A Natural Hybrid Zone Between Divergent Ecotypes of a Wood-Decay Fungus in Fennoscandia

Postglacial migration from multiple glacial refugia has led to secondary contact between diverging lineages and the formation of numerous hybrid zones. Such naturally occurring hybrid zones are, however, rarely reported in the fungal kingdom. In Fennoscandia, two distinct ecotypes of the wood-decay fungus  Meruliopsis taxicola  co-occur: a Coastal lineage associated with southern arid habitats and a Continental lineage found in inland old-growth forests. Previous studies have indicated admixture between these ecotypes. Using population genomics and distribution modelling, we investigate the population structure, demographic history, and hybrid zone dynamics of the two ecotypes in Fennoscandia. We reveal clear genomic and ecological divergence between the Continental and Coastal populations, as well as a distinct and narrow hybrid zone with ongoing admixture. Demographic inference supports divergence of the ecotypes in separate glacial refugia during the last ice age, followed by secondary contact after postglacial immigration into Fennoscandia. Gene flow across the hybrid zone is predominantly unidirectional, from the Continental population into the Coastal population, generating admixed samples. Our results support the existence of a natural hybrid zone in Fennoscandia between two genomically and ecologically divergent lineages of  M. taxicola . Given the postglacial migration routes into Fennoscandia of many organisms, we hypothesize that similar patterns may be common in other fungal species in the region.