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

International audience; Melampsora larici-populina is a partially asexual basidiomycete fungus responsible for poplar rust, with its sexual stage on larch (Larix sp.) and its asexual stage on poplar (Populus sp.). Resistant 7 poplar cultivars were widely planted in northern France until a major resistance breakdown which occurred in 1994. This selective event lead to a rapid and complete shift in the pathogen population. Two independent mutations are responsible for the virulent recessive allele: a SNP and a deletion. Here we study the dynamics of virulence allele fixation both theoretically and empirically, by modeling virulence fixation in different evolutionary scenarios, and by tracing back the polymorphism evolution at the candidate locus before, during and after the resistance breakdown. The modeling part highlighted the interplay between the life cycle (host alternation or not) and the extent of resistance deployment on the probability and dynamics of fixation. Fluorescence ratio methods allowed us to successfully characterize genotypes of reference strains. This method will be applied to nearly 300 isolates collected from 1988 to 2016. By linking our model results and the observed polymorphism evolution in the studied populations, we will investigate whether 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.