Latitudinal cline in allele length provides evidence for selection in a circadian rhythm gene

Divergent natural selection across a heterogeneous landscape can drive the evolution of locally adapted populations in which phenotypic variation is fine-tuned to the environment. At the molecular level, such processes can be inferred by identifying correlations between genetic variation and environmental variables. We demonstrate that allele length and allele frequency at a regulatory circadian rhythm gene, OtsClock1b, are highly correlated (R-2 = 0.86, P = 1.25 x 10(-5)) with latitude (a surrogate for photoperiod) in kokanee, the freshwater resident form of sockeye salmon (Oncorhynchus nerka). Two OtsClock1b alleles were identified that differed in length by seven amino acids, with the frequency of the shorter allele increasing from 50% in southern British Columbia (49 degrees N) to complete fixation in Alaska (62 degrees N). No such associations were detected for neutral microsatellite loci. In addition, a kokanee population sampled from Kamchatka, Russia (55 degrees N) fits within the North American latitudinal cline, suggesting that this pattern may be convergent across large longitudinal spatial scales. This correlation provides evidence that natural selection rather than demographic processes may drive the distribution of genetic variation at OtsClock1b in kokanee. (c) 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 869-877.