Postglacial History of a Widespread Wood-Decaying Fungus in Europe Suggests Migration out of Multiple Refugia Followed by Admixture.

While postglacial migration patterns have been well-studied in plants and animals in Europe, the same is not true for microorganisms such as fungi, and it remains unclear whether fungi have followed the same postglacial migration trajectories. In this study, we infer the postglacial history of the widespread wood-decay fungus Trichaptum abietinum in Europe. We investigate whether this fungus resided in multiple glacial refugia, as observed in many plants and animals, and how it migrated following the retreat of the ice. Population genomic analyses of T. abietinum suggest at least two glacial refugia in Europe: a southern refugium extending from Portugal to the Caucasus (potentially formed by several earlier distinct refugia), an eastern refugium and a potential third western refugium. Expansion from these refugia followed by migration and secondary contact has led to a complex genomic pattern of what we call the Mediterranean, Boreal, Atlantic and Central European admixed groups. From the southern and potential western refugia there have likely been northwards expansions resulting in the Mediterranean and Atlantic groups, while the Boreal group probably expanded westwards from the eastern refugium. A contact zone is present in Central Europe, where the Mediterranean and Boreal groups are admixing. In these admixed individuals, the second half of their largest scaffold is entirely inherited from the Boreal group, indicating either strong selection or genomic incompatibilities. In Scandinavia, the Atlantic and Boreal groups are in close contact but with limited admixture. Tajima's D and analyses of linkage disequilibrium decay support a recent history of contraction and expansion of this fungus in Europe. We conclude that the postglacial migration patterns in T. abietinum resemble those observed in several plants and animals, suggesting multiple glacial refugia followed by admixture during northward and westward expansions.