Résumé
Cold-adapted organisms with current arctic-alpine distributions have
persisted during the last glaciation in multiple ice-free refugia, leaving
footprints in their population structure that contrast with temperate
plants and animals. However, pathogens that live within hosts having
arctic-alpine distributions have been little studied. Here, we therefore
investigated the geographical range and population structure of a fungus
parasitizing an arctic-alpine plant. A total of 1437 herbarium specimens
of the plant Silene acaulis were examined, and the anther smut pathogen
Microbotryum silenes-acaulis was present throughout the host’s geographic
range. There was significantly greater incidence of anther smut disease in
more northern latitudes and where the host locations were less dense,
indicating a major influence of environmental factors and/or host
demographic structure on the pathogen distribution. Genetic analyses with
seven microsatellite markers on recent collections of 195 M.
silenes-acaulis individuals revealed three main genetic clusters, in North
America, northern Europe and southern Europe, likely corresponding to
differentiation in distinct refugia during the last glaciation. The lower
genetic diversity in northern Europe indicates postglacial recolonization
northwards from southern refugia. This study combining herbarium surveys
and population genetics thus uniquely reveals the effects of climate and
environmental factors on a plant pathogen species with an arctic-alpine
distribution.