Dispersal, gene flow and adaptive differentiation in hierarchically structured populations

Balancing the need for local adaptation and gene flow has proven difficult to quantify in hierarchically differentiated populations. This challenge was addressed using 1342 Atlantic salmon individuals from 21 rivers along the French Atlantic and Channel coasts, analyzed with a 60k SNP array. Through genome-wide scans, the study revealed five genetic clusters, with differentiation among groups being five times higher than within them.

Assignment tests showed a clear contrast between clusters, but unassigned individuals accounted for a significant proportion within some clusters, suggesting gene flow had blurred differences between populations. Both among-cluster and within-cluster adaptive differentiation were observed; however, the involved loci only partially overlapped across scales and clusters, indicating distinct sets of variants are required for adaptation at each hierarchical level.

Dispersal provided little insight into where these adaptations persisted: retaining potential dispersers narrowed the candidate set more than threefold, implying that immigrant genotypes can quickly dilute allele-frequency contrasts within a single generation. Nevertheless, differentiation among populations remained strong, as predicted if dispersers had low reproductive success.

Understanding the interplay between local adaptation and gene flow therefore necessitates considering dispersal and adaptive differentiation at different spatial scales, with important implications for conserving hierarchically structured populations at climatically vulnerable range edges.