Riverscape genetics in brook lamprey: genetic diversity is less influenced by river fragmentation than by gene flow with the anadromous ecotype

Understanding the effect of human-induced landscape fragmentation on gene flow and evolutionary potential of wild populations has become a major concern. Here, we investigated the effect of riverscape fragmentation on patterns of genetic diversity in the freshwater resident European brook lamprey (Lampetra planeri) that has a low ability to pass obstacles to migration. We tested the hypotheses of (i) asymmetric gene flow following water current and (ii) an effect of gene flow with the closely related anadromous river lamprey (L. fluviatilis)ecotype onL. planerigenetic diversity. We genotyped 2472 individuals, including 225L. fluviatilis, sampled from 81 sites upstream and downstream barriers to migration, in 29 western European rivers. Linear modelling revealed a strong positive relationship between genetic diversity and the distance from the river source, consistent with expected patterns of decreased gene flow into upstream populations. However, the presence of anthropogenic barriers had a moderate effect on spatial genetic structure. Accordingly, we found evidence for downstream-directed gene flow, supporting the hypothesis that barriers do not limit dispersal mediated by water flow. DownstreamL. planeripopulations in sympatry withL. fluviatilisdisplayed consistently higher genetic diversity. We conclude that genetic drift and slight downstream gene flow drive the genetic make-up of upstreamL. planeripopulations whereas gene flow between ecotypes maintains higher levels of genetic diversity inL. planeripopulations sympatric withL. fluviatilis. We discuss the implications of these results for the design of conservation strategies of lamprey, and other freshwater organisms with several ecotypes, in fragmented dendritic river networks.

Automated summary

The study investigated the impact of riverscape fragmentation on genetic diversity in freshwater resident European brook lamprey. Results showed a positive relationship between genetic diversity and distance from the river source, with downstream populations sympatric with anadromous river lamprey displaying higher genetic diversity. This suggests that gene flow between ecotypes may play a role in maintaining genetic diversity in fragmented river networks.