Sweptaway – Part 3 – Adaptation genomics of White Sands Lizards

Recent colonization events offer juicy insights into the adaptive evolution of species in response to natural selection of novel habitats – however, they are confounded by demographic changes (eg. bottlenecks, differential migration). In a recent study, Laurent et al. (2015) analyze the population genomics of convergent color morphs (dark and light, adaptations for crypsis against predators) of two lizard species – Sceloporus cowlesi and Aspidoscelis inornata in the White Sands, southern New Mexico.

‘Dark’ morph of A. inornata – image courtesy: Simone Des Roches, http://evoecowhitesandslizards.blogspot.com/ Do check out her blog for more cool pictures from the White Sands lizards project.

Using a sequence capture assay, and screening for the Mc1r gene clones across fosmids generated from dark and light populations of both species, Laurent et al. analyze diversity, differentiation and genetic structure, demography, and selective sweeps between them. Additionally, they also estimate allele ages, and selection coefficients for putatively selected mutations in both species. Using >20k SNP’s in S. cowlesi, and >13k SNP’s in A. inornata, they determined (a) similar levels of nuclear diversity amongst three sampling locations in both species, (b) genetic structuring among locations (3 subpopulations in S. cowlesi, >2 in A. inornata), and (c) support for a model of no migration between populations in S. cowlesi, versus one of migration between all three locations in A. inornata.
Mc1r regions showed high differentiation (compared to other genomic regions) in both species, with lower nuclear diversity in light color morphs, predicting reduction in diversity following a sweep. Mc1r regions also showed negative values for Tajima’s D in the lighter morphs, versus darker morphs, and high frequency white alleles in the lighter morphs. The mutant “white” allele was also dated to be very young in both species (900 – 1200 years).

Our study provides a number of important insights into the process of parallel ecological adaptation in a novel and geologically young environment. While the times of colonization may differ between the focal species, both appear to have colonized the White Sands area after their geological formation, utilizing different genetic mechanisms within Mc1r.

Laurent, Stefan, et al. “The population genomics of rapid adaptation: disentangling signatures of selection and demography in white sands lizards.”Molecular ecology (2015). DOI: http://dx.doi.org/10.1111/mec.13385

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