Résumé
Understanding the processes by which new diseases are introduced in
previously healthy areas is of major interest in elaborating prevention
and management policies as well as in understanding the dynamics of
pathogen diversity at large spatial scale. In this study, we aimed to
decipher the dispersal processes that have led to the emergence of the
plant pathogenic fungus Microcyclus ulei, which is responsible for the
South American Leaf Blight (SALB) that has affected rubber trees across
Latin America since the beginning of the twentieth century. As only
imprecise historical information is available, the study of population
evolutionary history based on population genetics appeared most
appropriate. The distribution of genetic diversity in a continental
sampling of four countries (Brazil, Ecuador, Guatemala and French Guiana)
was studied using a set of 16 microsatellite markers developed
specifically for this purpose. A very strong genetic structure was found
(Fst = 0.70), demonstrating that there has been no regular gene flow
between Latin American M. ulei populations. Strong bottlenecks probably
occurred at the foundation of each population. The most likely scenario of
colonization identified by the Approximate Bayesian Computation (ABC)
method implemented in DIYABC suggested two independent sources from the
Amazonian endemic area. The Brazilian, Ecuadorian and Guatemalan
populations might stem from serial introductions through human-mediated
movement of infected plant material from an unsampled source population,
whereas the French Guiana population seems to have arisen from an
independent colonization event through spore dispersal.