Natural colonization and localized admixture across the Adriatic–Danube divide: insights from Telestes souffia and Telestes muticellus

Along the Adriatic–Danube divide, Pleistocene climatic oscillations intermittently permitted faunal exchange across closely spaced headwaters. We investigated two non-commercial cyprinids, Telestes souffia and T. muticellus, across the Soča drainage (Slovenia–Italy) and adjacent Sava reaches using mitochondrial (cytochrome b) and nuclear (microsatellite) markers. We reveal clear genetic differentiation between T. souffia populations from the Sava and Soča drainages and show that its occurrence within the Soča drainage is restricted to the middle Soča. Genetic patterns indicate that its presence there reflects natural postglacial colonization from the upper Sava rather than recent anthropogenic translocations, highlighting the historical permeability of the Soča–Sava drainage boundary. We further confirm T. muticellus in previously unverified parts of Slovenia, including the Nadiža and Brda watercourses, providing evidence for its native status and supporting refinement of Natura 2000 site designations. A genetically distinct T. muticellus individual from the Vipava river system suggests a historically broader distribution in the lower Soča. Microsatellite data reveal localized introgressive hybridization between T. souffia and T. muticellus in the Volarja Stream, a middle Soča tributary, consistent with recent human-mediated translocations. No evidence for a broad or persistent hybrid zone was detected within the sampled sections of the Soča drainage.