bovis parasites[END_REF][START_REF] Platt | Ancient Hybridization and Adaptive Introgression of an Invadolysin Gene in Schistosome Parasites[END_REF]. These enable the assessment of the genetic composition of hybrids across the whole genome allowing us to decipher different generations of hybrids and backcrossed / introgressed forms by the assessment of the different genomic levels from the two parental forms. From this we can determine if hybridization is active, rare and / or ancient and can also identify directional hybridization/introgression [START_REF] Platt | Ancient Hybridization and Adaptive Introgression of an Invadolysin Gene in Schistosome Parasites[END_REF]. This relates directly to any pre-and postreproductive isolating barriers that may be in place that help maintain species integrities and prevent hybrid speciation [START_REF] Kincaid-Smith | Whole genome sequencing and morphological analysis of the humaninfecting schistosome emerging in Europe reveals a complex admixture between Schistosoma haematobium and Schistosoma bovis parasites[END_REF][START_REF] Oey | Whole-genome sequence of the bovine blood fluke Schistosoma bovis supports interspecific hybridization with S. haematobium[END_REF][START_REF] Platt | Ancient Hybridization and Adaptive Introgression of an Invadolysin Gene in Schistosome Parasites[END_REF].

C. Natural S. mansoni group specieshybridisation (S. mansoni X S. rodhaini)

As described previously,S. mansoni and S. rodhainiare closely related sister species and both have the ability to infect rodents. This provides the opportunity for these two species to encounter each other. S. rodhainiis highly specific to rodents and has also developed phenotypic traits supporting its transmission via nocturnal emergence patterns. S. mansoni is a human pathogen that has important zoonotic reservoirs including non-human primates and rodents (Standley et al., 2012b;Standley et al., 2012c).

Natural hybrids between S. mansoni and S. rodhainihave been identified in sympatric East African zones [START_REF] Morgan | First report of a natural hybrid between Schistosoma mansoni and S. rodhaini[END_REF]Steinauer et al., 2008b) (Table 2). S. rodhainihas a more focal distribution compared to S. mansoni and is currently only found transmitted in Rwanda, Kenya, Tanzania, Burundi, Uganda, Zaire and also there is a report from Nigeria. S. mansoni overlaps with S. rodhaini in several of these countries and natural hybrids have been identified in Tanzania and Kenya [START_REF] Morgan | First report of a natural hybrid between Schistosoma mansoni and S. rodhaini[END_REF]Steinauer et al., 2008b). Hybrids have only ever been found emitted from Biomphalaria snails with the molecular data concluded that no F1 hybrids were present with allappearing as introgressed forms. The data suggested a relatively low hybridisation rate and thathybrid backcrossing was rapidly occurring. True hybridisation appeared episodic and strongly influenced by the transmission dynamics of the different species, particularly related to S. rodhainiwith transmission and the opportunity to hybridise being highly seasonal. There also appeared to be strong directional introgression of genetic material from S. rodhainiinto S. mansoni.

D. Natural S. haematobium group hybridisation

The S. haematobium group is the most diverse Schistosomaspecies group with nine species several of which are sympatric (Figure 1). Three closely related species are human pathogens providing opportunities for interactions and hybridisation. Several of the others also have opportunities to interact within their non-human hosts particularly bovids. Interestingly hybridisation within this group centres around the most widespread, prevalentand medically important species S. haematobium. Particularly its hybridisation with bovid species is attracting attention due to the zoonotic components that may be facilitating this or the potential risk of the emergence of zoonotic strains and the creation of zoonotic reservoirs [START_REF] Leger | Prevalence and distribution of schistosomiasis in human, livestock, and snail populations in northern Senegal: a One Health epidemiological study of a multi-host system[END_REF].

S. haematobium and S. guineensis

Until 2003 S. guineensis and S. intercalatum were considered one species (S. intercalatum) but were split based on their mitochondrial divergence [START_REF] Pagès | Reconnaissance de deux espèces jumelles au sein du taxon Schistosoma intercalatum Fisher, 1934, agent de la schistosomose humaine rectale en Afrique. Description de Schistosoma guineensis n. sp[END_REF][START_REF] Webster | A revision of the interrelationships of Schistosoma including the recently described Schistosoma guineensis[END_REF].When S. guineensis and S. intercalatum were split into their separate species,S. intercalatum was recognised as being restricted to more central African regions, namely the Democratic Republic of Congo (DRC) whereas S. guineensis is found on South West coastal countries and Sao Tome(Figure 1). In the literature many of the earlier reports related to S. guineensis will refer to S. intercalatum and so for ease of understanding going forward, this will be described as S. guineensis (intercalatum).

