Testing the geographical dimensions of genetic diversity following range expansion in a North American snake

Abstract

Spatial and demographic expansion can alter patterns of genetic variation and have predictable spatial and temporal consequences. Two-dimensional range expansion should result in genetic variation that is correlated with the geographical axis of expansion. Notably, populations across the range of a geographically widespread species may experience expansion and contraction events asynchronously. We tested for the genetic consequences of range expansion in the flat-headed snake, Tantilla gracilis, which inhabits the North American Great Plains and has few barriers to terrestrial dispersal. We sequenced mitochondrial DNA (mtDNA) and nuclear DNA from across the range of T. gracilis, inferred phylogenies and constructed haplotype networks. We observed multiple geographically circumscribed mtDNA clades that varied greatly in their spatial extent, but little range-wide variation in nuclear DNA. Mitochondrial sequence variation was negatively associated with latitude across their geographical range, the expected pattern of northward expansion. Finally, the three largest mtDNA clades exhibited varying demographic patterns, with only one being consistent with recent expansion. Despite substantial mtDNA cladogenesis, the predicted signature of northern range expansion was still detectable in T. gracilis. Our results bolster the notion that post-Pleistocene range expansion was a dominant force shaping the genetic diversity of vertebrates in central North America.

The effects of climate and demographic history in shaping genomic variation across populations of the Desert Horned Lizard (Phrynosoma platyrhinos)

“…Range expansion can be viewed as a series of founder events where subsets of individuals colonize new areas, leading to thinning of genotypes in the direction of the expansion (Excoffier et al, 2009; Hewitt, 2000). The signal of serial founder events can be especially apparent in mitochondrial DNA (mtDNA) population structure due to combined effects of small effective population size, maternal inheritance, and fast mutation rates (Cox et al, 2018; Excoffier, Foll, et al, 2009; Jezkova et al, 2015, 2016; Streicher et al, 2016). Additionally, a phenomenon known as allele surfing occurs when populations on the expansion front are small and results in individuals of those populations disproportionately contributing their alleles to the expansion (Hoban et al, 2016) which can enable mutations to establish at expanding range margins and reach high frequencies.…”