Population connectivity and phylogeography of the Mediterranean endemic skate Raja polystigma and evidence of its hybridization with the parapatric sibling R. montagui

The genetic structure and population connectivity of the Mediterranean endemic speckled skate Raja polystigma were investigated in 10 population samples (N = 232) at seven exon-primed nuclear microsatellites and at three mitochondrial DNA sequence markers. The phylogeographical and population genetic analyses revealed that R. polystigma in Western and Central Mediterranean represents a near-panmictic population, with a subtle but significant mitochondrial divergence of the Adriatic deme. Nuclear genotypes revealed that 2.5% of the total individuals exhibited an admixed ancestry with the sibling species R. montagui (spotted ray). Individuals with admixed ancestry were detected along with purebred individuals in the Algerian, Southern Tyrrhenian, Sicilian and Adriatic R. polystigma population samples, but they were absent or rare in Sardinian and Northern Tyrrhenian ones. Since the two species co-occurs in the South-Western Mediterranean, we suggested that this area may act as a secondary hybrid zone.