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Abstract Although the publication of high-quality reference genomes is steadily increasing, many clades remain chronically neglected. Skinks (order, Squamata; family, Scincidae) are one of the most diverse lizard families (1,785 species), yet there are currently just six published chromosome-level skink genomes. Here, we present the first telomere-to-telomere, chromosome-level reference genome for one of the most abundant lizards in the eastern United States, the common five-lined skink (Plestiodon fasciatus). Through the sequencing of RNA, long-read DNA, and Hi-C chromatin interactions, we produced an annotated reference genome (N50 = 227MB, L50 = 3) consisting of 6 macrochromosome pairs and 7 microchromosome pairs with 98% of BUSCO genes represented (lineage, Sauropsida; 7480 BUSCO markers), providing one of the most complete skink genomes to date: rPleFas1.1. Functional annotation predicts 32,520 protein-coding genes (16,100 unique, named genes) with an average gene length of 9,372 bp. Repeat annotations estimate that transposable elements comprise 46.7% of the genome, for which we show the amount and content is remarkably conserved across Scincidae. 

Abstract Phylogeographically structured lineages are a common outcome of range-wide population genetic studies. In the southeastern United States, disconnection between populations found at the intersection of the southeastern coastal plains of peninsular Florida and the southeastern plains of the adjacent continent is readily apparent among many plants and animals. However, the timing and maintenance of species boundaries between these distinctly different subtropical and temperate regions remains unknown for all organisms studied there. Using genome-scale data, we examine the timing of origins, gene flow, and the movement of genes under selection in unique ecoregions within the North American racers (Coluber constrictor). Isolation-migration models along with tests of genome-wide selection, locus-environment associations, and spatial and genomic clines demonstrate that two unrecognized species are present and are in contact at the boundary of these two ecoregions. We show that selection at several loci associated with unique environments have maintained species boundaries despite constant levels of gene flow between these lineages over thousands of generations. This research provides a new avenue of research to examine speciation processes in poorly studied biodiversity hotspots. 

ABSTRACT Glacial cycles during the Pleistocene had profound impacts on local environments and climatic conditions. In North America, some regions that currently support diverse biomes were entirely covered by ice sheets, while other regions were environmentally unsuitable for the organisms that live there now. Organisms that occupy these regions in the present day must have expanded or dispersed into these regions since the last glacial maximum, leading to the possibility that species with similar geographic distributions may show temporally concordant population size changes associated with these warming trends. We examined 17 lineages from 9 eastern North American snake species and species complexes to test for a signal of temporally concordant coexpansion using a machine learning approach. We found that the majority of lineages show population size increases towards the present, with evidence for coexpansion in five out of fourteen lineages, while expansion in others was idiosyncratic. We also examined relationships between genetic distance and current environmental predictors and showed that genomic responses to environmental predictors are not consistent among species. We, therefore, conclude that Pleistocene warming resulted in population size increases in most eastern North American snake species, but variation in environmental preferences and other species‐specific traits results in variance in the exact timing of expansion. 

Abstract The outcomes of speciation across organismal dimensions (e.g., ecological, genetic, phenotypic) are often assessed using phylogeographic methods. At one extreme, reproductively isolated lineages represent easily delimitable species differing in many or all dimensions, and at the other, geographically distinct genetic segments introgress across broad environmental gradients with limited phenotypic disparity. In the ambiguous gray zone of speciation, where lineages are genetically delimitable but still interacting ecologically, it is expected that these lineages represent species in the context of ontology and the evolutionary species concept when they are maintained over time with geographically well‐defined hybrid zones, particularly at the intersection of distinct environments. As a result, genetic structure is correlated with environmental differences and not space alone, and a subset of genes fail to introgress across these zones as underlying genomic differences accumulate. We present a set of tests that synthesize species delimitation with the speciation process. We can thereby assess historical demographics and diversification processes while understanding how lineages are maintained through space and time by exploring spatial and genome clines, genotype‐environment interactions, and genome scans for selected loci. Employing these tests in eight lineage‐pairs of snakes in North America, we show that six pairs represent 12 “good” species and that two pairs represent local adaptation and regional population structure. The distinct species pairs all have the signature of divergence before or near the mid‐Pleistocene, often with low migration, stable hybrid zones of varying size, and a subset of loci showing selection on alleles at the hybrid zone corresponding to transitions between distinct ecoregions. Locally adapted populations are younger, exhibit higher migration, and less ecological differentiation. Our results demonstrate that interacting lineages can be delimited using phylogeographic and population genetic methods that properly integrate spatial, temporal, and environmental data.