Abstract
Investigating the extent (or the existence) of local adaptation is crucial to understanding how populations adapt. When experiments or fitness measurements are difficult or impossible to perform in natural populations, genomic techniques allow us to investigate local adaptation through the comparison of allele frequencies and outlier loci along environmental clines. The thick‐billed murre (Uria lomvia) is a highly philopatric colonial arctic seabird that occupies a significant environmental gradient, shows marked phenotypic differences among colonies, and has large effective population sizes. To test whether thick‐billed murres from five colonies along the eastern Canadian Arctic coast show genomic signatures of local adaptation to their breeding grounds, we analyzed geographic variation in genome‐wide markers mapped to a newly assembled thick‐billed murre reference genome. We used outlier analyses to detect loci putatively under selection, and clustering analyses to investigate patterns of differentiation based on 2220 genomewide single nucleotide polymorphisms (SNPs) and 137 outlier SNPs. We found no evidence of population structure among colonies using all loci but found population structure based on outliers only, where birds from the two northernmost colonies (Minarets and Prince Leopold) grouped with birds from the southernmost colony (Gannet), and birds from Coats and Akpatok were distinct from all other colonies. Although results from our analyses did not support local adaptation along the latitudinal cline of breeding colonies, outlier loci grouped birds from different colonies according to their non‐breeding distributions, suggesting that outliers may be informative about adaptation and/or demographic connectivity associated with their migration patterns or nonbreeding grounds.
Highlights
Natural selection is multidimensional in that selective forces can vary both spatially and temporally
ΔK showed a peak at K = 5, indicating that individuals are more similar within colonies than among colonies and suggesting hierarchical population structure (Figure 2b,d)
Lositan identified a total of 252 outlier single nucleotide polymorphisms (SNPs) from the randomized dataset, but both Structure and the discriminant analysis of principal components (DAPC) failed to detect population structure in this dataset, indicating that the apparent distinctiveness of the colonies is not an artifact of the analyses
Summary
Natural selection is multidimensional in that selective forces can vary both spatially and temporally. Thick-billed murres are exposed to environmental differences at their breeding areas, including differences in photoperiod, air and sea surface temperature, and timing of spring ice breakup and fall freeze-up (see Appendix S1). Some of these environmental factors have documented fitness effects, either direct or indirect. Thick-billed murres provide a good system to investigate local adaptation because the factors that might confound the signatures of selection in the genome, such as genetic drift or population structure, are minimized, and the time of divergence between colonies is known. From a conservation perspective, estimating levels of differentiation and standing genetic variation will help us to understand the potential of thick-billed murres to adapt to future environmental change
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