Differences in gene flow in a twofold secondary contact zone of pond turtles in southern Italy (Testudines: Emydidae:Emys orbicularis galloitalica,E. o. hellenica,E. trinacris)

Using virtually range-wide sampling for three pond turtle taxa (Emys orbicularis galloitalica, E. o. hellenica, E. trinacris), we analyse gene flow across their southern Italian contact zone. Based on population genetic analyses of 15 highly polymorphic microsatellite loci and a mitochondrial marker, we show that the general genetic pattern matches well with the current taxon delimitation. Yet, single individuals with conflicting genetic identity suggest translocation of turtles by humans. In addition, we identify in south-western France and the vicinity of Rome populations being heavily impacted by introduced turtles. Cline analyses reveal that the major genetic break between E. o. galloitalica and E. o. hellenica corresponds well with the currently accepted intergradation zone in southern Italy. However, introgression is largely unidirectional from E. o. galloitalica into E. o. hellenica. In the distribution range of the latter subspecies, genetic footprints of E. o. galloitalica are evident along most of the Italian east coast. Our results corroborate that E. o. galloitalica was introduced long ago in Corsica and Sardinia and naturalized there. Gene flow between E. orbicularis and E. trinacris is negligible, with the Strait of Messina matching well with the narrow cline centre between the two species. This contrasts with other Mediterranean freshwater turtle species with extensive transoceanic gene flow. Compared to the two subspecies of E. orbicularis, the Sicilian E. trinacris shows an unexpectedly strong population structuring, a finding also of some relevance for conservation. The differences between the two taxon pairs E. orbicularis/E. trinacris and E. o. galloitalica/E. o. hellenica support their current taxonomic classification and make them attractive objects for follow-up studies to elucidate the underlying mechanisms of speciation by comparing their properties.