Using estimates of migration from mtDNA haplotypes between dark- and light-colored sampling sites and Mclr allele frequencies at each site, we estimated selection coefficients against mismatched Mclr alleles, assuming a simple model of migration-selection balance. We found a strong correlation between Mclr allele frequency and habitat color and no correlation between mtDNA markers and habitat color. We also sequenced two neutral mtDNA genes, COIII and ND3, in the same individuals. To determine the frequency of the two Mclr allelic classes across the dark-co1ored lava and neighboring light-colored granite, we sequenced the Mclr gene in 175 individuals from a 35-km transect in the Pinacate lava region.
Previous work has demonstrated that two Mclr alleles, D and d, differ by four amino acids, and are responsible for the color polymorphism: DD and Dd genotypes are melanic whereas dd genotypes are light colored. This color polymorphism is a presumed adaptation to avoid predation. Pocket mice inhabiting volcanic lava have dark coats with unbanded, uniformly melanic hairs, whereas mice from nearby light-colored granitic rocks have light coats with banded hairs. Here, we compare phenotypic variation in color, allele frequencies of a pigmentation gene (the melanocortin-1 receptor, Mc1r), and patterns of neutral mitochondrial DNA (mtDNA) variation in rock pocket mice (Chaetodipus intermedius) across a habitat gradient in southern Arizona.
Patterns of geographic variation in phenotype or genotype may provide evidence for natural selection.
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