Mitochondrial phylogeography, contact zones and taxonomy of grass snakes (<i><scp>N</scp>atrix natrix</i>,<i><scp>N</scp>. megalocephala</i>)

Abstract

Grass snakes ( N atrix natrix ) represent one of the most widely distributed snake species of the P alaearctic region, ranging from the N orth A frican M aghreb region and the I berian P eninsula through most of E urope and western A sia eastward to the region of L ake B aikal in C entral A sia. Within N . natrix , up to 14 distinct subspecies are regarded as valid. In addition, some authors recognize big‐headed grass snakes from western T ranscaucasia as a distinct species, N . megalocephala . Based on phylogenetic analyses of a 1984‐bp‐long alignment of mt DNA sequences ( ND 4+t RNA s, cyt b ) of 410 grass snakes, a nearly range‐wide phylogeography is presented for both species. Within N . natrix , 16 terminal mitochondrial clades were identified, most of which conflict with morphologically defined subspecies. These 16 clades correspond to three more inclusive clades from (i) the Iberian P eninsula plus N orth A frica, (ii) E ast E urope and A sia and (iii) West E urope including C orso‐ S ardinia, the A pennine P eninsula and S icily. Hypotheses regarding glacial refugia and postglacial range expansions are presented. Refugia were most likely located in each of the southern E uropean peninsulas, C orso‐ S ardinia, North A frica, A natolia and the neighbouring N ear and M iddle E ast, where the greatest extant genetic diversity occurs. Multiple distinct microrefugia are inferred for continental Italy plus S icily, the B alkan P eninsula, A natolia and the N ear and M iddle E ast. Holocene range expansions led to the colonization of more northerly regions and the formation of secondary contact zones. Western E urope was invaded from a refuge within southern F rance, while C entral E urope was reached by two distinct range expansions from the B alkan P eninsula. In C entral E urope, there are two contact zones of three distinct mitochondrial clades, and one of these contact zones was theretofore completely unknown. Another contact zone is hypothesized for E astern E urope, which was colonized, like north‐western A sia, from the C aucasus region. Further contact zones were identified for southern Italy, the B alkans and T ranscaucasia. In agreement with previous studies using morphological characters and allozymes, there is no evidence for the distinctiveness of N . megalocephala . Therefore, N . megalocephala is synonymized with N . natrix .