Abstract
Parallel changes in genotype and phenotype in response to similar selection pressures in different populations provide compelling evidence of adaptation. House mice (Mus musculus domesticus) have recently colonized North America and are found in a wide range of environments. Here we measure phenotypic and genotypic differentiation among house mice from five populations sampled across 21° of latitude in western North America, and we compare our results to a parallel latitudinal cline in eastern North America. First, we show that mice are genetically differentiated between transects, indicating that they have independently colonized similar environments in eastern and western North America. Next, we find genetically-based differences in body weight and nest building behavior between mice from the ends of the western transect which mirror differences seen in the eastern transect, demonstrating parallel phenotypic change. We then conduct genome-wide scans for selection and a genome-wide association study to identify targets of selection and candidate genes for body weight. We find some genomic signatures that are unique to each transect, indicating population-specific responses to selection. However, there is significant overlap between genes under selection in eastern and western house mouse transects, providing evidence of parallel genetic evolution in response to similar selection pressures across North America.
Highlights
A central goal of evolutionary biology is to understand how organisms adapt to novel environments
To distinguish between these possibilities, we reconstructed the history of the 10 populations in both transects using exome data generated here from 50 wild mice across 21 ̊ of latitude in western North America (Fig 1A, collecting localities and samples are given in S1 Table, and exome data are summarized in S2 and S3 Tables), and exome data from the five eastern populations in [33]
A neighbor-joining tree generated from a distance matrix based on the exome sequences of mice in both transects depicts the relationships among these 10 populations using mice from the ancestral European range to root the tree (Fig 1B)
Summary
A central goal of evolutionary biology is to understand how organisms adapt to novel environments. To understand the predictability of adaptive evolution in response to spatial variation in selection pressures, several studies have looked for repeated patterns of evolution across different populations that have experienced similar environments and selection pressures. Comparison of pairs of freshwater and marine populations has led to the discovery of genes that show parallel changes in independent transects [15]. Parallel patterns of clinal variation across independent transects provide strong evidence that the traits in question are adaptive. In sticklebacks, most freshwater populations share a suite of common phenotypic changes. Genetic patterns of variation reveal both parallel clines and some transect-specific clines, suggesting that adaptation to a freshwater environment may involve a mix of both shared and unique genetic changes [15,20]
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