Phylogeography of Agkistrodon piscivorus with Emphasis on the Western Limit of Its Range
The Cottonmouth, Agkistrodon piscivorus, is a semi-aquatic pitviper that occupies the southeastern U.S. west into Texas. Several previous studies have investigated the biogeographic history of A. piscivorus. It has been hypothesized that A. piscivorus was split into two separate populations during the last glacial maximum and climate change has impacted its distribution. Additionally, a geographically isolated population of A. piscivorus occurs at the western limit of the species' range in the Concho Valley of Texas. To investigate biogeography and population structure within A. piscivorus in Texas and throughout its range, we utilized amplified fragment length polymorphism (AFLP) and sequence data from cytochrome b (cyt-b). The AFLP data indicate a lack of gene flow between the population of A. piscivorus in the Concho Valley and other nearby populations. However, based on cyt-b, there is no genetic differentiation. The AFLP data for the entire species show a signature of two historic populations that have recently come into secondary contact. Finding two historic populations is consistent with previously published data based on mitochondrial DNA analyses; however, due to the rapid evolution rate of AFLP data, we were able to detect a high level of gene flow between these populations. We conclude that it is possible Texas and Florida served as refugia for A. piscivorus during the last glacial maximum, and, as the glaciers receded, the two populations expanded, coming into secondary contact. The subsequent gene flow has resulted in shared loci across the two populations. The difference between the conclusions drawn between our two markers and previous research is due to the different time scales that AFLP and cyt-b markers measure. The AFLP data provided a contemporary marker and cyt-b indicated historic separation.
Distribution map of the Cottonmouth, Agkistrodon piscivorus, showing the two current views on its taxonomy. The patterns designate the subspecies view and the dashed line splits the Florida and continental groups discussed in text. Shapes indicate sampling localities and correspond to the shapes in Figure 4.
Three-dimensional principal coordinate analysis (PCoA) of all samples used in the analysis. Rattlesnakes fall out on the third axis away from the four species within Agkistrodon. Ellipses were used to help delineate groups.
Neighbor-joining phylogram with terminal branches condensed from all samples using AFLP data. One thousand bootstrap pseudoreplicates were performed, and those with support over 50 percent are shown.
Two-dimensional PCoA of Agkistrodon piscivorus from their entire range based on AFLP data. The first two axes explain 73.40% of the variation, and the pattern indicates a west to east trend in genetic variation.
Agkistrodon piscivorus posterior mean estimates of the proportion of each individual's genome that belongs to each of the two estimated populations from STRUCTURE.
Two-dimensional PCoA of only Texas Agkistrodon piscivorus based on AFLP data. The first two axes explain 56.88% of the variation, and the pattern indicates that the Concho Valley population is less genetically variable than other populations in Texas.
Contributor Notes
Associate Editor: D. S. Siegel.