Population genetic structure of Mugil cephalus in the Mediterranean and Black Seas: a single mitochondrial clade and many nuclear barriers

Abstract

The population structure and evolutionary history of Mugil cephalus were investigated across 18 sampling sites in the NE Atlantic Ocean, Mediterranean and Black Seas, us- ing 2 classes of genetic markers: sequence polymorphism of an 857 bp fragment of mitochondrial (mtDNA) cytochrome b, and allele size variation at 7 nuclear loci. The level of nucleotide diversity recovered with the mtDNA marker was very low (~0.6% divergence), indicating the presence of a single clade over the entire area. Mismatch distribution, Bayesian skyline plots and associated sta- tistics revealed a recent demographic crash followed by population expansion, but nuclear data in- dicated population constancy in the area covered in this study. While a single clade was detected, significant mtDNA genetic differentiation was, however, observed between the samples from the Black Sea and the samples from other (sub-) basins (ΦST = 0.17; p = 0.029). The nuclear loci also re- vealed significant genetic differentiation and isolation-by-distance in M. cepha lus. Patterns of ge- netic structure were, however, significantly more pronounced with nuclear than with mtDNA markers; the former indicated the presence of 3 (Bayesian clustering) to 6 (Monmonnier's method) populations. The highest levels of genetic heterogeneity at nuclear markers occurred at the well- known Almeria-Oran Front, but also in the Bosporus Strait. Thus, both sets of markers revealed the importance of this strait as a barrier to gene flow, probably during the Pleistocene. The results also revealed genetic heterogeneity in the eastern Mediterranean basin, and suggested that the popula- tion expanded from this sub-basin towards the Atlantic Ocean and Black Sea.