Genetic Population Structure of Cagle's Map Turtle (Graptemys caglei) in the Guadalupe and San Marcos Rivers of Texas—A Landscape Perspective
We investigated genetic variability in 101 individuals of the Cagle's Map Turtle (Graptemys caglei) collected from across most of its extant distribution in the Guadalupe River and from a site on the San Marcos River of Texas. Analyses of allelic variation in microsatellite loci indicated individuals from the San Marcos River were genetically similar to conspecifics from the middle Guadalupe River. Turtles from the upper Guadalupe River, on the other hand, were genetically divergent from middle Guadalupe River individuals. Isolation by distance appears to play a major role in genetic structuring within the Guadalupe River. Anthropogenic features (e.g., dams and reservoirs), while they may be expected to restrict gene flow, appear to have had little impact on genetic structure.
Estimated population structure generated by the program STRUCTURE (Pritchard et al., 2000). Each individual is represented by a vertical line partitioned into two segments that represent the individual's estimated probability of membership in each of two hypothetical populations. Heavy vertical lines separate individuals based on sampling site (upper Guadalupe River, middle Guadalupe River from above Canyon Lake to Victoria, Texas, and San Marcos River).
(A) Plot of likelihood scores derived from the assignment test implemented in ARLEQUIN 3.0 (Excoffier et al., 2005) for 101 individual Graptemys caglei from the upper and middle Guadalupe River and the San Marcos River, Texas. (B) Graphic representation of detrended correspondence analysis (DCA) performed on ten microsatellite composite genotypes by the GENETIX software (Belkhir, 2001) on 101 individual Graptemys caglei from the upper and middle Guadalupe River and the San Marcos River, Texas. Open circles = upper Guadalupe River samples. Black circles = middle Guadalupe River samples. Shaded circles = San Marcos River samples.
Contributor Notes
Associate Editor: D. Siegel.