It is apparent that S. haematobium and S. guineensis readily hybridise in nature. This is probably not surprising given the fact that they are sister species and human pathogens, however they do utilize different anatomical sites of infection (urogenital and intestinal respectively) probably reducing interactions. Evidence of hybridisation between S. haematobium and S. guineensis (intercalatum), has been reported in several areas of Cameroon (Loum, KindingNdjabi, Kumba, BarombiKotto [START_REF] Ratard | A new focus of Schistosoma haematobium/S. intercalatum hybrid in Cameroon[END_REF][START_REF] Ratard | Human schistosomiasis in Cameroon. I. Distribution of schistosomiasis[END_REF][START_REF] Southgate | Schistosomiasis at Loum, Cameroun; Schistosoma haematobium, S. intercalatum and their natural hybrid[END_REF]Webster et al., 2003;[START_REF] Wright | Hybrids between Schistosoma haematobium and S. intercalatum in Cameroon[END_REF]predominately based on polymorphic eggs excreted in the urine of patients and this was later backed up with molecular data obtained from preserved isolates (Webster et al., 2003[START_REF] Webster | A single-strand conformation polymorphism (SSCP) approach for investigating genetic interactions of Schistosoma haematobium and Schistosoma guineensis in Loum, Cameroon[END_REF]. Molecular analyses, using mtcox1 and nrDNA ITS2 + IGS genetic markers, of samples collected from various time periods during the fluctuation ofS. haematobium and S. guineensis (intercalatum)in Cameroon clearly showed the presence of S. haematobium X S. guineensis (intercalatum)hybrids, with a predominance of hybrids probably resulting from interactions between S. haematobium ♂'s with S. guineensis (intercalatum) ♀'s (Webster and Southgate, 2003;[START_REF] Webster | The interaction of Schistosoma haematobium and S. guineensis in Cameroon[END_REF][START_REF] Webster | A single-strand conformation polymorphism (SSCP) approach for investigating genetic interactions of Schistosoma haematobium and Schistosoma guineensis in Loum, Cameroon[END_REF]. The data also clearly showed a decline in numbers of hybrids over time suggesting a replacement of S. guineensis (intercalatum)by S. haematobium through introgressive hybridization [START_REF] Tchuem Tchuente | Schistosoma intercalatum: an endangered species in Cameroon?[END_REF][START_REF] Webster | The interaction of Schistosoma haematobium and S. guineensis in Cameroon[END_REF].

The natural occurrence of S. haematobium X S. guineensis (intercalatum) hybridisation has also be reported in Gabon and also Benin [START_REF] Mone | Natural Interactions between S. haematobium and S. guineensis in the Republic of Benin[END_REF][START_REF] Zwingenberger | Mixed Schistosoma haematobium/Schistosoma intercalatum infection[END_REF]. The Gabon account was based on egg morphology and remains unconfirmed. [START_REF] Mone | Natural Interactions between S. haematobium and S. guineensis in the Republic of Benin[END_REF]molecularly identifiedS. haematobium X S. guineensishybrid miracidia collected from human urine samples in Benin. All samples showed maternal mtDNA cox1 S. haematobium genetic profiles suggesting that these were not early generation hybrids but probably introgressed forms. Current foci for S. guineensis remain severely underreported due to their scarcity and lack of knowledge related to this species. However, foci do still exist in Central African/South Western regions and are likely in some areas to overlap with S. haematobium.

S. haematobium and S. mattheei

Many sympatric zones exist for these species however, compared to S. haematobium S. mattheei has a more restricted distribution to East African regions and typically below a latitude of 10º South (Figure 1). S. mattheei is considered to be a potential pathogen of humans although it is primarily a pathogen of bovids (Standley et al., 2012a). It still needs to be elucidated if S. mattheei actually infects humans or is only found in humans as part of mixed infections and/or in a hybrid form.

