Genetic structure and reproductive isolation among two sympatric cryptic sole species, Solea solea and S. aegyptiaca, in Mediterranean demes

In the Mediterranean, the common (Solea solea) and Egyptian (S. aegyptiaca) soles are two of the most valuable flatfish fishery resources. In the past, S. aegyptiaca was erroneously synonymised with Solea solea because of the great similarity in external morphology. Recently molecular markers have enabled their reliable identification, showing that the two species seem to co-occur in several areas of the Mediterranean, forming sympatric demes. The sympatric distribution and the close phylogenetic relationship allow a scenario for potential ecological and evolutionary interactions. Here, we aimed at providing new insights on the taxonomy, phylogeography and evolutionary interactions of the Mediterranean soles by i) developing PCR tests for the rapid screening of the two cryptic species, ii) analysing the species composition at several geographical locations, iii) assessing the phylogeographic pattern and genetic structure of populations and iv) the possible occurrence of interspecific hybridization in sympatric populations. In total 435 sole individuals were sequenced for a mitochondrial fragment (cytB gene) and 217 genotyped at two nuclear marker classes (ITS rDNA gene and 8 microsatellite loci). The analysis of species composition revealed a wider distribution of S. aegyptiaca in the Mediterranean than previously described. The observed frequent sympatry with S. solea could have lead to their frequent mis-identification. All nuclear markers however consistently revealed a clear genetic separation of the two species. Both mitochondrial and nuclear data showed the structuring of populations according to a longitudinal gradient, between the Western and Eastern basin, with different degree of differentiation in the two sole species. Higher genetic diversity was observed in S. solea for all markers. Further multidisciplinary data (combining data from morphology, reproductive biology and life history with genetic) are needed to unravel distribution and ecology of S. aegyptiaca in the Mediterranean and its ecological interactions with the cryptic species S. solea. This assessment will have important implications for sustainable management and conservation of fishery resources, because different species can differently respond to environmental pressures and changes.