Abstract
The evolutionary past of lineages can be intricate despite well-resolved phylogenies. When speciation occurs on a continuous scale, reproductive barriers can remain incomplete and give way to introgression. If species have diverged over large time spans, the signs of introgression can get blurred by recombination and genetic drift, leaving only small traces of admixture in the genomes. The kingdom of Fungi originated over a billion years ago, and it might contain many species exhibiting signs of ancient introgression. In this study, I investigate introgression between two fungal sister species, Trichaptum abietinum and T. fuscoviolaceum. The species constitute monophyletic taxa but are morphologically and ecologically similar, with overlapping habitats. I aimed at investigating the possibility of introgression between these species by conducting whole genome sequencing of individuals from populations in North America and Europe. I applied divergence analyses (FST and dXY) to assess the genome wide nucleotide differences between the species, and ABBA-BABA analyses (D and f statistics) to investigate introgression. This study is one of few conducting such analyses to examine introgression among mushroom-forming fungi. I also performed crossing experiments to assess reproductive barriers between the species. The results reveal T. abietinum and T. fuscoviolaceum to be highly divergent sister species with genome wide high FST and dXY values. The crossing experiments further show the species incompatible in vitro. Despite the large genetic differences and incompatibility, the species show signs of introgression with small regions of high or low fdM values scattered throughout the genomes. Ghost populations (both unsampled extant and extinct populations) may be involved in the introgression. Moreover, the introgression is most likely ancient and might have affected the evolutionary trajectory of the species. This study demonstrates that ancient introgression can be found among mushroom-forming fungi, but the implications of gene transfer across species and possible retention of introgressed genes from extinct lineages remain unknown.