S Gourbière

P Dorn

F Tripet

Professor E Dumonteil

Genetics and evolution of triatomines: from phylogeny to vector control

Keywords: Chagas disease, phylogeny, population genetics, community epidemiology, Trypanosoma TRIATOMINES

Triatomines are hemipteran bugs acting as vectors of the protozoan parasite Trypanosoma cruzi. This parasite causes Chagas disease, one of the major parasitic diseases in the Americas. Studies of triatomine genetics and evolution have been particularly useful in the design of rational vector control strategies, and are reviewed here. The phylogeography of several triatomine species is now slowly emerging, and the struggle to reconcile the phenotypic, phylogenetic, ecological and epidemiological species concepts makes for a very dynamic field. Population genetic studies using different markers indicate a wide range of population structures, depending on the triatomine species, ranging from highly fragmented to mobile, interbreeding populations. Triatomines transmit T. cruzi in the context of complex interactions between the insect vectors, their bacterial symbionts and the parasites; however, an integrated view of the significance of these interactions in triatomine biology, evolution and in disease transmission is still lacking. The development of novel genetic markers, together with the ongoing sequencing of the Rhodnius prolixus genome and more integrative studies, will provide key tools to expanding our understanding of these important insect vectors and allow the design of improved vector control strategies.

 [START_REF] Silva | Comparative ultrastructural analysis of the antennae of Triatoma guazu and Triatoma jurbergi (Hemiptera: Reduviidae) during the nymphal stage development[END_REF][START_REF] Catala | Antennal phenotype of Triatoma dimidiata populations and its relationship with species of Phyllosoma and protracta complexes[END_REF]and rostrum [START_REF] Silva | Comparative study of the stridulatory sulcus, buccula and rostrum of the nymphs of Triatoma guazu and Triatoma jurbergi by scanning electron microscopy (Hemiptera, Reduviidae)[END_REF]. Within the triatomines, for example, the monophylly of the Rhodiniini tribe has been supported by apical antenna insertion, body forms, post-ocular callosities, male genital characteristics, egg surface architecture and salivary gland protein composition [START_REF] Tartarotti | Phylogenetic approach to the study of triatomines (Triatominae, Heteroptera)[END_REF].

Morphological characters can distinguish the vast majority of triatomine species (Carbajal de la [START_REF] Carbajal De La Fuente | Misidentification of two Brazilian triatomes, Triatoma arthurneivai and Triatoma wygodzinskyi, revealed by geometric morphometrics[END_REF]. Recently diverged species can sometimes be differentiated [START_REF] Gurgel-Goncalves | Geometric morphometrics and ecological niche modelling for delimitation of near-sibling triatomine species[END_REF] or be remarkably isomorphic, making the detection and description of cryptic taxa difficult [START_REF] Abad-Franch | Molecular research and the control of Chagas disease vectors[END_REF][START_REF] Nouvellet | Effects of genetic factors and infection status on wing morphology of Triatoma dimidiata species complex[END_REF]. A good example of the underlying taxonomic Structure, which could not be resolved by morphological studies alone, is the Triatoma sordida species complex whose Argentinean, Brazilian and Bolivian member species could only be described thanks to additional tools such as cytogenetics and molecular markers (Dujardin et al., 1999b). Other triatomine species show such high levels of phenotypic plasticity that they were erroneously split into several species. For example, chromatic variants of T. infestans (one of the most important vectors found in Peru, Bolivia, Brazil, Chile, Paraguay, Uruguay and Argentina) were considered a different species (Triatoma melanosoma in Argentina) or variants (a 'dark morph' in Bolivia). However, these 'species' or variants produced fertile offsprings with T. infestans and no difference was found using cytogenetics and molecular markers [START_REF] Noireau | Detection of sylvatic dark morphs of Triatoma infestans in the Bolivian Chaco[END_REF]Dujardin et al., 1999a;[START_REF] Monteiro | Mitochondrial DNA variation of Triatoma infestans populations and its implication on the specific status of T melanosoma[END_REF][START_REF] Bargues | Origin and phylogeography of the Chagas disease main vector Triatoma infestans based on nuclear rDNA sequences and genome size[END_REF]. Similarly, revisions are underway in the Phyllosoma complex, which includes important vectors species in Mexico, all of which are interfertile, so also suspected of incorrect species-level designations [START_REF] Martinez-Ibarra | Occurrence of hybrids and laboratory evidence of fertility among three species of the Phyllosoma complex (Hemiptera: Reduviidae) in Mexico[END_REF][START_REF] Martinez-Hernandez | Natural crossbreeding between sympatric species of the Phyllosoma complex (Insecta: Hemiptera: Reduviidae) indicate the existence of only one species with morphologic and genetic variations[END_REF].

There are countless examples in which experimental crosses have been used to complement morphological studies when the latter lead to inconclusive results [START_REF] Perez | Cytogenetics as a tool for triatomine species distinction (Hemiptera-Reduviidae)[END_REF], and these still have an important role in clarifying the specific and sub-specific status in triatomine species complexes. For example, crosses were instrumental in establishing the specific status of Triatoma pseudomaculata, an important sylvatic vector in Brazil (Correa and Espınola, 1964). They also helped in distinguishing Rhodnius prolixus and Rhodnius robustus, the main vectors of Chagas disease in Venezuela, Colombia and some other regions of central America [START_REF] Galındez-Giron | Relaciones reproductivas entre Rhodnius prolixus Stal y Rhodnius robustus Larrousse (Hemiptera, Reduviidae, Triatominae) bajo condiciones de laboratorio[END_REF]. As mentioned previously, crosses were important in clarifying taxonomy in the Infestans complex [START_REF] Abalos | Sobre hı´bridos naturales y experimentales de Triatoma[END_REF][START_REF] Franca | Vectors in Uruguay. Factores Biologicos y Ecologicos en la Enfermedad de Chagas[END_REF][START_REF] Monteiro | Mitochondrial DNA variation of Triatoma infestans populations and its implication on the specific status of T melanosoma[END_REF], and more recently, the Phyllosoma complex [START_REF] Martinez-Ibarra | Occurrence of hybrids and laboratory evidence of fertility among three species of the Phyllosoma complex (Hemiptera: Reduviidae) in Mexico[END_REF][START_REF] Martinez-Hernandez | Natural crossbreeding between sympatric species of the Phyllosoma complex (Insecta: Hemiptera: Reduviidae) indicate the existence of only one species with morphologic and genetic variations[END_REF].

As in the Diptera, cytogenetics has been an essential tool in Triatominae systematics since the early 1900s [START_REF] Payne | Some new types of chromosome distribution and their relation to sex[END_REF] and is still very much in use in combination with molecular markers [START_REF] Bargues | Origin and phylogeography of the Chagas disease main vector Triatoma infestans based on nuclear rDNA sequences and genome size[END_REF][START_REF] Dos Santos | Evolutionary relationships based on genetic and phenetic characters between Triatoma maculata, Triatoma pseudomaculata and morphologically related species (Reduviidae: Triatominae)[END_REF]. An interesting application involves cytogenetics used in combination with crosses to study karyotypic evolution or 're-patterning' [START_REF] Perez | Cytogenetic analysis of experimental hybrids in species of Triatominae (Hemiptera-Reduviidae)[END_REF].