Many historical observations exist forS. mattheei shaped eggs being excreted by humans in South and South Eastern regions of Africa, where S. haematobium and S. mattheei are sympatric(reviewed in [START_REF] Leger | Hybridizations within the Genus Schistosoma: implications for evolution, epidemiology and control[END_REF] (Table 2). At the time molecular methods could not be used and only observational data are available from these earlier studies. Wright and Ross (1980), followed up this earlier work and confirmed natural hybrids between S. haematobium and S. mattheei using species-specific biochemical markers, with field isolates showing the same enzymatic patterns as laboratory produced hybrid offspring (Wright and Ross, 1980).

In the late 1980's Kruger (Kruger, 1987(Kruger, , 1988(Kruger, , 1990a;;[START_REF] Kruger | Do all human urinary infections with Schistosoma mattheei represent hybridization between S. haematobium and S. mattheei?[END_REF][START_REF] Kruger | Scanning electron microscope studies of miracidia suggest introgressive hybridization between Schistosoma haematobium and S. haematobium x S. mattheei in the Eastern Transvaal[END_REF][START_REF] Kruger | Phenotypic differences in Schistosoma mattheei ova from populations sympatric and allopatric to S. haematobium[END_REF]and colleagues further investigatedS. mattheei X S. haematobiuminteractions,using both morphological and biochemical methods, identifyingS. mattheei X S. haematobium hybrids from a patient from Nelspruit, in the north east of South Africa. Phenotypic differences suggested that the S. mattheei populations that were sympatric with S. haematobium, may have been infiltrated by genetic introgression from S. haematobium in these areas. However,it was evident that the proportion of hybrid forms was low and that parental backcrossing with introgression of S. mattheei genetic material into the S. haematobium populations was probable.

In 1994, De Bontand colleagues (De Bont et al., 1994), found evidence, based on egg morphology and biochemical markers, for suspected S. haematobium-mattheei hybrids in cattle however, they could not conclude if there were either S. mattheei x S. haematobium or S. mattheei x S. leiperi hybrids. Since then, very few studies have followed up on the occurrence of the natural hybridisation between S. haematobium and S. mattheei, but very recentlymolecular data (mt cox1, ITS and 18S nrDNA) has provided indisputable evidence for the occurrence of S. haematobium-mattheei in Malawi [START_REF] Webster | Schistosome Interactions within the Schistosoma haematobium Group, Malawi[END_REF]. The study was conducted due to the observations of atypical eggs (resembling S. mattheei and also S. bovis) excreted in the urine of children atspecificS. haematobium foci in Malawi. Additionally, very recently S. haematobium-mattheei mix / hybrid infections were alsoidentified, via the molecular analysis of serum,in a group of tourists who became infected in the north east of South Africa in 2016/2017 [START_REF] Cnops | Acute Schistosomiasis with A S. mattheei x S. haematobium Hybrid Species in a Cluster of 34 Travelers Infected in South Africa[END_REF]. As with the more historical accounts these more recent findings still point to asymmetric interactions highlighting the extreme biases observed in these hybridisation systems with pre-and postzygotic reproductive barriers in place to reduce prolific admixing.

S. haematobium and S. bovis

These two species are widespread across Africa and are sympatric in many countries. S. bovis has a slightly more restricted distribution primarily above a latitude of 10º South below which it is replaced by S. mattheei. Natural hybridisation between S. haematobium X S. bovis has become the most recent subject of several research groups mainly due to the identification of high numbers of hybrids excreted by humans in West African countriesand the possible implications for schistosomiasis control, in relation to zoonotic transmission (Table 2).

The first observations of natural hybridisation were made by Brémondet al. in 1993 who identified schistosomes excreted by humans in Niger, with allozyme profiles that were intermediate between S. bovis and S. haematobium or S. curassoni, but more sensitive markers were needed to discriminate S. haematobium from S. curassoni [START_REF] Brémond | Arguments en faveur d'une modification du génome (introgression) du parasite humain Schistosoma haematobium par des gènes de S. bovis, au Niger[END_REF].

