Maps of Drymarchon couperi (sensu lato) sampling sites represented as (A) pie charts of percent ancestry within population clusters identified by Structure analysis K = 8 populations,and (B) cluster membership from the Geneland analysis with K = 6 populations. (2016b), Figure borrowed from Folt et al. The black dashed lines indicates the boundary between the Atlantic and Gulf lineages from Krysko et al. For both panels, percent ancestry and/or cluster membership was assigned given the number of populations K that received the highest support during the analysis. (2019) concluded that genetic structure among populations is best described as continuous isolation by distance along a north-south geographic axis within a single species. ![]() Genetic distance was strongly correlated with geographic distance across the range when samples were separated into both north-south clusters and Gulf-Atlantic clusters. ![]() These genetic analyses supported multiple populations within eastern indigo snakes however the geographic pattern suggested a north-south orientation rather than a Gulf-Atlantic orientation and the contemporary gene flow was widespread across this geographic pattern (Figure 5) (Folt et al. (2019) using microsatellite (nuclear) DNA (nDNA) from 428 tissue samples of eastern indigo snakes from across the species range, including the 20 samples used by Krysko et al. Genetic diversity was further evaluated by Folt et al. More recent data bring into question the validity of splitting the eastern indigo snake into two species. Atlantic Lineage (blue triangles with dots = genetic samples, without dots = this study using morphology based on cluster analysis) Gulf Lineage (red circles with dots = genetic samples, without dots = this study using morphology based on cluster analysis) and stars represent holotype specimens for D. Distribution of Indigo Snake (Drymarchon) samples on the subaerially exposed Florida Platform in Florida and Georgia. 566) as a hybrid zone between the two lineages. This region was described by Krysko et al. Nevertheless, in certain areas of Florida, this potential classification would place the two eastern indigo snake lineages in close enough proximity that no barrier to gene flow would exist between them (Figure 4). These two lineages illustrate a similar biogeographic pattern previously identified for other plants and animals that have come in and out of contact with each other many times during historic sea level changes. 2016a, b, entire), and described differences in scalation which they assert provide a method to morphologically distinguish between the two species. The authors hypothesized that these two lineages represent two different species of indigo snakes (Krysko et al. (2016b, entire) described a divergence of the species into two genetic lineages, an Atlantic lineage occupying southeastern Georgia and northeast Florida and a Gulf lineage occupying southern Florida, the central Lake Wales Ridge of Florida, the Gulf Coast drainage of Florida, and the panhandle region of western Florida (Figure 4). (2016b, entire) evaluated the genetic diversity of 20 eastern indigo snakes across Florida and southern Georgia using mitochondrial DNA (mtDNA) derived from tissue samples. (2010, entire) used 22 nuclear microsatellite markers to successfully differentiate individual snakes from Fort Stewart, Georgia, and suggested the technique used in their genetic analysis could also prove valuable in conducting population level studies. Ongoing genetic studies further evaluating taxonomic classification have only recently been conducted. In addition to the eastern indigo snake, other common names include blue indigo snake and blue gopher snake. ![]() The Service adopted this change in nomenclature in 2008 (USFWS 2008, p.23). ![]() Subsequent work supported this designation and the eastern indigo snake was accepted by the scientific community as its own species, Drymarchon couperi (Wüster et al. Post-listing, Collins (1991, p.43) elevated this lineage to species status based on geographic isolation and morphology. At the time of listing under the Endangered Species Act in 1978, the eastern indigo snake was considered a subspecies of indigo snake, Drymarchon corais couperi (USFWS 1978).
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