Triatomines show high chromosome homogeneity; nearly all have 20 autosomes and most South American species show two sex chromosomes (XY). Secondary fragmentation is thought to have led to the few instances of 2 or 3 X-chromosomes (X1X2Y and X1X2X3Y) [START_REF] Perez | Cytogenetics as a tool for triatomine species distinction (Hemiptera-Reduviidae)[END_REF] found in some Central and North American species [START_REF] Panzera | Cytogenetics and genome evolution in the subfamily Triatominae (Hemiptera, Reduviidae)[END_REF]. Triatomine chromosomes are holocentric (that is, centromeric activity is spread over the entire chromosomal arm) and unlike the large polytenic metacentric chromosomes of the Diptera, which show clear euchromatic banding patterns, they yield very little information in terms of chromosomal polymorphism. Cytogeneticists have concentrated instead on comparing different stages of meiosis between putative species using cbanding staining, which marks constitutive heterochromatin and allows the identification of distinct heterochomatic blocks along chromosomes [START_REF] Perez | Cytogenetics as a tool for triatomine species distinction (Hemiptera-Reduviidae)[END_REF].

Cytogenetic comparisons have helped clarify the T. sordida complex of 'isomorphic' species. They highlighted chromosomal differences between the smaller and darker Argentinean populations of T. sordida (previously known as Triatoma garciabesi) and Brazilian populations, as well as between T. sordida and other closely related species of the complex [START_REF] Panzera | Genetic variability and geographic differentiation among three species of triatomine bugs (Hemiptera-Reduviidae)[END_REF].

Interestingly, chromosomes are often marked by intraspecific polymorphism in heterochromatic content [START_REF] Panzera | C-heterochromatin polymorphism in holocentric chromosomes of Triatoma infestans (Hemiptera, Reduviidae)[END_REF][START_REF] Panzera | Genomic changes of Chagas disease vector, South America[END_REF]. For example, the Andean populations of T. infestans present a 50% higher constitutive heterochromatin content than the non-Andean populations. This led to the suggestion that recent colonization of new habitats has been accompanied by genome reduction [START_REF] Panzera | Genomic changes of Chagas disease vector, South America[END_REF], but intermediate forms have been described in northern Argentina, which complicates phylogeographic interpretation [START_REF] Monteiro | Population genetics of triatomines[END_REF]. In recent studies, measurements of haploid genome size have been used to complement c-banding karyotype comparisons [START_REF] Panzera | Genomic changes of Chagas disease vector, South America[END_REF] or to corroborate molecular data [START_REF] Bargues | Origin and phylogeography of the Chagas disease main vector Triatoma infestans based on nuclear rDNA sequences and genome size[END_REF].

Cytogenetics also had a crucial role in the ongoing description of T. dimidiata. This species presents morphologically variable populations, but early attempts to delineate geographic boundaries between them failed, and clinal variation was proposed to explain this species' extensive polymorphism (reviewed by [START_REF] Dorn | Triatoma dimidiata (Latreille, 1811): a review of its diversity across its geographic range and the relationship among populations[END_REF]. Later on however, a combination of morphology, cytogenetics and molecular data led to the unambiguous detection of a cryptic taxon and the recognition of the Dimidiata species complex [START_REF] Marcilla | The ITS-2 of the nuclear rDNA as a molecular marker for populations, species, and phylogenetic relationships in Triatominae (Hemiptera: Reduviidae), vectors of Chagas disease[END_REF][START_REF] Panzera | Chromosomal variation and genome size support existence of cryptic species of Triatoma dimidiata with different epidemiological importance as Chagas disease vectors[END_REF][START_REF] Dorn | Triatoma dimidiata (Latreille, 1811): a review of its diversity across its geographic range and the relationship among populations[END_REF][START_REF] Dorn | Two distinct Triatoma dimidiata (Latreille, 1811) taxa are found in sympatry in Guatemala and Mexico[END_REF].

In the 1980s, isozyme analysis was first applied to population genetic questions of triatomines. Fixed allelic differences characterizing certain sympatric populations were quickly recognized as proof of their reproductive isolation, hence potential specific status. A good example was the 'chance' discovery of two reproductively isolated sylvatic populations of T. sordida in a study of genetic divergence between T. sordida and a closely related species, T. guasayana [START_REF] Noireau | Cryptic speciation in Triatoma sordida (Hemiptera: Reduviidae) from the Bolivian Chaco[END_REF]. Another fine example was the realization of the close relationship between the small Rhodnius ecuadoriensis and its 'large cousin' R. prolixus from Colombia, which morphological studies of the Rhodnius sub-tribe described as unrelated [START_REF] Chavez | Isoenzyme electrophoresis of Rhodnius species: a phenetic approach to relationships within the genus[END_REF]Dujardin et al., 1999a, b). Isozymes are still used, mostly because of their affordability and have recently helped clarify species complexes such as the Triatoma rubrovaria and the Triatoma maculata complexes, which include several morphologically similar sylvatic vectors distributed throughout Brazil and Northern South-America [START_REF] Dos Santos | Evolutionary relationships based on genetic and phenetic characters between Triatoma maculata, Triatoma pseudomaculata and morphologically related species (Reduviidae: Triatominae)[END_REF].

Origin of triatomines: monophyly versus polyphyly

Since the late 1990s, DNA sequence data have been used to test phylogenetic relationships between putative taxa, complementing morphological studies based on morphology. Morphological and molecular phylogenies are often used to validate one another and they can generate remarkably similar trees particularly at higher taxonomic levels [START_REF] Weirauch | Cladistic analysis of Reduviidae (Heteroptera: Cimicomorpha) based on morphological characters[END_REF][START_REF] Weirauch | Molecular phylogeny of the assassin bugs (Hemiptera: Reduviidae), based on mitochondrial and nuclear ribosomal genes[END_REF]. However, at lower taxonomic levels, the extensive plasticity of some species can result in morphological convergence or divergence, thus complicating taxonomic assignations [START_REF] Lent | Revision of the Triatominae (Hemiptera, Reduviidae), and their significance as vectors of Chagas' disease[END_REF]Dujardin et al., 1999b). This illustrates the importance of using as many characters as possible for robust taxonomies. A recent cladistic analysis of the assassin bugs (Reduviidae), which includes the Triatominae as well as non-hematophagous families, used as many as 162 morphological characters [START_REF] Weirauch | Cladistic analysis of Reduviidae (Heteroptera: Cimicomorpha) based on morphological characters[END_REF]. DNA sequences are not immune to convergence (homoplasy) and phylogenetic analyses may depend on the number and types of sequence used, the amount of polymorphic sites they include and a variety of assumption about the rates of mutations (insertions, deletions, transitions, transversions), although these can be estimated independently. Importantly, the possibility of introgression may lead to gene phylogenies that do not reflect the species phylogeny. Future phylogenetic analyses are likely to include whole or nearly whole genomes, and thus provide insights into historical hybridization events.

Perhaps the best illustration of the complexity of phylogenetic interpretations is the long-lasting question of monophyly versus polyphyly of the Triatominae. Morphological studies have been unable to establish if, within the Triatominae, the Rhodniini and the Triatomini tribes evolved from one or more than one bloodsucking ancestor, and whether the Rhodnius and Triatoma genera are paraphyletic with respect to other genera of the Triatominae. The initial sequence-based studies could also not clearly reject one or the other hypothesis because of lack of statistical power [START_REF] Hypsa | Phylogeny and biogeography of Triatominae (Hemiptera: Reduviidae): molecular evidence of a New World origin of the Asiatic clade[END_REF]Schofield and Galva˜o, 2009). Nonetheless, they illustrated the potential pitfalls of phylogenies based on sequence data and led to a progressive increase in the number of taxa, sequence data and analytical approaches involved.

The latest phylogenetic studies of the Triatominae and other Reduviidae combine several nuclear and mitochondrial DNA (mtDNA) sequences, comprise large numbers of taxa and explore extensive parameter spaces by using parsimony, maximum likelihood and Bayesian analyses [START_REF] Weirauch | Molecular phylogeny of the assassin bugs (Hemiptera: Reduviidae), based on mitochondrial and nuclear ribosomal genes[END_REF][START_REF] Patterson | Phylogenetic multi-locus codon models and molecular clocks reveal the monophyly of haematophagous reduviid bugs and their evolution at the formation of South America[END_REF]. The resulting trees recover the Triatominae as a monophyletic taxon with good statistical support, particularly from Bayesian and maximum likelihood inferences [START_REF] Weirauch | Molecular phylogeny of the assassin bugs (Hemiptera: Reduviidae), based on mitochondrial and nuclear ribosomal genes[END_REF][START_REF] Patterson | Phylogenetic multi-locus codon models and molecular clocks reveal the monophyly of haematophagous reduviid bugs and their evolution at the formation of South America[END_REF].

