Résumé
Pathogen introductions into novel areas can lead to the emergence of new
fungal diseases of plants. Understanding the origin, introduction
pathways, possible changes in reproductive system and population size of
fungal pathogens is essential in devising an integrated strategy for the
control of these diseases. We used minisatellite markers to infer the
worldwide invasion history of the fungal plant pathogen Leptosphaeria
maculans, which causes stem canker (blackleg) of oilseed and vegetable
brassicas. Clustering analyses partitioned genotypes into distinct
populations corresponding to major geographic regions, along with two
differentiated populations in Western Canada. Comparison of invasion
scenarios using Approximate Bayesian Computation suggested an origin of
the pathogen in the USA, the region where epidemics were first recorded,
and independent introductions from there over the last few decades into
Eastern Canada (Ontario), Europe and Australia. The population in Western
Canada appeared to be founded from a source in Ontario and the population
in Chile resulted from an admixture between multiple sources. A bottleneck
was inferred for the introduction into Western Canada but not into Europe,
Ontario or Australia. Clonality appeared high in Western Canada, possibly
because environmental conditions there were less conducive to sexual
reproduction. Leptosphaeria maculans is a model invasive pathogen with
contrasting features in different regions: shallow population structure,
high genetic variability and regular sexual recombination in some regions,
by comparison with reduced genetic variability, high rates of asexual
multiplication, strong population structure or admixture in others.