Evolutionary history of the conifer root rot fungus Heterobasidion annosum sensu lato

We investigated two hypotheses for the origin of the root rot fungus Heterobasidionannosum species complex: (i) that geology has been an important factor for the speciation (ii) that coevolutionaryprocesses with the hosts drove the divergence of the pathogen species. The H. annosumspecies complex consists of five species: three occur in Europe, H. annosum s.s., Heterobasidionparviporum and Heterobasidion abietinum, and two in North America, Heterobasidion irregulare andHeterobasidion occidentale; all with different but partially overlapping host preferences. Theevolution of the H. annosum species complex was studied using six partially sequenced genes,between 10 and 30 individuals of each species were analysed. Neighbour-joining trees wereconstructed for each gene, and a Bayesian tree was built for the combined data set. In addition,haplotype networks were constructed to illustrate the species relationships. For three of the genes, H.parviporum and H. abietinum share haplotypes supporting recent divergence and⁄or possible geneflow. We propose that the H. annosum species complex originated in Laurasia and that the H.annosum s.s.⁄H. irregulare and H. parviporum⁄H. abietinum⁄H. occidentale ancestral species emergedbetween 45 and 60 Ma in the Palaearctic, well after the radiation of the host genera. Our data implythat H. irregulare and H. occidentale were colonizing North America via different routes. Inconclusion, plate tectonics are likely to have been the main factor influencing Heterobasidionspeciation and biogeography.