Additionally, sequence data have been used to clarify taxonomy at different levels. The phylogeny and taxonomy of the Rhodniini tribe, for example, was derived early on from mitochondrial and nuclear sequences [START_REF] Monteiro | Phylogeny and molecular taxonomy of the Rhodniini derived from mitochondrial and nuclear DNA[END_REF]. Ribosomal DNA in combination with haploid genome size comparisons was used to clarify the Infestans complex [START_REF] Bargues | Origin and phylogeography of the Chagas disease main vector Triatoma infestans based on nuclear rDNA sequences and genome size[END_REF]. Recent studies based on ribosomal and mitochondrial genes have helped described several putative species and sub-species within the Dimidiata complex [START_REF] Marcilla | The ITS-2 of the nuclear rDNA as a molecular marker for populations, species, and phylogenetic relationships in Triatominae (Hemiptera: Reduviidae), vectors of Chagas disease[END_REF]Ramirez et al., 2005;[START_REF] Bargues | Origin and phylogeography of the Chagas disease main vector Triatoma infestans based on nuclear rDNA sequences and genome size[END_REF][START_REF] Dorn | Triatoma dimidiata (Latreille, 1811): a review of its diversity across its geographic range and the relationship among populations[END_REF][START_REF] Dorn | Two distinct Triatoma dimidiata (Latreille, 1811) taxa are found in sympatry in Guatemala and Mexico[END_REF][START_REF] Tamay-Segovia | Two Triatoma dimidiata clades (Chagas disease vector) associated with different habitats in southern Mexico and Central America[END_REF].

Sequence data have several other important advantages over other markers. First, they document sequence polymorphisms that can be readily used to develop simple high-throughput PCR diagnostics for discriminating putative taxa, thus greatly facilitating the work with recently diverged isomorphic species group [START_REF] Pavan | A multiplex PCR assay that separates Rhodnius prolixus from members of the Rhodnius robustus cryptic species complex (Hemiptera: Reduviidae)[END_REF][START_REF] Herrera-Aguilar | Identification of a large hybrid zone between sympatric sibling species of Triatoma dimidiata in the Yucatan peninsula, Mexico, and its epidemiological importance[END_REF]. Cryptic species are not common in the Triatominae, however they have been detected in T. dimidiata [START_REF] Marcilla | The ITS-2 of the nuclear rDNA as a molecular marker for populations, species, and phylogenetic relationships in Triatominae (Hemiptera: Reduviidae), vectors of Chagas disease[END_REF][START_REF] Panzera | Chromosomal variation and genome size support existence of cryptic species of Triatoma dimidiata with different epidemiological importance as Chagas disease vectors[END_REF], T. sordida [START_REF] Noireau | Cryptic speciation in Triatoma sordida (Hemiptera: Reduviidae) from the Bolivian Chaco[END_REF][START_REF] Panzera | Chromosomal variation and genome size support existence of cryptic species of Triatoma dimidiata with different epidemiological importance as Chagas disease vectors[END_REF] and R. robustus [START_REF] Monteiro | Molecular phylogeography of the Amazonian Chagas disease vectors Rhodnius prolixus and R. robustus[END_REF]. Natural hybrids also occur, for example, between T. infestans and Triatoma platensis, and these appear to remain separated by occupying distinct ecological niches [START_REF] Abalos | Sobre hı´bridos naturales y experimentales de Triatoma[END_REF]. In the case of T. dimidiata and the cryptic species, decreased hybrid fitness [START_REF] Herrera-Aguilar | Identification of a large hybrid zone between sympatric sibling species of Triatoma dimidiata in the Yucatan peninsula, Mexico, and its epidemiological importance[END_REF] may be what is maintaining the 45-million-year separation, as determined using an ITS-2 molecular clock [START_REF] Bargues | Phylogeography and genetic variations of Triatoma dimidiata, the main Chagas disease vector in Central America, and its position within the genus Triatoma[END_REF]. R. robustus was a concern as a possibly epidemiologically important sylvatic species nearly identical in appearance to R. prolixus [START_REF] Monteiro | Molecular phylogeography of the Amazonian Chagas disease vectors Rhodnius prolixus and R. robustus[END_REF].

Second, when phylogenies are combined with the known age of geological events to build a so-called 'molecular clock', one can match the age of a putative tree branching event with geological events to generate phylogeographic scenarios. These elegant studies (for example, [START_REF] Monteiro | Nested clade and phylogeographic analyses of the Chagas disease vector Triatoma brasiliensis in Northeast Brazil[END_REF][START_REF] Bargues | Origin and phylogeography of the Chagas disease main vector Triatoma infestans based on nuclear rDNA sequences and genome size[END_REF] further illuminate the ecological and geological context of the Triatominae extensive species radiation in the New World, and which in turn facilitates our understanding of their systematics.

POPULATION GENETICS

Population genetics has been an important tool in understanding the epidemiology of Chagas disease and in designing effective control methods to reduce local triatomine populations and interrupt transmission. Such studies have provided estimates of the level of genetic diversity present in populations, which are required to predict vector's adaptability (for example, domestication or development of pesticide resistance). Studies of gene flow among populations can help define the geographic coverage needed for successful control, the importance of sylvan populations in house infestation or the sources of reinfestation following insecticide treatment.

Genetic diversity

T. infestans and R. prolixus are two of the most medically important vectors, in large part because they are domestic species across most of their ranges. These two species generally show low genetic diversity in natural populations as well as in laboratory colonies, as determined by allozymes [START_REF] Harry | Isozyme variability and differentiation between Rhodnius prolixus, R. robustus and R. pictipes, vectors of Chagas disease in Venezuela[END_REF][START_REF] Harry | Isozyme variability in Rhodnius prolixus populations, vectors of Chagas disease in Venezuela[END_REF][START_REF] Lopez | Genetic variability and differentiation between populations of Rhodnius prolixus and R. pallescens, vectors of Chagas' disease in Colombia[END_REF]Dujardin et al., 1998b;[START_REF] Monteiro | Allozyme relationships among ten species of Rhodniini, showing paraphyly of Rhodnius including Psammolestes[END_REF] and confirmed by mtDNA and nuclear DNA sequence analyses (Table 1). By contrast, populations of T. dimidiata and T. brasiliensis, both of which are widespread across domestic, peri-domestic and sylvan habitats, generally show higher diversity, although nearby T. dimidiata populations in Costa Rica showed a single ITS-2 haplotype (Table 1). Survival in these very diverse habitats, and movement among habitats, may have resulted in distinct selective pressures thus maintaining genetic diversity. Reduced genetic repertoire owing to founder effects during domestication could explain the low genetic diversity in T. infestans and R. prolixus (Schofield et al., 1999). However, domestication per se may not be sufficient to reduce genetic diversity as domestic populations showed nearly identical allozyme [START_REF] Dujardin | Isozyme evidence of lack of speciation between wild and domestic Triatoma infestans (Heteroptera: Reduviidae) in Bolivia[END_REF], or similar or even greater mtDNA diversity [START_REF] Fitzpatrick | Molecular genetics reveal that silvatic Rhodnius prolixus do colonise rural houses[END_REF][START_REF] Piccinali | Molecular population genetics and phylogeography of the Chagas disease vector Triatoma infestans in South America[END_REF] as nearby sylvan populations. Of course sylvan populations may represent feral derivatives of domestic populations [START_REF] Dujardin | Isozyme evidence of lack of speciation between wild and domestic Triatoma infestans (Heteroptera: Reduviidae) in Bolivia[END_REF]. An alternative hypothesis is that low genetic variability reflects rapid and recent expansion owing to passive dispersal (Dujardin et al., 1998a).

