Individual‑based population genomics reveal diferent drivers of adaptation in sympatric fish

Abstract

Connectivity and local adaptation are two contrasting evolutionary forces highly infuencing population structure. To evaluate the impact of early-life traits and environmental conditions on genetic structuring and adaptation, we studied two sympatric fsh species in the Western Mediterranean Sea: Symphodus tinca and S. ocellatus. We followed an individual-based approach and measured early-life history traits from otolith readings, gathered information on environmental variables and obtained genome-wide markers from genotyping-by-sequencing (GBS). The two species presented contrasting population structure across the same geographic gradient, with high and signifcant population diferentiation in S. ocellatus, mostly determined by oceanographic fronts, and low diferentiation and no front efect in S. tinca. Despite their diferent levels of genetic diferentiation, we identifed in both species candidate regions for local adaptation by combining outlier analysis with environmental and phenotypic association analyses. Most candidate loci were associated to temperature and productivity in S. ocellatus and to temperature and turbulence in S. tinca suggesting that diferent drivers may determine genomic diversity and diferentiation in each species. Globally, our study highlights that individual-based approach combining genomic, environmental and phenotypic information is key to identify signals of selection and the processes mediating them.