Post-glacial colonization and isolation shape fine-scale population structure in Venustaconcha ellipsiformis: implications for population-level conservation.

The spatial distribution of genetic diversity in freshwater organisms reflects a complex interplay of historical and contemporary evolutionary processes. In North America, repeated Pleistocene glaciations reshaped aquatic landscapes, influencing dispersal, isolation, and secondary contact. Freshwater mussels (order Unionida), which rely on host-mediated larval dispersal and often occupy fragmented habitats, are particularly sensitive to these processes. Venustaconcha ellipsiformis, a widespread but imperiled mussel species, provides a model for evaluating how glacial history and river evolution shape genetic structure across landscapes. We conducted a range-wide population genetic study using mitochondrial DNA sequences and microsatellite markers from 426 individuals across 27 localities to assess colonization history, population structure, and genetic diversity. Significant genetic structure was detected at local and regional scales, with a latitudinal gradient in genetic diversity. Ozark Plateau populations exhibited the highest haplotype and allelic richness, while populations in formerly glaciated northern regions had less variation. Cluster analysis identified four major genetic groups with further subdivision by river drainages and strong isolation by distance. These patterns support northward colonization via stepping-stone dispersal from southern refugia, followed by regional isolation. We recommend designation of three evolutionarily significant units (ESUs): Western Ozarks, Northern Ozarks, and Northern Region (Upper Mississippi-Illinois-Great Lakes), and five management units (MUs) corresponding to major drainages for V. ellipsiformis. These findings highlight the importance of conserving diversity hotspots and demographically independent populations and demonstrate the value of a genetically informed conservation framework for freshwater mussels.