Genetic data to describe the hybrid zone between Bufo bufo (Linnaeus, 1758) and Bufo verrucosissimus (Pallas, 1814) in northeastern Türkiye

Hybrid zones are geographic areas where individuals from distinct taxa meet, mate, and hybridize. These zones may have complex histories, but many of them originated relatively recently, during climatic oscillations in the Quaternary, following range shifts of formerly isolated, well-differentiated lineages. The Bufo bufo species group comprises four species distributed over the Western Palearctic. Whereas the contact zone between Bufo bufo and B. spinosus in western Europe has been well characterized, little is known about other species contacts. Here we focused on the contact between B. verrucosissimus and B. bufo in northeastern Türkiye, using mtDNA and microsatellite markers to describe genetic structure and patterns of admixture in the hybrid zone based on Bayesian clustering and cline analyses. Both species meet in a narrow contact zone at Rize province, with restricted introgression suggesting barriers to hybridization consistent with species status. Spatial population genetic analyses of microsatellite data pinpoint a possible enclave population of B. bufo at the Borçka district in Artvin province, isolated within the B. verrucosissimus range. The centers of the microsatellite and mtDNA-based clines are slightly displaced, with B. verrucosissimus mtDNA introgressing about 33 km W of the nuclear contact. Hybrid zone dynamics seem to be associated with range shifts mediated by Pleistocene glacial cycles and/or sex-biased dispersal.