A relatively higher genetic diversity in the T. infestans populations in the Bolivian Andes was used to propose this region as the origin of dispersal (based on allozymes (Dujardin et al., 1998a) and cytb sequence [START_REF] Giordano | Genetic diversity of Triatoma infestans (Hemiptera: Reduviidae) in Chuquisaca, Bolivia based on the mitochondrial cytochrome b gene[END_REF]). This hypothesis is supported by cytogenetic differences in the Andean as compared with non-Andean T. infestans, DNA sequence differences (ITS-2 [START_REF] Bargues | Origin and phylogeography of the Chagas disease main vector Triatoma infestans based on nuclear rDNA sequences and genome size[END_REF] and cytb [START_REF] Monteiro | Mitochondrial DNA variation of Triatoma infestans populations and its implication on the specific status of T melanosoma[END_REF][START_REF] Giordano | Genetic diversity of Triatoma infestans (Hemiptera: Reduviidae) in Chuquisaca, Bolivia based on the mitochondrial cytochrome b gene[END_REF]), as well as a highly significant FST=0.36 between populations from the two areas (based on cytb [START_REF] Giordano | Genetic diversity of Triatoma infestans (Hemiptera: Reduviidae) in Chuquisaca, Bolivia based on the mitochondrial cytochrome b gene[END_REF]). Recent studies have shown considerable diversity in the Argentinean T. infestans populations as well (cytochrome oxidase sequence [START_REF] Piccinali | Molecular population genetics and phylogeography of the Chagas disease vector Triatoma infestans in South America[END_REF]). The speculation is that, perhaps multiple waves of immigration and rapid spread in Argentina, followed by genetic drift in isolated populations, has increased the genetic variability.

Genetic structure of populations

Vector migration is important for genetic mixing, re-infestation following pesticide treatment, the spread of genetically modified symbionts [START_REF] Beard | Bacterial symbionts of the triatominae and their potential use in control of Chagas disease transmission[END_REF] and pesticide resistance genes. Although adults generally fly poorly (for example, [START_REF] Barbu | Characterization of the dispersal of nondomiciliated Triatoma dimidiata through the selection of spatially explicit models[END_REF], some species have been known to fly as far as 1km, and wingless nymphs can walk tens of meters (Nu´n˜ez, 1987). In addition to this active dispersion, passive dispersion by human activity, and perhaps carriage on animals and birds, is important over longer distances.

Over long distances, most studies show variation among populations consistent with the 'isolation by distance' model [START_REF] Wright | Isolation by distance[END_REF][START_REF] Caldero´n | A preliminary assessment of genetic differentiation of Triatoma dimidiata (Hemiptera: Reduviidae) in Guatemala by RAPD-PCR[END_REF]Pe´rez de Rosas et al., 2007, 2008;[START_REF] Piccinali | Molecular population genetics and phylogeography of the Chagas disease vector Triatoma infestans in South America[END_REF]; Table 2). At shorter distances, the picture differs among species and localities, and is often affected by pesticide treatment history.

T. infestans populations are generally highly structured as shown by allozyme analyses [START_REF] Dujardin | Isozyme evidence of lack of speciation between wild and domestic Triatoma infestans (Heteroptera: Reduviidae) in Bolivia[END_REF](Dujardin et al., , 1998a;;Brenie`re et al., 1998) and microsatellite markers (Table 2), as might be expected for a mostly domestic species. Interestingly, in areas where T. infestans has extensive sylvan populations, gene flow was also restricted among these sylvan populations and largely absent between the sylvan and domestic populations [START_REF] Richer | Active dispersal by wild Triatoma infestans in the Bolivian Andes[END_REF] andTable 2). Recent analysis of genomic [START_REF] Perez De Rosas | Molecular phylogeography of the Chagas' disease vector Triatoma infestans in Argentina[END_REF] and mtDNA (Torres- [START_REF] Perez De Rosas | Molecular phylogeography of the Chagas' disease vector Triatoma infestans in Argentina[END_REF] both support the hypothesis of two independent migration events of T. infestans in South America, and confirmed the existence of two distinct lineages.

By contrast, T. dimidiata populations show more extensive gene flow among houses within a village as well as among nearby villages in areas where it seasonally enters homes [START_REF] Dumonteil | Assessment of Triatoma dimidiata dispersal in the Yucatan Peninsula of Mexico by morphometry and microsatellite markers[END_REF]; Table 2). Surprisingly, in areas apparently lacking sylvan populations, high levels of migration among houses within a village and nearby villages are inferred by high gene flow [START_REF] Dorn | The Chagas vector, Triatoma dimidiata (Hemiptera: Reduviidae), is panmictic within and among adjacent villages in Guatemala[END_REF]Stevens et al., unpublished data;Table 2). A finding of mostly unrelated or distantly related individuals within a house by random amplification of polymorphic DNA-PCR (RAPD-PCR) [START_REF] Melgar | The number of families of Triatoma dimidiata in a Guatemalan house[END_REF]) also supports substantial movement by T. dimidiata. R. prolixus also shows movement between most sylvan and domestic populations, indicating that sylvan populations can pose a risk for human infection [START_REF] Fitzpatrick | Molecular genetics reveal that silvatic Rhodnius prolixus do colonise rural houses[END_REF]Table 2).

INTERACTIONS VECTOR-PARASITE-HOST

The genetics and evolution of diseases vectors are influenced by relationships with their parasites [START_REF] Schaub | Parasitogenic alterations of vector behaviour[END_REF], hosts [START_REF] Sacks | Molecular aspects of parasite-vector and vector-host interactions in Leishmaniasis[END_REF], symbionts [START_REF] Dillon | The gut bacteria of insects: nonpathogenic interactions[END_REF] and with congeneric vectors [START_REF] Pereira | Competitive displacement in Triatominae: the Triatoma infestans success[END_REF]. These relationships are two-way interactions as vectors also have impacts on the other members of these 'vector-borne disease communities'. An integrated view of the significance of these interactions on the genetic and evolution of triatomines is largely unknown, but some bits of the puzzle are being pieced together.

Triatomine-Trypanosoma interactions

Once ingested during a blood meal, T. cruzi parasites undergo two major transformations in the vector. Trypomastigotes differentiate into the epimastigote stage, to multiply asexually, before differentiating into infectious metacyclic trypomastigotes (Figure 2). Parasite viability, development and multiplication are then influenced by physiological and biochemical interactions with the vector, according to its life stage and location inside the vector (see Azambuja et al., 2005b;[START_REF] Garcia | Exploring the role of insect host factors in the dynamics of Trypanosoma cruzi-Rhodnius prolixus interactions[END_REF][START_REF] Garcia | Trypanosoma rangeli: a new perspective for studying the modulation of immune reactions of Rhodnius prolixus[END_REF].