However, these findings did indicate that some sort of inter-species interaction had or was taking place. No further studies investigated these hybridisations in nature occurred until 2009 where [START_REF] Huyse | Bidirectional introgressive hybridization between a cattle and human schistosome species[END_REF] using molecular methods (mt cox1 and nuclear ITS analysis) identified the high abundance and wide distribution of S. haematobium X S. bovis hybrids along the Senegal River Basin. These hybrids were found in multiple sites and were collected from both humans and snail hosts (B. truncatus and B. globosus). An expanded study was conducted in Senegal (both in the North and South of Senegal) in 2009-2010 (Webster et al., 2013), that also examined infections in domestic livestock and humans. It was found that high levels of hybrids occurred in the human population in areas of sympatry but no hybrids or S. haematobium were found in livestock. This clearly showed the biases within the hybrid system, with humans being the primary host for the hybrids withstrongcorrelation with urinary excretion of the hybrids. Further studies by [START_REF] Boon | Detecting hybridization in African schistosome species: does egg morphology complement molecular species identification?[END_REF][START_REF] Boon | Barcoding hybrids: heterogeneous distribution of Schistosoma haematobium x Schistosoma bovis hybrids across the Senegal River Basin[END_REF]showed the heterogeneity of these hybrids and also the close association with co-infections with S. mansoni. In 2013, a surprise came with the outbreak of urogenital schistosomiasis on the Mediterranean Island of Corsica (France) [START_REF] Boissier | Schistosomiasis reaches Europe[END_REF], with hybrids molecular identified within the schistosome populations excreted from several infected tourists [START_REF] Boissier | Outbreak of urogenital schistosomiasis in Corsica (France): an epidemiological case study[END_REF][START_REF] Mone | [END_REF]. It was also concluded, via molecular phylogenetic analyses that the parasites had been imported from West Africa and in particular Senegal, where there are high levels of migration between these French colonies [START_REF] Boissier | Outbreak of urogenital schistosomiasis in Corsica (France): an epidemiological case study[END_REF]. Interestingly, a single egg excreted from one of the infected tourists showed a genetic profile suggesting a pure S. bovis origin (both nuclear ITS and mt cox1 DNA giving a S. bovis profile) presenting some evidence for potential S. bovis zoonotic infection however, more in-depth genomic analysis would be needed to determine the genetic make-up of this egg. Sporadic eggs like this have also been found in studies in Senegal and also suggest a pure S. bovis origin however, they are very rare and their viability needs to be determined. Whole genome analyses of miracidia from Senegal and Niger failed to identify F1 hybrids within the samples analysed and suggested that the hybrid populations are highly introgressed forms, with hybridisation being ancient and not active. Additionally, genetic microsatellite analyses of hybrids in Senegal and Niger have shown clear population differentiation between the schistosomes infecting humans (S.

haematobium and the S. haematobium-bovis hybrids versus S. bovis) suggesting no or very limited active interactions/hybridisation and gene flow between the populations [START_REF] Boon | No barrier breakdown between human and cattle schistosome species in the Senegal River Basin in the face of hybridisation[END_REF][START_REF] Pennance | Interactions between Schistosoma haematobium group species and their Bulinus spp. intermediate hosts along the Niger River Valley[END_REF]. Further areas of hybridisation between S. haematobium and S.

bovis have now been molecularly identified, including areas in Niger, Benin, Mali, Cote D'Ivoire and also Malawi [START_REF] Leger | Prevalence and distribution of schistosomiasis in human, livestock, and snail populations in northern Senegal: a One Health epidemiological study of a multi-host system[END_REF][START_REF] Pennance | Interactions between Schistosoma haematobium group species and their Bulinus spp. intermediate hosts along the Niger River Valley[END_REF][START_REF] Savassi | Cattle as natural host for Schistosoma haematobium (Bilharz, 1852) Weinland, 1858 x Schistosoma bovis Sonsino, 1876 interactions, with new cercarial emergence and genetic patterns[END_REF][START_REF] Soentjens | Diagnosis and Clinical Management of Schistosoma haematobium-Schistosoma bovis Hybrid Infection in a Cluster of Travelers Returning From Mali[END_REF][START_REF] Tian-Bi | Molecular characterization and distribution of Schistosoma cercariae collected from naturally infected bulinid snails in northern and central Cote d'Ivoire[END_REF]. The Benin study [START_REF] Savassi | Cattle as natural host for Schistosoma haematobium (Bilharz, 1852) Weinland, 1858 x Schistosoma bovis Sonsino, 1876 interactions, with new cercarial emergence and genetic patterns[END_REF] indicated the possible occurrence of S. haematobium-bovis hybrids in cattle, identified by the shift in cercarial shedding patterns of the resultant progeny isolated from infected cattle together with atypical genetic profiles of the adult worms.It is clear from the data from multiple studies that this hybridisation is common but it is not random and there are specific biological and genetic influences taking place, which may also differ between foci. This raises several questions with regard to the impact of this hybridisation which, only further detailed epidemiological and in depth genetic/genomic studies can help unravel.

