Abstract
The way environmental variation shapes neutral and adaptive genetic variation in natural populations is a key issue in evolutionary biology. Genome scans allow the identification of the genetic basis of local adaptation without previous knowledge of genetic variation or traits under selection. Candidate loci for divergent adaptation are expected to show higher FST than neutral loci influenced solely by random genetic drift, migration and mutation. The comparison of spatial patterns of neutral markers and loci under selection may help disentangle the effects of gene flow, genetic drift and selection among populations living in contrasting environments. Using the gastropod Radix balthica as a system, we analyzed 376 AFLP markers and 25 mtDNA COI haplotypes for candidate loci and associations with local adaptation among contrasting thermal environments in Lake Mývatn, a volcanic lake in northern Iceland. We found that 2% of the analysed AFLP markers were under directional selection and 12% of the mitochondrial haplotypes correlated with differing thermal habitats. The genetic networks were concordant for AFLP markers and mitochondrial haplotypes, depicting distinct topologies at neutral and candidate loci. Neutral topologies were characterized by intense gene flow revealed by dense nets with edges connecting contrasting thermal habitats, whereas the connections at candidate loci were mostly restricted to populations within each thermal habitat and the number of edges decreased with temperature. Our results suggest microgeographic adaptation within Lake Mývatn and highlight the utility of genome scans in detecting adaptive divergence.
Highlights
Temperature influences all physiological processes from the molecular level to that of the whole organism [1,2], affects individual fitness and exerts a profound impact on the structure, dynamics and functioning of populations [2,3,4]
We found evidence for divergent selection among R. balthica populations with ca. 2% of the loci under directional selection, a proportion that fits within the 2–5% intervals reported for similar AFLP genome scans in different taxa [14,15,16,87,88]
Our results suggest fine-grained adaptation to local thermal conditions and emphasize the potential of combining information from neutral and candidate loci with ecological data to identify contrasting patterns of genetic variation related to different environmental conditions
Summary
Temperature influences all physiological processes from the molecular level to that of the whole organism [1,2], affects individual fitness and exerts a profound impact on the structure, dynamics and functioning of populations [2,3,4]. Because of these influences, the way organisms adapt to thermal variation has long captivated the attention of scientists. Identifying the way in which neutral genomic regions and regions putatively under thermal selection are distributed among thermal environments provides insights into the genetic properties that allow and constrain thermal adaptation
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