Studies of the molecular interactions between triatomines and T. cruzi remain scarce as compared with other vector-parasite systems as they lie at the intersection between two neglected fields of research: (i) The immune system of hemipterans, less studied than in higher orders of insects [START_REF] Ursic-Bedoya | Rhodnius prolixus: identification of immunerelated genes upregulated in response to pathogens and parasites using suppressive subtractive hybridization[END_REF], and (ii) the role of the innate immunity of insects in flagellate infections, less studied than the response to other microbial organisms [START_REF] Boulanger | Antimocrobial peptides in the interaction between insects and flagellate parasites[END_REF]. Most studies focus on mechanisms involved in the humoral response (but see [START_REF] Garcia | Effects of eicosanoid biosynthesis inhibitors on the prophenoloxidase-activating system and microaggregation reactions in the hemolymph of Rhodnius prolixus infected with Trypanosoma rangeli[END_REF] for insights into the cellular response), their genetic basis and, to a lesser extent, their effects on parasite life history and within-host dynamics (Table 3). For example, lectins and antimicrobial peptides such as defensins and lysozymes have been described in several triatomine species (Table 3). Many of the corresponding genes are mostly expressed in the stomach (but see [START_REF] Lopez | Isolation and characterization of a novel insect defensin from Rhodnius prolixus, a vector of Chagas disease[END_REF], where symbionts grow intensively after blood meals (Arau jo et al., 2006;[START_REF] Balczun | Sequence characterization of an unusual lysozyme gene expressed in the intestinal tract of the reduviid bug Triatoma infestans (Insecta)[END_REF]. Importantly, defensin is strongly upregulated in the mid-gut of T. brasiliensis following infection with T. cruzi [START_REF] Waniek | Trypanosoma cruzi infection modulates the expression of Triatoma brasiliensis def1 in the midgut[END_REF]. Lysozyme genes are expressed in both the fat body and the mid-gut of triatomines. Trypanolytic factors, the prophenoloxidase cascade and the impact of various oxides have also been studied, but almost exclusively in the hemocel of R. prolixus infected by Trypanosoma rangeli.

One of the most intriguing outcomes of triatomines-Trypanosoma interactions is that closely related parasites have very different transmission modes. T. rangeli spreads from the gut to the hemocoel [START_REF] Mello | Trypanosoma cruzi and Trypanosoma rangeli: interplay with hemolymph components of Rhodnius prolixus[END_REF] and, at least for some (KP1-) populations, to the salivary gland of Rhodnius species, and is transmitted by bites [START_REF] Guhl | Trypanosoma (Herpetosoma) rangeli Tejera, 1920: an updated review[END_REF]. By contrast, T. cruzi infection is limited to the gut, so that it is transmitted through the vector feces. It is still unclear whether such a difference results from (1) an adaptation of T. cruzi to limit its spread from the gut and avoid the insect immune response primarily located in the hemocoel [START_REF] Whitten | Role of superoxide and reactive nitrogen intermediates in Rhodnius prolixus (Reduviidae)/ Trypanosoma rangeli interactions[END_REF][START_REF] Lopez | Isolation and characterization of a novel insect defensin from Rhodnius prolixus, a vector of Chagas disease[END_REF], or (2) an adaptation of T. rangeli to reduce the impact of the humoral response of the insect, for example, by infecting hemocytes and inhibiting the proPo pathway that leads to melanization [START_REF] Gregorio | The phenoloxidase system and in vitro interaction of Trypanosoma rangeli with Rhodnius prolixus and Triatoma infestans[END_REF][START_REF] Gomes | Rhodnius prolixus infected with Trypanosoma rangeli: in vivo and in vitro experiments[END_REF][START_REF] Garcia | Exploring the role of insect host factors in the dynamics of Trypanosoma cruzi-Rhodnius prolixus interactions[END_REF]. Interestingly, whereas the first hypothesis involves a trade-off between an increase in T. cruzi growth rate within its vector and a reduced transmission to mammalian hosts, the second hypothesis involves an increase in T. rangeli development rate in the hemolymph, with no detrimental impact on transmission.

There is also evidence that, according to variations in their immune response, vectors are likely to be biological filters transmitting differently lineages/genotypes of T. cruzi and T. rangeli [START_REF] Vallejo | Triatominae-Trypanosoma cruzi/T. rangeli: vector-parasite interactions[END_REF]. Lectins produced in the stomach of R. prolixus agglutinate Dm28 clones but not the Y-strain of T. cruzi [START_REF] Mello | Differential in vitro and in vivo behaviour of three strains of Trypanosoma cruzi in the gut and hemolymph of Rhodnius prolixus[END_REF][START_REF] Ratcliffe | Trypanosoma cruzi and erythrocyte agglutinins: a comparative study of occurrence and properties in the gut and hemolymph of Rhodnius prolixus[END_REF]. The presence of trypanolytic factors in the R. prolixus hemolymph impedes the development of T. rangeli KP1-but not the development of T. rangeli KP1+ [START_REF] Vallejo | kDNA markers define two major Trypanosoma rangeli lineages in Latin-America[END_REF][START_REF] Vallejo | Parity between kinetoplast DNA and mini-exon gene sequences supports either clonal evolution or speciation in Trypanosoma rangeli strains isolated from Rhodnius colombiensis, R. pallescens and R. prolixus in Colombia[END_REF].

The above mechanisms and outcomes of vector-parasite interactions can be thought of in terms of coevolutionary adaptations, especially within the Rhodnius genus, where T. rangeli is known to be pathogenic to its vector, and where parasite/vector association appear to be more specific [START_REF] Vallejo | Triatominae-Trypanosoma cruzi/T. rangeli: vector-parasite interactions[END_REF]. By contrast, a common view is that T. cruzi has little impact on its vector's life history (Zeledon, 1981;[START_REF] Schaub | Does Trypanosoma cruzi stress its vectors?[END_REF][START_REF] Schaub | Parasitogenic alterations of vector behaviour[END_REF][START_REF] Vallejo | Triatominae-Trypanosoma cruzi/T. rangeli: vector-parasite interactions[END_REF], and thus produces no or little selective pressure on its vector. Such a view is supported by a lack of change in developmental time (Zeledon, 1981;[START_REF] Schaub | Developmental time and mortality of larvae of Triatoma infestans infected with Trypanosoma cruzi[END_REF], life span and fertility (Zeledon, 1981;[START_REF] Schaub | Does Trypanosoma cruzi stress its vectors?[END_REF], and feeding behavior [START_REF] Schaub | Parasitogenic alterations of vector behaviour[END_REF][START_REF] Garcia | Exploring the role of insect host factors in the dynamics of Trypanosoma cruzi-Rhodnius prolixus interactions[END_REF] when triatomines are infected by T. cruzi in the laboratory. But there is counter-evidence [START_REF] Schaub | Does Trypanosoma cruzi stress its vectors?[END_REF][START_REF] Schaub | Pathogenicity of trypanosomatids on insects[END_REF][START_REF] Botto-Mahan | Chagas disease parasite induces behavioural changes in the kissing bug Mepraia spinolai[END_REF][START_REF] Botto-Mahan | Trypanosoma cruzi induces life-history trait changes in the wild kissing bug Mepraia spinolai: implications for parasite transmission[END_REF][START_REF] Nouvellet | Effects of genetic factors and infection status on wing morphology of Triatoma dimidiata species complex[END_REF] and laboratory experiments often suffer biases, thus weakening their applicability to natural populations [START_REF] Schaub | Does Trypanosoma cruzi stress its vectors?[END_REF]. Typically, natural conditions involve long periods of starvation potentially leading to competition for rare nutrients between a vector and its parasites [START_REF] Kollien | The development of Trypanosoma cruzi in Triatominae[END_REF]. In fact, resistance to starvation decreases after T. cruzi infection in both T. dimidiata [START_REF] Vargas | Effect of fasting on Trypanosoma cruzi infection in Triatoma dimidiata (Hemiptera: Reduviidae)[END_REF] and T. infestans [START_REF] Schaub | Parasite/host-interrelationships of the trypanosomatids Trypanosoma cruzi and Blastocrithidia Triatomae and the reduviid bug Triatoma infestans: influence of starvation of the bug[END_REF]. Nonetheless, as T. cruzi develops in the gut rather than in the hemolymph, it is thought to interfere less with the vector than T. rangeli [START_REF] Lopez | Isolation and characterization of a novel insect defensin from Rhodnius prolixus, a vector of Chagas disease[END_REF][START_REF] Ursic-Bedoya | Identification and characterization of two novel lysozymes from Rhodnius prolixus, a vector of Chagas disease[END_REF]. However, the mid-gut (and not only the hemocoel) of triatomines has been shown to be immune-reactive, and (artificial) infections of the hemolymph by T. cruzi do induce an immune response (Ursic-Bedoya and Lowenberger, 2007). As T. cruzi is able to enter the hemocoel [START_REF] Zeledon | Life cycle of Trypanosoma cruzi in the insect vector[END_REF][START_REF] Mello | Trypanosoma cruzi and Trypanosoma rangeli: interplay with hemolymph components of Rhodnius prolixus[END_REF], 'mid-gut habitat preference' observed for T. cruzi, could readily be the outcome of a (potentially costly) immune process preventing the invasion of the hemocoel [START_REF] Whitten | Role of superoxide and reactive nitrogen intermediates in Rhodnius prolixus (Reduviidae)/ Trypanosoma rangeli interactions[END_REF]. A detailed understanding of the mechanism blocking the transit of T. cruzi across the mid-gut is thus critically needed to convincingly evaluate the actual cost of T. cruzi infection to the vector.

