Résumé
Biologic invasions can have important ecological, economic and social
consequences, particularly when they involve the introduction and spread
of plant invasive pathogens, as they can threaten natural ecosystems and
jeopardize the production of human food. Examples include the grapevine
downy mildew, caused by the oomycete Plasmopara viticola, an invasive
species native to North America, introduced into Europe in the 1870s. We
investigated the introduction and spread of this invasive pathogen, by
analysing its genetic structure and diversity in a large sample from
European vineyards. Populations of P. viticola across Europe displayed
little genetic diversity, consistent with the occurrence of a bottleneck
at the time of introduction. Bayesian coalescent analyses revealed a clear
population expansion signal in the genetic data. We detected a weak, but
significant, continental-wide population structure, with two
geographically and genetically distinct clusters in Western and Eastern
European vineyards. Approximate Bayesian computation, analyses of clines
of genetic diversity and of isolation-by-distance patterns provided
evidence for a wave of colonization moving in an easterly direction across
Europe. This is consistent with historical reports, first mentioning the
introduction of the disease in Bordeaux vineyards (France) and
sub-sequently documenting its rapid spread across Europe. This initial
introduction in the west was probably followed by a ‘leap-frog’ event into
Eastern Europe, leading to the formation of the two genetic clusters we
detected. This study shows that recent population genetics methods within
the Bayesian and coalescence frameworks are extremely powerful for
increasing our understanding of pathogen population dynamics and invasion
histories.