S. haematobium and S. curassoni

Interactions between these two species are rarer, mainly due to the more restricted distribution ofS. curassoni to specific foci in a few West African countries with most transmission in Senegal [START_REF] Leger | Prevalence and distribution of schistosomiasis in human, livestock, and snail populations in northern Senegal: a One Health epidemiological study of a multi-host system[END_REF][START_REF] Leger | Hybridizations within the Genus Schistosoma: implications for evolution, epidemiology and control[END_REF][START_REF] Webster | [END_REF] (Table 2). In the early 1990's suspected hybrids between S. haematobium and S. curassoni, based on egg morphology and biochemical markers,were observed excreted by humans in Niger but the species involved could not decisively be determined as either S. bovis or S. curassoni [START_REF] Brémond | Arguments en faveur d'une modification du génome (introgression) du parasite humain Schistosoma haematobium par des gènes de S. bovis, au Niger[END_REF].More recent work in Senegal has providedevidence of S. haematobium-curassoni hybrids excreted via the urine of patients in sympatric areas of Senegal, although more in-depth genetic analysis would support our understanding of these interactions in these endemic zones [START_REF] Webster | [END_REF] (Table 2). Particularly the distinction between S. bovis and S. curassoni using traditional nuclear markers is not always robust with a single species-specific SNP in the ITS1 nrDNA that allows distinction between the two species [START_REF] Pennance | Interactions between Schistosoma haematobium group species and their Bulinus spp. intermediate hosts along the Niger River Valley[END_REF][START_REF] Webster | [END_REF], with the use of other nrDNA regions such as the 18S needed for further clarification. A very recent report by [START_REF] Leger | Prevalence and distribution of schistosomiasis in human, livestock, and snail populations in northern Senegal: a One Health epidemiological study of a multi-host system[END_REF]showed no S. haematobium and S. curassoni interactions in sympatric zones in Senegal suggesting that interactions between these species are rare and unstable. More detailed studies into the dynamics of these interactions and hybridisation events in different settings is certainly warranted as it is likely that differences occur at both micro and macro geographical scales [START_REF] Stothard | Future schistosome hybridizations: Will all Schistosoma haematobium hybrids please stand-up![END_REF].

S. haematobium and S. mansoni

S. haematobium and S. mansoni represent two co-endemic pathogens that are both highly prevalent in humans but highly divergent from each other, a key postzygotic reproductive isolating barrier retaining these species integrities. There have been many reports of the possible interactions between these species in their human hosts based on atypical egg excretion, ectopic egg deposition and host morbidity assessments and competitive exclusion of S. mansoni by S. haematobium in some areas [START_REF] Cote D'ivoire | [END_REF][START_REF] Koukounari | The impact of single versus mixed schistosome species infections on liver, spleen and bladder morbidity within Malian children pre-and post-praziquantel treatment[END_REF][START_REF] Stothard | Future schistosome hybridizations: Will all Schistosoma haematobium hybrids please stand-up![END_REF] (Table 2). Lateral-spined eggs, with a typical S. mansonimorphology, have often been observed in urine samples and terminal-spined S. haematobium shapedeggs have been observed in stool samples in several co-endemic areas including Kenya, Egypt, Senegal, Malawi, Mali and Cote D'Ivoire. However, the occurrences of these atypical egg excretions in relation to heterospecific interactions are dynamic and are not observed in all co-endemic areas and in all patients. (Huyse et al., 2013a) conducted the first molecular analyses of eggs / miracidia using nrDNAITS and mt DNA cox1 markers and showed that the offspring contained genetic material from both parental species. This disputes parthenogenesis and suggests hybridisation has occurred between these two distantly related species however, the viability of the offspring to infect snails remains unknown. Additionally, a case of S. haematobium-mansoni hybrids were molecularly identified from atypical eggs excreted from a migrant boy who had migrated from Cote D'Ivoire, via West Africa to France, but again the viability of the offspring was not able to be tested(Le [START_REF] Govic | Schistosoma haematobium-Schistosoma mansoni Hybrid Parasite in Migrant Boy, France, 2017[END_REF]. The impact of such inter-species interactions, even if they are not viable, on human health should not be dismissed as they are likely to cause increased pathology due to ectopic egg deposition and a higher abundance of eggs being deposited in organs and tissues.