Overall, although studies of the molecular aspects of the vector immune responses are developing, they still lack quantitative assess ments of their effect on parasite life history. Such measurements are critically needed to further understand the mechanisms as well as the evolution of these interactions; especially, the assessment of the effects of T. cruzi under biologically realistic conditions, that is, when infection is considered in synergy with other stress factors such as starvation or co-infection with additional strains. Quantitative assessments would make key contributions to the understanding of the interactions between triatomines and trypanosomes, and ultimately the transmission of Chagas disease.

Triatomine-Trypanosoma-microbiota interactions

Experimental studies indicate that a community of intestinal microorganisms might regulate the development of parasites in the insect mid-gut (and thus vector competence), although the underlying mechanisms remain largely unknown (Azambuja et al., 2005a). Numerous species of bacteria inhabit the gut of triatomines (listed in [START_REF] Vallejo | Triatominae-Trypanosoma cruzi/T. rangeli: vector-parasite interactions[END_REF] and they typically undergo massive population size expansion after a blood meal (102to 104-fold increases; [START_REF] Azambuja | Isolation of Serratia marcescens in the midgut of Rhodnius prolixus: impact on the establishment of the parasite, Trypanosoma cruzi, in the vector[END_REF], whereas trypanosomes sharply decrease in number (Azambuja et al., 2005a). The trypanolytic activity of two strains of Serratia marcescens (SM365 and RPH) has been shown recently to be associated with its attachment to the membrane of the T. cruzi Y-strain [START_REF] Azambuja | Isolation of Serratia marcescens in the midgut of Rhodnius prolixus: impact on the establishment of the parasite, Trypanosoma cruzi, in the vector[END_REF]. The rapid bacterial expansion could then contribute to decreasing parasite abundance in a direct manner. Indirect effects are also expected as increase of bacterial populations activates the production of lysozyme [START_REF] Kollien | Isolation and characterization of a cDNA encoding for a lysozyme from the gut of the reduviid bug Triatoma infestans[END_REF] and defensin [START_REF] Araujo | Sequence characterization and expression patterns of defensin and lysozyme encoding genes from the gut of the reduviid bug, Triatoma brasiliensis[END_REF], which are likely to affect the multiplication of trypanosomes. Furthermore, intestinal microorganisms have significant effects on the vectors. Typically, asymbiotic triatomines suffer important developmental delays, and increased mortality [START_REF] Eichler | Development of symbionts in triatomine bugs and the effects of infections with trypanosomatids[END_REF][START_REF] Vallejo | Triatominae-Trypanosoma cruzi/T. rangeli: vector-parasite interactions[END_REF]. A potential explanation is that triatomine blood meals lack essential vitamins and nutrients, which are supplied by symbionts [START_REF] Beard | Bacterial symbionts of the triatominae and their potential use in control of Chagas disease transmission[END_REF]. Microorganisms may thus influence the vector-parasite interaction through their relationships with both triatomines and trypanosomes. It has indeed been hypothesized that R. prolixus has evolved a lower antimicrobial peptide production in the intestine to allow essential symbionts to develop. This could have induced the evolution of a midgut habitat for T. cruzi [START_REF] Ursic-Bedoya | Identification and characterization of two novel lysozymes from Rhodnius prolixus, a vector of Chagas disease[END_REF], leading to the relatively inefficient mode of transmission by feces, with key implications for the triatomine-trypanosome interaction.

Triatomine species interaction

Studies of interaction between triatomines are rare, and focus on the effect of T. infestans on other Triatominae. T. infestans is thought to have competitively displaced P. megistus, T. sordida, T. brasiliensis and T. pseudomaculata from different parts of South America (Figure 1) [START_REF] Pereira | Competitive displacement in Triatominae: the Triatoma infestans success[END_REF]. Such ecological displacement has indeed been observed in laboratory experiments where T. infestans consistently excluded T. sordida after a few months [START_REF] Bar | Population interactions between Triatoma infestans and Triatoma sordida[END_REF][START_REF] Oscherov | Interactions between Triatoma infestans and Triatoma sordida (Hemiptera: Reduviidae) in artificial ecotopes: population growth and age structure[END_REF][START_REF] Oscherov | Competition between vectors of Chagas disease, Triatoma infestans and T. sordida: effects on fecundity and mortality[END_REF]. Indirect field evidence of competition includes observations of higher house infestation and abundance of T. sordida [START_REF] Canale | Peridomestic distribution of Triatoma garciabesi and Triatoma guasayana in north-west Argentina[END_REF][START_REF] Oscherov | Competition between vectors of Chagas disease, Triatoma infestans and T. sordida: effects on fecundity and mortality[END_REF], P. megistus [START_REF] Villela | Entomological surveillance for Chagas disease in the mid-western region of Minas Gerais State, Brazil, from 2000 to 2003[END_REF] and T. brasiliensis [START_REF] Pereira | Competitive displacement in Triatominae: the Triatoma infestans success[END_REF] after removal of T. infestans by control programs. Intraspecific and interspecific competition for blood meals appears to be the key mechanisms to explain such exclusion. In mixed experimental populations, the T. sordida blood-meal size is reduced, leading to lower larval survival rates and reduced population growth, whereas T. infestans remains unaffected. Similarly, the competitive advantage of T. infestans could result from its ability to ingest larger blood meals, while having a lower requirement to complete its (shorter) life cycle [START_REF] Pereira | Competitive displacement in Triatominae: the Triatoma infestans success[END_REF]. Clearly, interactions between (domestic) T. infestans and (peri-domestic and sylvatic) other triatomine species also involve a dispersal-competitive trade-off [START_REF] Oscherov | Competition between vectors of Chagas disease, Triatoma infestans and T. sordida: effects on fecundity and mortality[END_REF], and could thus be better understood if studied in a meta-population dynamic framework such as the ones available for triatomines (Gourbiere et al., 2008;[START_REF] Barbu | Optimization of control strategies for nondomiciliated Triatoma dimidiata, Chagas disease vector in the Yucata´n peninsula, Mexico[END_REF][START_REF] Barbu | Characterization of the dispersal of nondomiciliated Triatoma dimidiata through the selection of spatially explicit models[END_REF][START_REF] Barbu | Optimization of spatially targeted vector control interventions for non-domiciliated triatomines[END_REF]Slimi et al., 2009, and references therein). Understanding the balance between competitive and dispersal abilities will allow understanding the impact of competition between triatomines on parasite transmission, as suggested by theoretical studies [START_REF] Gourbiere | Competition between unit-restricted fungi: a metapopulation model[END_REF][START_REF] Begon | Effects of host diversity on disease dynamics[END_REF].

