Macrochelys

Folt, Brian & Guyer, Craig, 2015, Evaluating recent taxonomic changes for alligator snapping turtles (Testudines: Chelydridae), Zootaxa 3947 (3), pp. 447-450 : 447

Macrochelys phylogenetics

Geographic variation in Macrochelys morphology has been described among populations (e.g., number of supramarginal scutes, skull shape; Pritchard 1989), but populations have historically been treated as comprising a single, wide-ranging species. However, because other highly aquatic organisms in Gulf Coastal drainages exhibit patterns of drainage-specific endemism (e.g., Graptemys ; Ennen et al. 2010), two studies in the last 16 years have explored population genetic structure of Macrochelys and systematic implications of that structure.

Roman et al. (1999; hereafter ‘Roman et al.’) sequenced two partial genes of the mitochondrial genome (tRNA PRO, 5’ end of the control region) and found populations to exhibit drainage-specific haplotypes; a gene tree generated from these data recovered three major clades of Macrochelys temminckii (sensu lato): a western clade including populations from the Trinity River to the drainages of Pensacola Bay, a central clade from the Choctawhatchee River to the Ochlockonee River, and an eastern clade restricted to the Suwannee River. In this hypothesis, the Eastern (Suwannee) population (hereafter referred to as the Eastern (Suwannee) assemblage, for consistency with literature) was basal and sister to a well-supported monophyletic group comprising populations from the central and western distribution (hereafter, central and western assemblages, respectively). However, because mtDNA is maternally inherited and fails to detect male-mediated dispersal, Echelle et al. (2010; hereafter Echelle et al.) analyzed microsatellites from the nuclear genome to further test for population genetic structure, compare phylogeographic patterns between nuclear and mtDNA, and test for past population bottlenecks. Comparison of a neighbor-joining tree summarizing microsatellite variation (F ST values) and a parsimony tree summarizing mtDNA haplotypes presented generally similar relationships, except for the drainages of Pensacola Bay, which were described by microsatellite data as being so deeply divergent as to question their membership in either the central or western clades of Roman et al. Both Roman et al. and Echelle et al. suggested low dispersal among drainages, high population structure among drainages, and potential for cryptic species within Macrochelys , but, in the absence of a thorough morphological investigation and because of nuclear-mitochondrial discordance, no taxonomic changes were made.