Gene flow among deeply divergent mtDNA lineages of Testudo graeca (Linnaeus, 1758) in Transcaucasia

Abstract

Using 10 polymorphic microsatellite loci and sequences of the mitochondrial cytochrome b gene, we examine gene flow in more than 100 spur-thighed tortoises from Transcaucasia and compare our findings with previously published AFLP and mtDNA data. While mtDNA sequences correspond to three deeply divergent clades and AFLP data suggest two distinct groups, microsatellite data indicate weak differentiation and extensive gene flow. We conclude that each marker system reflects a distinct episode in the evolutionary history of the Testudo graeca complex, corresponding to phases of vicariance and extensive gene flow. We hypothesize that the reciprocally monophyletic and deeply divergent mtDNA lineages reflect old vicariance events, while the conflicting nuclear markers are the legacy of younger episodes of extensive gene flow. The differentiation pattern found in the AFLP markers, with AFLP groups matching allopatrically or parapatrically distributed mitochondrial lineages, is likely to be older than the microsatellite differentiation, which could correspond to Holocene range expansions into Caucasian valleys and lower altitudes. Owing to the largely mutually exclusive distribution ranges of the deeply divergent mtDNA lineages, their identification with distinct subspecies is a reasonable and straightforward classification that facilitates communication and acknowledges the conspecifity of the involved evolutionary units. Consequently, Transcaucasia constitutes an intergradation zone of T. g. armeniaca, T. g. buxtoni and T. g. ibera.