The interaction between triatomines, their symbionts and trypanosomes establishes a community network with many potential positive and negative feedbacks (Figure 3). In this context, the genetics and evolution of triatomines are likely to follow non-trivial dynamics. Better knowledge of this complex dynamic will surely be gained by studying the R. prolixus-T. rangeli and T. infestans-T. cruzi interactions, two systems with interesting transmission differences, and by looking at the influence of major triatomine life history on pathogen dynamics [START_REF] Menu | Adaptive developmental delay in Chagas disease vectors: an evolutionary ecology approach[END_REF]. Additionally, studies of other important vectors of T. cruzi and Chagas disease will be needed to reach the general level of understanding required to transfer basic knowledge into safe and efficient control strategies.

VECTOR CONTROL: BEYOND MASSIVE INSECTICIDE SPRAYING

Most studies of triatomines have been motivated by the need for vector control interventions to prevent Chagas disease transmission to humans. The general aims were thus to identify vector species present in human dwellings, to understand the success or failure of indoor insecticide spraying, as well as to design more rational and evidencebased vector control interventions.

Because the medical entomological importance of vectors was traditionally assumed to be associated with the species, efforts have focused on taxonomic identification, for example, resolving the issue of the sylvatic dark morph of T. infestans [START_REF] Noireau | Detection of sylvatic dark morphs of Triatoma infestans in the Bolivian Chaco[END_REF] and or evaluating gene flow between sylvatic and domestic R. robustus and R. prolixus in Venezuela, to show that R. robustus and R. prolixus are distinct species (with rare introgression), and implicating the sylvatic R. prolixus populations as potential contributors to human transmission.

T. dimidiata phenotypic variations have also puzzled entomologists for many years, the more so because it was found in a wide variety of habitats, and presented widely different levels of adaptations to human dwellings, ranging from highly domiciliated to preferentially sylvatic [START_REF] Dorn | Triatoma dimidiata (Latreille, 1811): a review of its diversity across its geographic range and the relationship among populations[END_REF]. The existence of a species complex is now emerging, supported by molecular, morphometric and cytogenetic data [START_REF] Bustamante | Metric variation among geographic populations of the Chagas vector Triatoma dimidiata (Hemiptera: Reduviidae: Triatominae) and related species[END_REF][START_REF] Lehmann | Morphometric analysis of Triatoma dimidiata populations (Reduviidae: Triatominae) from Mexico and Northern Guatemala[END_REF][START_REF] Panzera | Chromosomal variation and genome size support existence of cryptic species of Triatoma dimidiata with different epidemiological importance as Chagas disease vectors[END_REF][START_REF] Bargues | Phylogeography and genetic variations of Triatoma dimidiata, the main Chagas disease vector in Central America, and its position within the genus Triatoma[END_REF], Dorn et al., 2009), but the association of the different species/subspecies with particular levels of adaptation to the domiciles and vectorial competence is still unclear. For example, two of the distinct T. dimidiata cryptic species behave in an identical nondomiciliated manner in the Yucatan peninsula, although one was thought to be much more domiciliated [START_REF] Dorn | Two distinct Triatoma dimidiata (Latreille, 1811) taxa are found in sympatry in Guatemala and Mexico[END_REF][START_REF] Herrera-Aguilar | Identification of a large hybrid zone between sympatric sibling species of Triatoma dimidiata in the Yucatan peninsula, Mexico, and its epidemiological importance[END_REF]. This suggests that domiciliation may rather be associated with ecological environments than with species status, which emphasizes the need for more detailed population genetic studies to identify the relevant vector populations.

Indeed, population genetics has been a key tool to define panmictic populations and dispersal range, and thus explain outcomes of vector control efforts. Control strategies based on indoor insecticide spraying began to be tested in the field in the 1940s, and national programs were initiated in the 1970s in several countries such as Brazil, Argentina, Venezuela and Chile, and culminated with several international initiatives such as the Southern Cone or the Central American initiatives in the 1990s [START_REF] Schofield | The Southern Cone initiative against Chagas disease[END_REF][START_REF] Dias | The impact of Chagas disease control in Latin America: a review[END_REF].

The success of such interventions, which resulted in a 490% reduction of house infestation by triatomines and a similar reduction in vectorial transmission of T. cruzi to humans in most of the southern cone region [START_REF] Dias | The impact of Chagas disease control in Latin America: a review[END_REF][START_REF] Schofield | The future of Chagas disease control[END_REF], relied mostly on the marked genetic structure of T. infestans populations and the lack of gene flow among them. However, the elimination of T. infestans from houses opened the way for secondary species to start invading this habitat, but again, little is known about triatomine inter-species competition.

As expected, a reduction in the genetic diversity of triatomine populations has been observed following pesticide application, presumably as a result of severe bottlenecks [START_REF] Garcia | Short report: variation in mitochondrial 12S and 16S ribosomal DNA sequences in natural populations of Triatoma infestans (Hemiptera: Reduviidae)[END_REF]. However, some studies have reported an unchanged or even greater genetic diversity after insecticide spraying (Pe´rez de [START_REF] Perez De Rosas | Microsatellite analysis of genetic structure in natural Triatoma infestans (Hemiptera: Reduviidae) populations from Argentina: its implication in assessing the effectiveness of Chagas' disease vector control programmes[END_REF][START_REF] Perez De Rosas | Macrogeographic and microgeographic genetic structure of the Chagas' disease vector Triatoma infestans (Hemiptera: Reduviidae) from Catamarca, Argentina[END_REF]. The authors speculate that the split of a population into several ones, each with different combination of alleles, followed by genetic drift and population expansion could retain the diversity. The effects of anthropogenic changes such as deforestation and global climate change are yet unknown, although the Amazon is becoming a new area for Chagas transmission, coincident with massive deforestation [START_REF] Diotaiuti | Chapter 4: triatomines vectors of Trypanosoma cruzi infections[END_REF].

In the Gran Chaco region (which encompasses Northern Argentina and parts of Bolivia and Paraguay; Figure 1), vector control measures have notoriously failed. Insects present some months after pesticide treatment are nearly always survivors or migrants from nearby peridomestic populations as revealed by allozyme, mtDNA and microsatellite analyses [START_REF] Dujardin | Genetic analysis of Triatoma infestans following insecticidal control interventions in central Bolivia[END_REF][START_REF] Garcia | Short report: variation in mitochondrial 12S and 16S ribosomal DNA sequences in natural populations of Triatoma infestans (Hemiptera: Reduviidae)[END_REF][START_REF] Perez De Rosas | Microsatellite analysis of genetic structure in natural Triatoma infestans (Hemiptera: Reduviidae) populations from Argentina: its implication in assessing the effectiveness of Chagas' disease vector control programmes[END_REF][START_REF] Pizarro | Microsatellites reveal a high population structure in Triatoma infestans from Chuquisaca, Bolivia[END_REF]. Heterogeneity in peri-domestic environments (from wood piles to granaries to animal corrals) makes control very difficult [START_REF] Dohna | Re-establishment of local populations of vectors of Chagas disease after insecticide spraying[END_REF]. In addition, pesticide resistance has now been detected in several of these populations [START_REF] Gonzalez Audino | Role of enhanced detoxication in a deltamethrin-resistant population of Triatoma infestans (Hemiptera, Reduviidae) from Argentina[END_REF][START_REF] Picollo | High resistance to pyrethroid insecticides associated with ineffective field treatments in Triatoma infestans (Hemiptera: Reduviidae) from Northern Argentina[END_REF][START_REF] Toloza | Differential patterns of insecticide resistance in eggs and first instars of Triatoma infestans (Hemiptera: Reduviidae) from Argentina and Bolivia[END_REF], Germano et al., 2010a;[START_REF] Lardeux | Insecticide resistance of Triatoma infestans (Hemiptera, Reduviidae) vector of Chagas disease in Bolivia[END_REF]. The molecular mechanisms of resistance evolved by triatomines may rely on various detoxification pathways [START_REF] Orihuela | Relative contribution of monooxygenase and esterase to pyrethroid resistance in Triatoma infestans (Hemiptera: Reduviidae) from Argentina and Bolivia[END_REF]. Recent crossing experiments indicate that deltamethrin resistance inheritance is autosomal and semi-dominant, and seems to involve up to three different genes [START_REF] Cardozo | Inheritance of resistance to pyrethroids in Triatoma infestans, the main Chagas disease vector in South America[END_REF]Germano et al., 2010b). However, more studies are required to further understand the development of insecticide resistance in these species.

