Affiliations: 1) Genetics Department, University of Wisconsin-Madison, Madison, WI; 2) Biology Department, Suffolk University, Boston, MA.
Keywords: Population history
Island populations often exhibit departures from their mainland counterparts in key behavioral, morphological, and life-history traits– a phenomenon termed the “island syndrome”. Populations of white-footed mice (Peromyscus leucopus) inhabiting two islands within the Boston Harbor archipelago provide compelling examples of this rule, measuring between 40 and 50% larger in body weight than mainland mice. In contrast to the ecological and geographical conditions often thought to drive island syndrome phenotypes, the Boston Harbor islands are situated close to the mainland and harbor both predators and interspecific competitors of P. leucopus. In addition, these islands have undergone dramatic ecological transformations over the past 400 years owing to a history of human usage that spans agriculture, industry, and recreation. Yet, despite such physical proximity and human-mediated connectivity to the mainland, island populations maintain a significant difference in body size. To understand how such phenotypic differences can persist under these seemingly permeable barriers to gene flow, we performed whole-genome sequencing of wild-caught individuals sampled from Peddocks and Bumpkin Island together with mainland representatives from World’s End. Preliminary analyses of genome pairs sampled from each location suggest lower per-site nucleotide diversity in the island populations (Peddocks = 0.00573; Bumpkin = 0.00614) compared to the mainland (World’s End = 0.00762), consistent with reduced effective population sizes on the islands, and differences in the extent of genetic differentiation between the populations (World’s End vs. Bumpkin Fst = 0.079; World’s End vs. Peddocks Fst = 0.107; Peddocks vs. Bumpkin Fst = 0.189). We extend this analysis to larger sample sizes and use the frequency spectra of putatively neutrally evolving variants to reconstruct the colonization history of the islands and estimate migration rates among the island and mainland populations. In the future, we will leverage the wealth of information encoded within whole genome sequences to examine how measures of genetic diversity and differentiation compare across coding, regulatory, and evolutionarily conserved regions of the genome. Together, these genetic comparisons of island and mainland mice will establish important aspects of their demographic history that will be key to interpreting their stark phenotypic differences and identifying the genomic regions that maintain them.