Postglacial Range Dynamics of the Common Roach (Rutilus rutilus) in the Baltic Region: Integrating Palaeodistribution Modelling and Mitochondrial Haplogroup Geography

Quaternary climatic cycles reshaped freshwater fish distributions, with postglacial recolonisation influenced by climatic suitability and hydrological connectivity. We reconstructed the postglacial history of the common roach (Rutilus rutilus) in the Baltic region by integrating palaeodistribution modelling and mitochondrial haplogroup geography. A MaxEnt model calibrated with present-day occurrence records was projected onto eight periods from the Last Glacial Maximum (LGM) to the present. Mitochondrial haplogroups were inferred from concatenated ATP6 and D-loop sequences from three Lithuanian and Latvian drainages and the Ukrainian Dnieper River basin. Projected climatically suitable area changed non-monotonically, reaching its minimum during Heinrich Stadial 1 and increasing approximately 14-fold between the LGM and the present. Using the P10 suitability threshold (0.481) and MESS-masking criteria, no candidate cold-stage climatic refugia were predicted in the Baltic region. Haplogroup A predominated in the Lithuanian and Latvian drainages and comprised 96.7% of the Ukrainian Dnieper basin sample. It also occurred in Portuguese, British, Czech and Romanian references, whereas haplogroup D occurred only in Russian and Chinese reference sequences. Together, the results are consistent with postglacial recolonisation from populations outside the Baltic region, potentially facilitated during freshwater stages of the Baltic basin, but do not identify a unique refugial source or dispersal route.