When triatomine populations are present in domestic, peri-domestic and sylvatic ecotopes, and gene flow among ecotopes is significant, insecticide spraying is of limited efficacy [START_REF] Dumonteil | Re-infestation of houses by Triatoma dimidiata after intra-domicile insecticide application in the Yucatan peninsula, Mexico[END_REF][START_REF] Barbu | Optimization of control strategies for nondomiciliated Triatoma dimidiata, Chagas disease vector in the Yucata´n peninsula, Mexico[END_REF], likely because populations can be rapidly rebuilt by migrants. Thus, the control of such non-domiciliated or autochthonous triatomines remains a major problem for vector control [START_REF] Guhl | Sylvatic triatominae: a new challenge in vector control transmission[END_REF], and novel approaches such as screens, bednets and house improvements will be necessary to interrupt transmission (Figure 4) [START_REF] Monroy | Wall plastering and paints as methods to control vector of Chagas disease in Guatemala[END_REF][START_REF] Monroy | House improvements and community participation in the control of Triatoma dimidiata re-infestation in Jutiapa, Guatemala[END_REF][START_REF] Barbu | Optimization of control strategies for nondomiciliated Triatoma dimidiata, Chagas disease vector in the Yucata´n peninsula, Mexico[END_REF][START_REF] Barbu | Optimization of spatially targeted vector control interventions for non-domiciliated triatomines[END_REF][START_REF] Ferral | Comparative field trial of alternative vector control strategies for non-domiciliated Triatoma dimidiata in the Yucatan peninsula, Mexico[END_REF].

Another potential alternative is to manipulate the vector-parasite interaction and modified vector symbionts may allow reducing the parasite transmission rate. For example, Rhodococcus rhodnii can be transformed so that it produces peptides of Lepidoptera [START_REF] Beard | Bacterial symbiosis in arthopods and the control of disease transmission[END_REF][START_REF] Beard | Bacterial symbionts of the triatominae and their potential use in control of Chagas disease transmission[END_REF] or a mammalian antibody [START_REF] Durvasula | Expression of a functional antibody fragment in the gut of Rhodnius prolixus via transgenic bacterial symbiont Rhodococcus rhodnii[END_REF] that strongly reduces the number of T. cruzi parasites in the vector. This parasite control strategy has been referred to as paratransgenesis, and may provide new options for Chagas disease control.

Overall, much integration is still needed to take advantage of the genetic and molecular knowledge available to move from the empirical insecticide spraying strategies to the design of more rational and evidence-based control interventions against triatomines. The examples detailed above illustrate how vector control can benefit from genetic studies, and point to gaps in our current understanding of some of these aspects.

CONCLUDING REMARKS

An increasing number of genetic studies have contributed significantly to our understanding of triatomine biology, from species identification to phylogeny and phylogeography and population genetics. However, a major drawback of man of these studies is that they do not address aspects of the vectorial capacity of the different populations/ species (Mas-Coma and [START_REF] Mas-Coma | Populations, hybrids and the systematic concepts of species and subspecies in Chagas disease triatomine vectors inferred from nuclear ribosomal and mitochondrial DNA[END_REF], making interpretation of their results in terms of parasite transmission and epidemiological relevance very speculative. Integrative studies including aspects such as T. cruzi infection rates, blood meal analysis or vector-parasite interactions with population genetic data are thus critically needed to clearly understand the links between population structure and parasite transmission. Such studies would also allow a better understanding of major processes such as the transition from predatory to hematophagous bugs, the domiciliation of triatomines or the impact of control interventions on vectorial transmission.

Whereas several microsatellite markers have been identified in major triatomine species such as T. infestans, T. dimidiata or R. prolixus, molecular markers are lacking for many other species. Also, genomewide single-nucleotide polymorphism genotyping approaches, which are now popular in other insect vector species [START_REF] Megy | Genomic resources for invertebrate vectors of human pathogens, and the role of VectorBase[END_REF][START_REF] Horton | Identification of three single nucleotide polymorphisms in Anopheles gambiae immune signaling genes that are associated with natural Plasmodium falciparum infection[END_REF][START_REF] Li | Allelic gene structure variations in Anopheles gambiae mosquitoes[END_REF], have not yet been developed in triatomines. In that respect, the ongoing sequencing of the R. prolixus genome [START_REF] Megy | Genomic resources for invertebrate vectors of human pathogens, and the role of VectorBase[END_REF] will provide a very valuable tool to further understand triatomine biology and genetics. Nonetheless, additional triatomine genomes will be required, particularly from the Triatoma genus, and major vector species such as T. infestans or T. dimidiata would be excellent candidates for future genome sequencing. Triatomine genomes would also open the way to extensive studies of vector-parasite interactions at the molecular level, an area where little is known, in spite of the uniqueness of the mechanism of T. cruzi transmission through insect feces. Thus, although triatomine genetics research has lagged somewhat compared with other insect species, these species represent unique models for key biological processes, and we can expect that several novel approaches will be developed in the near future to expand our understanding of T. cruzi vectorial transmission, vector-parasite interactions and the development of rational vector control strategies.

FIGURE 1

Distribution of triatomine species of major epidemiological relevance. Only about 20 triatomine species are responsible for T. cruzi transmission to humans, because of their ability to infest and, for some species, to colonize human habitat. The red areas indicate the approximate species geographic distribution. Species highlighted in red are considered the most important vectors of the parasite.

  FIGURE 2T. cruzi life cycle. Following a blood meal containing bloodstream trypomastigotes, T. cruzi parasites first differentiate into the epimastigote stage and multiply asexually, before differentiating into infectious metacyclic trypomastigotes. Upon entry of trypomastigotes in the mammalian host, those are able to invade cells and further replicate as intracellular amastigotes (from Andrade and Andrews, 2005, with permission).

FIGURE 3

 3 FIGURE 3 Interaction network. Triatoma, Trypanosoma and microorganisms (either symbiotic or not) interact with one another. Direct or indirect interactions, which have been documented in this review, are all included in this figure. The question marks indicate interactions, which have not yet been investigated.

FIGURE 4

 4 FIGURE 4 Innovative vector control interventions. Novel interventions, based on detailed knowledge of triatomine species population dynamics and genetics, may rely on improving wall plastering of houses using traditional methods, which prevents colonization (a), or on the installation of insect screens on windows, to prevent transient infestation (b).

TABLE 1

 1 DNA sequence population diversity of major triatomine species.

TABLE 2

 2 Microsatellite subdivision among triatomine populations.

TABLE 3

 3 Components of the humoral response of triatomines.

ACKNOWLEDGEMENTS

This work has been supported in part by Grant no. 2005-1-13790 from CONACYT-SALUD, by the UNDP/World Bank/WHO Special Program for Research and Training in Tropical Diseases (TDR)/International Development Research Center (IDRC), Project no. A90276, by the National Institutes of Health (USA) Grant 1R15 A1079672-01A1, and by EU-FP7-PEOPLE-Intra-European Fellowship for career development (IEF) grant no. 253483.

CONFLICT OF INTEREST

The authors declare no conflict of interest.