Méline Saubin

Clémentine Louet

Stéphane De Mita

Antoine Persoons

Axelle Andrieux

Jérémy Pétrowski

Bénédicte Fabre

Pascal Frey

Fabien Halkett

Evolutionary dynamics of virulence spread: effects of life cycle, genetic determinism and host resistance deployment

(1) Modeling evolutionary trajectory of a partially asexual organism

Context

Melampsora larici-populina is a basidiomycete fungus (diploid) responsible for poplar rust. This partially asexual organism has its sexual stage on larch (Larix sp.) and its asexual stage on poplar (Populus sp.), a life cycle that amounts to cyclical parthenogenesis in insects.

A poplar cultivar 'Beaupré' (R7) was selected to be completely resistant to M. larici-populina and planted widely in northern France between 1982 and 1996. A major resistance breakdown event occurred in 1994 when virulent rust genotypes (avr7) emerged [START_REF] Xhaard | The genetic structure of the plant pathogenic fungus Melampsora larici-populina on its wild host is extensively impacted by host domestication[END_REF]. Beaupré revealed to be highly susceptible to avr7 pathogen genotypes, and this rapid breakdown led to a replacement of the pathogen population by a new genetic group [START_REF] Persoons | The escalatory Red Queen: Population extinction and replacement following arms race dynamics in poplar rust[END_REF].

A candidate locus for the avr7 gene was identified in M. laricipopulina (Persoons et al., in prep., Maupetit et al., in prep.). The virulent allele is recessive and there are two independent causal mutations at the same locus: a single nucleotide polymorphism (T  C) and the deletion of this locus.

Research goal:

To study the dynamics of virulence allele fixation both theoretically and empirically, by (1) Modeling virulence fixation in different evolutionary scenarios.

(2) Tracing back the polymorphism evolution at the candidate locus before, during and after the resistance breakdown. • Larch plays a decisive role in virulence fixation: it increases both the probability of extinction and the speed of fixation. • The heterozygous state is highly vulnerable, being affected by genetic drift only (not selected on resistant cultivars). There is still to enrich the modeling part by taking into account both SNP and deletion and investigate in parallel the empirical evolution of virulence alleles dynamics. • Hypothesis: the combined presence of the deletion and the mutation conferring virulence may have led to such a rapid and complete replacement of the pathogen population. This hypothesis will be tested and our model results will be linked to the effective polymorphism evolution of the studied population. 2D

  of M. larici-populina Detection of the recessive C allele at the candidate locus Detection of the deletion at the candidate locus  TT or TØ • C and T  TC Heterozygous • C only  CC or CØ • Deleted  ØØ • H 2 O

M

  . Saubin, C. Louet, S. De Mita, A. Persoons, A. Andrieux, J. Pétrowski, B. Fabre, P. Frey, F. Halkett INRA, Université de Lorraine, UMR 1136 Interactions Arbres-Microorganismes, F-54280 Champenoux, France : with and without alternate host • Extinction of population • Fixation of allele T • Fixation of allele CFirst simulations were run with 1000 replicates using the library SIMUPOP[START_REF] Peng | SIMUPOP: a forward-time population genetics simulation environment[END_REF] on PYTHON, without the deletion:____ K S = K R = 10 000 _ _ _ K S = 10 000, K R = 90 000

  ■avr7 individuals detected ■ No avr7 individual detected □ Not surveyed area Persoons et al.with alternate host.-Polymorphism is more protected.Ratio of fluorescence from two specific probes for alleles C and T, validated on reference isolates.Relative quantification with α-tubulin as reference gene, present in two diploid copies (4 copies) in M. larici-populina's genome, validated on reference isolates.