Mitochondrial phylogeography of Testudo graeca in the Western Mediterranean: Old complex divergence in North Africa and recent arrival in Europe

Abstract

Abstract We investigated the mitochondrial phylogeography of spur-thighed tortoises (Testudo graeca) in the Western Mediterranean. In North Africa, four major lineages (A-D) occur that together constitute a well-supported clade corresponding to one of the six major clades within T. graeca; the North African clade is sister to a Caucasian clade representing the subspecies T. g. armeniaca. Phylogenetic relationships between the North African lineages are badly resolved. Lineage A is distributed in Tunisia and adjacent Algeria, lineage B in Algeria and northern Morocco, lineage C in the Libyan Cyrenaica Peninsula, and lineage D north of the High Atlas Mts. and in the Souss Valley (southern Morocco). Lineage B is subdivided into two subgroups, B1 (eastern Morocco and Algeria) and B2 (north-western Morocco). Italian tortoises harbour haplotypes of lineage A, Spanish tortoises of subgroup B1. Based on a relaxed molecular clock calibrated with fossil evidence, the six major mtDNA clades of T. graeca are estimated to have diverged approximately 4.2-1.8 Ma ago; the split between the clades representing the eastern subspecies T. g. ibera and T. g. terrestris is younger than the split between Western Mediterranean tortoises and T. g. armeniaca. The Western Mediterranean lineages A-D were dated to have diverged at least 1.4-1.1 Ma ago; B1 and B2 split approximately 0.7 Ma ago. Our results suggest that Italian and Spanish tortoises were either introduced or originated from trans-oceanic dispersal in historic or prehistoric times. Spur-thighed tortoises invaded North Africa probably across Near Eastern landbridges that emerged in the Late Tertiary. Their diversification in North Africa seems to be correlated with habitat aridization cycles during the Pleistocene. The ranges of the Western Mediterranean lineages largely correspond to the distribution of morphologically defined subspecies in North Africa, with exception of T. g. graeca and T. g. whitei, and of T. g. lamberti and T. g. marokkensis, which are not differentiated. We propose to lump the first two subspecies under the name of T. g. graeca and the latter under the name of T. g. marokkensis. The complex differentiation of spur-thighed tortoises in North Africa implies that the model of a bipartite east-west differentiation, as proposed for other Maghrebian amphibians and reptiles, may be too simplistic, reflecting incomplete locality sampling rather than actual phylogeographic differentiation.