Environmental selection and gene flow jointly determine the population genetic diversity and structure of Diaphanosoma dubium along a watershed elevation

Biodiversity of local communities has been found to generally decrease with elevation. However, the elevational patterns of intraspecific genetic diversity remain unclear, especially in passive dispersers and species with short life histories. In this study, we investigated the populations of a tropical dominant Cladoceran, Diaphanosoma dubium, along elevation in the largest watershed of South China. The studied populations were grouped into four regional groups based on elevation and habitat connectivity. No significant elevation pattern was observed for mitochondrial genetic diversity, but nuclear diversity was influenced by habitat type. The haplotype network showed a radial pattern with a common haplotype at the centre, and many private haplotypes existed in each regional group. Four clusters of D. dubium populations were identified and exactly corresponding to the four investigated regions, indicating a strong spatial genetic structure. Weak but significant isolation by distance (IBD) supported the critical role of environmental selection with elevation. Precipitation significantly explained population genetic variation at the watershed scale. Bidirectional gene flow was a common pattern that occurred among the four regional groups. Our results highlight that habitat selection with gene flow plays an important role in shaping the genetic pattern of tropical freshwater Cladocera along an elevational gradient.