Gene flow and genetic admixture across a secondary contact zone between two divergent lineages of the Eurasian Green Woodpecker Picus viridis

Secondary contact zones are natural systems which can be efficiently used to measure genetic differentiation and gene flow and thus provide a good opportunity to assess the level of reproductive isolation between divergent evolutionary lineages. In this study, we used ten Z-linked and nine autosomal loci from seven chromosomes and twenty males to evaluate gene flow across a secondary contact zone between two mitochondrial lineages of the Eurasian Green Woodpecker (Picus viridis), that diverged around 1 million years ago. One lineage (Picus viridis sharpei) is distributed throughout the Iberian Peninsula whereas the other one (Picus viridis viridis) is widespread across the Western Palearctic. These two lineages form a secondary contact zone in southern France. Formerly treated as two subspecies of Picus viridis, several authors have recently proposed assigning a specific rank to P. viridis sharpei and P. viridis viridis. Our results indicate no introgression of nuclear loci in allopatric populations located on both sides of the contact zone, which thus acts as an efficient barrier to gene flow. All males sampled within the contact zone and one male sampled near its eastern border were slightly admixed revealing that reproductive isolation between P. viridis sharpei and P viridis viridis has not been completely achieved. In accordance with the geographical range of each lineage, the two admixed males sampled near the western border of the contact zone harboured a large proportion of P. viridis sharpei alleles whereas admixed males sampled eastwardly near the Rhone Valley had a high proportion of P. viridis viridis alleles. Overall our results further support considering P. viridis sharpei and P. viridis viridis as two biological species.