Résumé
The competitive exclusion principle postulates that different species can
only coexist in sympatry if they occupy distinct ecological niches. The
goal of this study was to understand the geographical distribution of
three species of Microbotryum anther-smut fungi that are distantly related
but infect the same host plants, the sister species Silene vulgaris and S.
uniflora, in western Europe. We used microsatellite markers to investigate
pathogen distribution in relation to host specialization and ecological
factors. Microbotryum violaceo-irregulare was only found on S. vulgaris at
high elevations in the Alps. Microbotryum lagerheimii could be subdivided
into two genetically differentiated clusters, one on S. uniflora in the UK
and the second on S. vulgaris in the Alps and Pyrenees. The most abundant
pathogen species, M. silenes-inflatae, could be subdivided into four
genetic clusters, co-occurring in the Alps, the UK and the Pyrenees, and
was found on both S. vulgaris and S. uniflora. All three fungal species
had high levels of homozygosity, in agreement with the selfing mating
system generally observed in anther-smut fungi. The three pathogen species
and genetic clusters had large range overlaps, but occurred at sites with
different elevations, temperatures and precipitation levels. The three
Microbotryum species thus do not appear to be maintained by host
specialization or geographic allopatry, but instead may occupy different
ecological niches in terms of environmental conditions.