Abstract
Biogeographical patterns and large-scale genetic structure have been little studied in
ectomycorrhizal (EM) fungi, despite the ecological and economic importance of EM
symbioses. We coupled population genetics and phylogenetic approaches to understand
spatial structure in fungal populations on a continental scale. Using nine microsatellite
markers, we characterized gene flow among 16 populations of the widespread EM
basidiomycete Laccaria amethystina over Europe (i.e. over 2900 km). We also widened
our scope to two additional populations from Japan (104 km away) and compared them
with European populations through microsatellite markers and multilocus phylogenies,
using three nuclear genes (NAR, G6PD and ribosomal DNA) and two mitochondrial
ribosomal genes. European L. amethystina populations displayed limited differentiation
(average FST = 0.041) and very weak isolation by distance (IBD). This panmictic European
pattern may result from effective aerial dispersal of spores, high genetic diversity in
populations and mutualistic interactions with multiple hosts that all facilitate migration.
The multilocus phylogeny based on nuclear genes confirmed that Japanese and
European specimens were closely related but clustered on a geographical basis. By
using microsatellite markers, we found that Japanese populations were strongly
differentiated from the European populations (FST = 0.416), more than expected by
extrapolating the European pattern of IBD. Population structure analyses clearly
separated the populations into two clusters, i.e. European and Japanese clusters. We
discuss the possibility of IBD in a continuous population (considering some evidence for
a ring species over the Northern Hemisphere) vs. an allopatric speciation over Eurasia,
making L. amethystina a promising model of intercontinental species for future studies.