Conclusion

Molecular epidemiological studies on the diverse range of Schistosoma specieshave provided valuable insights into transmission dynamics, population and species level diversity over time and space, and how selective pressures such as drug treatmentmay shape and change the schistosome populations. Results obtained in the last decade clearly show, and in particular forS. mansoni and S. haematobium, that whatever the schistosome species involved only few barriers to gene flow exist at a local and regional scale. However, despite this, species have maintained their integrity with natural selection processes making them highly adapted to their environments with successful and ongoing transmission. It is acknowledged that our knowledge on the genetic distribution and the evolutionary history of other human-associated and all animal-associated African Schistosoma species is currently very limited if not completely absent at a large geographical scale. It is thus crucial to gather more information on the population genetic structuring of these schistosomes including the recently identified hybrids in order to; i) infer the role of the definitive hosts on the parasite gene flow and ii) to better understand their relative importance in the epidemiology and evolution of African Schistosoma species, withparasite genetic variation known to be an important factor in explaining the variation in host disease phenotype.

Another important aspect in studying animal infecting schistosomes, potential zoonotic hybrids and animal reservoirs of infection is the role they may play in spreading resistance or virulence alleles among and between populations. Indeed, despite several years of vast MDA coverage in Africa and extensive treatment campaigns, many areas characterized as "persistent hotspots" fail to substantially decrease transmission, with on-going high levels of prevalence, intensity and associated human morbidity.

Studies focusing on the genetic structure of Schistosoma populations and their response to MDA suggest that rapid re-infection of parasites from genetically differentiated source populations occurs and that MDA alone is not sufficient to control the disease transmission [START_REF] Van Den Broeck | A Darwinian outlook on schistosomiasis elimination[END_REF]. High levels of gene flow between parasite populations before and after treatment together with refugia may facilitate a rapid recovery of population genetic diversity with treatment having only a limited impact on parasite transmission at the community level. In addition, whether drug resistance or an increased genetic diversity brought about by inter-species/population genetic exchange, maybe influencing reduced drug efficacy in some foci needs to be investigated.

Broader and more fine-scale molecular epidemiological analyses of schistosome populations are warranted both to understand transmission and infection dynamics, but also in relation to treatmentresponses. Powerful genetic analysis tools, such as Whole-Genome Single Nucleotide Polymorphisms (SNP), now exist and are quickly being optimised to study natural Schistosoma populations and will provide greater insights into many aspects of schistosome biology, evolution, genetics, transmission dynamics, inter-species interactions and resistance/versus re-infection dynamics, all of which will support control and elimination of this debilitating disease.

Acknowledgement

This work was supported by the program HySWARM (ANR-18-CE35-0001) from the French Research National Agency respectively awarded to O. Rey and J. Boissier. Amos Onykwere is funded by Campus France. This study is set within the framework of the "Laboratoires d'Excellence (LABEX)" TULIP (ANR-10-LABX-41).

Table 2. Details of the detected natural hybridisation between different Schistosoma species combinations in Africa. In bold are the cases that have been confirmed using molecular data.

Other accounts shown in (), are from atypical/morphological observations and biochemical markers. *This is the suspected occurrence of S. haematobium-bovis hybrids in cattle based on cercarial circadium rhythms. The years are the first and last year of the reported hybridisation.

The host in the definitive host that the hybrids / introgressed forms were recovered from.

