A Molecular Phylogeny of Four Endangered Madagascar Tortoises Based on MtDNA Sequences

Abstract

Four of the five tortoise species in Madagascar,Pyxis arachnoides, P. planicauda, Geochelone radiata,andG. yniphora,are endemic and on the verge of extinction. Their phylogenetic relationships remain controversial and unresolved. Here we address the phylogeny of this group using DNA sequences for the 12S and 16S rDNA and cytbgenes in mitochondrial DNA. As outgroups we used two species ofGeochelone, pardalis(mainland Africa) andnigra(Galápagos), as well as a more distant North American tortoise,Gopherus polyphemus.We conclude that the twoPyxisspecies are sister taxa and are imbedded in the genusGeochelone,rendering this latter genus paraphyletic. There is moderate support for the sister status of the two MadagascarGeocheloneand for the monophyletic origin of all four endemics, suggesting a single colonization of the island. The separation of Madagascar from other land masses (90-165 mya) predates the origin of the endemic tortoises (estimated to be 14-22 mya). This suggests founding by rafting, a process known to have occurred with other tortoises. The derived morphological divergence of thePyxisspecies in a relatively short period of time (13-20 my) stands in contrast to the notoriously slow rate of morphological evolution in most lineages of Chelonia. © 1999 Academic Press.