Ancient mitogenomics elucidates diversity of extinct West Indian tortoises

We present 10 nearly complete mitochondrial genomes of the extinct tortoise Chelonoidis alburyorum from the Bahamas. While our samples represent morphologically distinct populations from six islands, their genetic divergences were shallow and resembled those among Galapagos tortoises. Our molecular clock estimates revealed that divergence among Bahamian tortoises began similar to 1.5 mya, whereas divergence among the Galapagos tortoises (C. niger complex) began similar to 2 mya. The inter-island divergences of tortoises from within the Bahamas and within the Galapagos Islands are much younger (0.09-0.59 mya, and 0.08-1.43 mya, respectively) than the genetic differentiation between any other congeneric pair of tortoise species. The shallow mitochondria! divergences of the two radiations on the Bahamas and the Galapagos Islands suggest that each archipelago sustained only one species of tortoise, and that the taxa currently regarded as distinct species in the Galapagos should be returned to subspecies status. The extinct tortoises from the Bahamas have two well-supported clades: the first includes one sample from Great Abaco and two from Crooked Island; the second Glade includes tortoises from Great Abaco, Eleuthera, Crooked Island, Mayaguana, Middle Caicos, and Grand Turk. Tortoises belonging to both clades on Great Abaco and Crooked Island suggest late Holocene inter-island transport by prehistoric humans.

Automated summary

The study found that the genetic divergences of extinct tortoises from the Bahamas were shallow, and the divergence among different islands began at a similar time as the Galapagos tortoises. The genetic differentiation within the tortoises of the Bahamas and the Galapagos Islands is younger compared to other congeneric pairs of tortoise species.