Résumé
The sexual stage of pathogens governs recombination patterns and often
also provides means of surviving the off-season. Despite its importance
for evolutionary potential and between-season epidemiology, sexual systems
have not been carefully investigated for many important pathogens, and
what generates variation in successful sexual reproduction of pathogens
remains unexplored. We surveyed the sexually produced resting structures
(chasmothecia) across 86 natural populations of fungal pathogen
Podosphaera plantaginis (Ascomycota) naturally infecting Plantago
lanceolata in the Åland archipelago, southwest of Finland. For this
pathosystem, these resting structures are a key life-history stage, as
more than half of the local pathogen populations go extinct every winter.
We uncovered substantial variation in the level of chasmothecia produced
among populations, ranging from complete absence to presence on all
infected leaves. We found that chasmothecia developed within clonal
isolates (single strain cultures). Additionally, these clonal isolates all
contained both MAT1-1-1 and MAT1-2-1 genes that characterize mating-types
in Ascomycetes. Hence, contrary to expectations, we conclude that this
species is capable of haploid selfing. In controlled inoculations we
discovered that pathogen genotypes varied in their tendency to produce
chasmothecia. Production of chasmothecia was also affected by ambient
temperature (E), and by the interaction between temperature and pathogen
genotype (G × E). These G, E and G × E effects found both at a European
scale, as well as within Åland, may partly explain the high variability
observed among populations in chasmothecia levels. Consequently, they may
be key drivers of the evolutionary potential and epidemiology of this
highly dynamic pathosystem.