Figure legend Figure 1 .

 legend1 Figure legend

Schistosoma mansoni with adult worms surviving praziquantel treatment at the start of mass drug administration in Uganda. Parasites & vectors 12, 607. Fenwick, A., 2006. New initiatives against Africa's worms. Transactions of the Royal Society of Tropical Medicine and Hygiene 100, 200-207. Fenwick, A., Webster, J.P., 2006. Schistosomiasis: challenges for control, treatment and drug resistance. Curr. Opin. Infect. Dis. 19, 577-582. Gasmelseed, N., Karamino, N.E., Abdelwahed, M.O., Hamdoun, A.O., Elmadani, A.E., 2014. Genetic diversity of Schistosoma haematobium parasite Iis not associated with severity of disease in an endemic area in Sudan. BMC infectious diseases 14, 469. Glenn, T.C., Lance, S.L., McKee, A.M., Webster, B.L., Emery, A.M., Zerlotini, A., Oliveira, G., Rollinson, D., Faircloth, B.C., 2013. Significant variance in genetic diversity among populations of Schistosoma haematobium detected using microsatellite DNA loci from a genome-wide database. Parasites & vectors 6, 300. Gouvras, A.N., Kariuki, C., Koukounari, A., Norton, A.J., Lange, C.N., Ireri, E., Fenwick, A., Mkoji, G.M., Webster, J.P., 2013. The impact of single versus mixed Schistosoma haematobium and S. mansoni infections on morbidity profiles amongst school-children in Taveta, Kenya. Acta Trop. 128, 309-317. Gower, C.M., Gabrielli, A.F., Sacko, M., Dembele, R., Golan, R., Emery, A.M., Rollinson, D., Webster, J.P., 2011. Population genetics of Schistosoma haematobium: development of novel microsatellite markers and their application to schistosomiasis control in Mali. Parasitology 138, 978-994. Gower, C.M., Gehre, F., Marques, S.R., Lamberton, P.H.L., Lwambo, N.J., Webster, J.P., 2017. Phenotypic and genotypic monitoring of Schistosoma mansoni in Tanzanian schoolchildren five years into a preventative chemotherapy national control programme. Parasites & vectors 10, 593. Gower, C.M., Gouvras, A.N., Lamberton, P.H., Deol, A., Shrivastava, J., Mutombo, P.N., Mbuh, J.V., Norton, A.J., Webster, B.L., Stothard, J.R., Garba, A., Lamine, M.S., Kariuki, C., Lange, C.N., Mkoji, G.M., Kabatereine, N.B., Gabrielli, A.F., Rudge, J.W., Fenwick, A., Sacko, M., Dembele, R., Lwambo, N.J., Tchuem Tchuente, L.A., Rollinson, D., Webster, J.P., 2013. Population genetic structure of Schistosoma mansoni and Schistosoma haematobium from across six sub-Saharan African countries: implications for epidemiology, evolution and control. Acta Trop. 128, 261-274. Gower, C.M., Shrivastava, J., Lamberton, P.H., Rollinson, D., Webster, B.L., Emery, A., Kabatereine, N.B., Webster, J.P., 2007. Development and application of an ethically and epidemiologically advantageous assay for the multi-locus microsatellite analysis of Schistosoma mansoni. Parasitology 134, 523-536. Hotez, P.J., Alvarado, M., Basanez, M.G., Bolliger, I., Bourne, R., Boussinesq, M., Brooker, S.J., Brown, A.S., Buckle, G., Budke, C.M., Carabin, H., Coffeng, L.E., Fevre, E.M., Furst, T., Halasa, Y.A., Jasrasaria, R., Johns, N.E., Keiser, J., King, C.H., Lozano, R., Murdoch, M.E., O'Hanlon, S., Pion, S.D., Pullan, R.L., Ramaiah, K.D., Roberts, T., Shepard, D.S., Smith, J.L., Stolk, W.A., Undurraga, E.A., Utzinger, J., Wang, M., Murray, C.J., Naghavi, M., 2014. The global burden of disease study 2010: interpretation and implications for the neglected tropical diseases. PLoS Negl Trop Dis 8, e2865. Huyse, T., Boon, N.A.M., Van den Broeck, F., Mbow, M., Chaturvedi, A., Meurs, L., Volckaert, F.A.M., Polman, K., 2018. Evolutionary epidemiology of schistosomiasis: linking parasite genetics with disease phenotype in humans. Int. J. Parasitol. 48, 107-115. Huyse, T., Van den Broeck, F., Hellemans, B., Volckaert, F.A., Polman, K., 2013a. Hybridisation between the two major African schistosome species of humans. Int. J. Parasitol. 43, 687-689.