Abstracts (first author)
Genetic basis and geographical patterns of adaptive divergence in an intertidal snail
At the onset of ecological speciation with gene flow, an ancestral species splits into ecotypes with incomplete reproductive isolation. In multiple systems, ecotypes coexist in many geographical locations, raising the question whether the genetic basis of divergence is identical across the whole range. While some loci may be under divergent selection on large geographical scales (reflecting a spread of favourable alleles), others might be involved in divergence only locally (potentially causing “parallel evolution”). Geographically close locations with a shared colonization history may be expected to share more divergently selected loci than distant ones. Our project aims at estimating the relative contribution of globally and locally selected alleles to divergence between ecotypes of the marine snail Littorina saxatilis. Using snails from Spain, Sweden and the United Kingdom, we performed an RNA-sequencing experiment and analysed the resulting allele frequency data. Because Swedish and British locations probably have a shared postglacial colonization history, we expected them to share loci under divergent selection. We found a large number of single nucleotide polymorphisms (SNPs) which showed differentiation between ecotypes only locally, indicating parallel evolution. In Spain there were more SNPs with significant differentiation between ecotypes than in Sweden and the UK, potentially reflecting stronger divergence between ecotypes. While transcriptome-wide differentiation between Swedish and UK samples was lower than their differentiation from Spain, we did not find a higher number of shared loci under selection. However, we identified >1000 SNPs showing increased differentiation between ecotypes across all three countries, suggesting a shared origin of divergence. Our results contribute to the understanding of the genome as a mosaic of loci with partly independent evolutionary histories and demonstrate how genomic patterns of selection vary across space.