Trachemys in Mexico and beyond: Beautiful turtles, taxonomic nightmare, and a mitochondrial poltergeist (Testudines: Emydidae)

Abstract

Abstract Trachemys is a speciose genus of freshwater turtles distributed from the Great Lakes in North America across the southeastern USA, Mexico and Central America to the Rio de la Plata in South America, with up to 13 continental American species and 11 additional subspecies. Another four species with three additional subspecies occur on the West Indies. In the present study, we examine all continental Trachemys taxa except for Trachemys hartwegi using mitochondrial and nuclear DNA sequences (3221 and 3396 bp, respectively) representing four mitochondrial genes and five nuclear loci. We also include representatives of all four West Indian species and discuss our results in the light of putative species-diagnostic traits in coloration and pattern. We provide evidence that one Mexican species, T. nebulosa, has captured a deeply divergent foreign mitochondrial genome that renders the mitochondrial phylogeny of Trachemys paraphyletic. Using nuclear markers, Trachemys including T. nebulosa represents a well-supported monophylum. Besides the mitochondrial lineage of T. nebulosa, there are six additional mitochondrial Trachemys lineages: (1) T. venusta, (2) T. ornata + T. yaquia, (3) T. grayi, (4) T. dorbigni + T. medemi, (5) T. gaigeae + T. scripta, and (6) West Indian Trachemys. These six mitochondrial lineages constitute a well-supported clade. Each mitochondrial Trachemys lineage is corroborated by our nuclear markers. For T. gaigeae another mitochondrial capture event is likely because its mitochondrial genome is sister to T. scripta, although T. gaigeae is deeply divergent in nuclear markers and resembles Mexican, Central and South American Trachemys species in morphology, sexual dimorphism and courtship behavior. The two subspecies of T. nebulosa and many Mexican and Central American subspecies of T. venusta are not clearly distinct in our studied genetic markers. Also, the putatively diagnostic coloration and pattern traits of the T. venusta subspecies are more variable than previously reported, challenging their validity. Our analyses fail to identify T. taylori as a lineage distinct from T. venusta and we propose to assign it as a subspecies to the latter species (Trachemys venusta taylori nov. comb.).