Genetic structure of Phoxinus bigerri (Cyprinidae) is shaped by historical isolation and contemporary fragmentation

Le résumé fourni par la source

Habitat fragmentation is a major threat to freshwater biodiversity, and in river networks it arises both from the intrinsic dendritic structure of drainage systems and from anthropogenic barriers that disrupt longitudinal connectivity, yet disentangling the relative contributions of these natural and human-driven processes remains challenging. We assessed how historical catchment isolation and contemporary river fragmentation jointly shape the genetic structure of a low-dispersal freshwater fish, the Pyrenean minnow (Phoxinus bigerri), across northern Spain by genotyping 281 individuals from five independent catchments at 26 microsatellite loci and analysing genetic structure at regional and within-catchment scales using STRUCTURE. The effects of fragmentation metrics on pairwise genetic differentiation were quantified with Maximum Likelihood Population Effects mixed models, and demographic histories were reconstructed using BOTTLENECK and DIYABC-RF. Strong genetic differentiation among catchments reflected long-term drainage isolation, while pronounced substructure within several catchments indicated restricted contemporary connectivity. Among fragmentation metrics, the number of barriers between river reaches emerged as the strongest predictor of genetic differentiation, whereas cumulative barrier height and geographic distance showed weaker effects. Demographic analyses revealed signatures of recent bottlenecks, likely dating to the post-medieval period and coinciding with intensified human modification of river systems. Together, these findings indicate that contemporary river fragmentation has amplified historical patterns of genetic isolation, highlighting the pervasive impact of barriers on connectivity and providing insights relevant for the conservation and management of fragmented freshwater ecosystems.

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