Hybridization of Two Megacephalic Map Turtles (Testudines: Emydidae: Graptemys) in the Choctawhatchee River Drainage of Alabama and Florida
Map turtles of the genus Graptemys are highly aquatic and rarely undergo terrestrial movements, and limited dispersal among drainages has been hypothesized to drive drainage-specific endemism and high species richness of this group in the southeastern United States. Until recently, two members of the megacephalic “pulchra clade,” Graptemys barbouri and Graptemys ernsti, were presumed to be allopatric with a gap in both species' ranges in the Choctawhatchee River drainage. In this paper, we analyzed variation in morphology (head and shell patterns) and genetics (mitochondrial DNA and microsatellite loci) from G. barbouri, G. ernsti, and Graptemys sp. collected from the Choctawhatchee River drainage, and we document the syntopic occurrence of those species and back-crossed individuals of mixed ancestry in the Choctawhatchee River drainage. Our results provide a first counter-example to the pattern of drainage-specific endemism in megacephalic Graptemys. Geologic events associated with Pliocene and Pleistocene sea level fluctuations and the existence of paleo-river systems appear to have allowed the invasion of the Choctawhatchee system by these species, and the subsequent introgression likely predates any potential human-mediated introduction.
Gulf Coastal Plain river systems of Alabama, Florida, and Georgia from which taxa of Graptemys were collected for the taxonomic assessment of the Choctawhatchee and Pea river population. Open squares represent collection localities for Graptemys ernsti from the Conecuh and Yellow rivers, Alabama; open triangles represent collection localities for Graptemys barbouri from Ichawaynochaway Creek, Georgia; solid diamonds represent collection localities for Graptemys sp. in the Choctawhatchee and Pea rivers, Alabama and Florida.
Diagnostic head characters of select Graptemys from the southeastern United States: (A) chin bar of Graptemys barbouri (Choctawhatchee River, Holmes County, FL), (B) chin spots of Graptemys ernsti (Yellow River, Covington County, AL), (C) POB-IOB separation, supraoccipital spots, and nasal trident of G. ernsti (Pea River, Geneva County, AL), (D) postorbital-interorbital (POB-IOB) connection, mid-dorsal “heart” pattern, and prefrontal arrow of G. barbouri (Choctawhatchee River, Geneva County, AL), and (E) intermediate dorsal head pattern of a Graptemys collected in the Choctawhatchee River drainage (Pea River, Geneva County, AL).
Plot of discriminant scores for mensural shell variables in male (A) and female (B) Graptemys. Refer to text for details. Symbols are as follows: squares = Graptemys ernsti, triangles = Graptemys barbouri, and solid circles = Graptemys from the Choctawhatchee and Pea rivers. Minimum convex polygons are drawn around each cluster of points.
Distribution of MHI scores of individuals from the Choctawhatchee and Pea rivers, Alabama and Florida, ranging from 2 (morphologically pure Graptemys ernsti) to 0 (morphologically pure Graptemys barbouri).
Principal coordinate analysis showing allelic ordination of Graptemys from Ichawaynochaway Creek, Georgia and the Conecuh, Yellow, Pea, and Choctawhatchee rivers of Alabama and Florida.
Bar plots showing the results of the mtDNA assay, STRUCTURE analysis, and NewHybrids analysis for 42 Graptemys from the Choctawhatchee and Pea rivers, Alabama and Florida. The order of individuals corresponds to that in Appendix 1. For the mtDNA, black represents G. barbouri and white represents G. ernsti haplotypes. The STRUCTURE results show the average ancestry scores (q) with gray representing the G. barbouri group and white the G. ernsti group. The NewHybrids plot shows the probability of an individual belonging to one of the six genotype classes with colors ranging from dark blue to dark red reflecting G. barbouri, G. barbouri-backcross, F2, F1, G. ernsti-backcross, and G. ernsti, respectively.
Contributor Notes
Associate Editor: J. Kerby.