Phylogeography and population genetics of the endemic Italian water voles, Arvicola italicus

The genus Arvicola has a wide palearctic distribution and present two ecological forms – terrestrial and aquatic – characterizing the genus. In Europe two species are present: the most widespread, A. amphibius and A. italicus, endemic to Italy. The aim of the study was to describe the genetic structure and variability in A. italicus and the genetic identity of populations in northeast Italy, an area of biogeographic importance as contact zone for small mammal species. Thirty-six Italian water voles, displaced in population from south to north of Italy, were analysed by two mitochondrial markers and 8 autosomal microsatellite loci. The phylogenetic analysis on the genus confirmed the presence of major groups mostly corresponding to the species and clades already described. About the Italian specimens, the individuals from one population in northeastern Italy resulted included in A. amphibius revealing the presence of this species in Italy and, in consequence, the possible presence of a putative contact zone between A. italicus and A. amphibius in the northeast. The phylogeny includes a second population from northeast Italy within A. amphibius, but in basal position and with a low support. The values of divergence of this lineage from A. italicus and A. amphibius lie on the threshold between intraspecific and interspecific divergence. Conversely, microsatellite data support a clear distinction between A. amphibius and A. italicus and includes the ambiguous mitochondrial lineage in this latter species. Furthermore, this northeastern population shows evidence of admixture of nuclear loci with the southcentral A. italicus populations, supporting the attribution of these specimens to A. italicus. This pattern could suggest that A. italicus in northeast Italy retained an ancestral haplotype and undergone a long period of isolation respect to the southern populations, without genetic exchange until recent times. Within A. italicus, the admixture shows northern population as the most differentiated showing limited evidence of admixture with the most diverse southcentral. The intraspecific diversity decreases from south to north as expected for an endemic small mammal in Italy. Finally, the low genetic diversity observed in the northern population can be challenging for a species as Arvicola living in fragmented habitats and susceptible to habitat loss; consequently, became important identifying suitable conservation measures for this barely known endemic small mammal.