Investigating the role of the Turkish straits system as a phylogeographical break in the mediterranean-black sea transition

Understanding the allopatric isolation and evolutionary processes in the marine realm can be difficult due to high dispersal potential of pelagic larvae. In addition, the role of barriers in shaping gene flow patterns between the populations of marine species can be less pronounced than their terrestrial counterparts. Straits are such potential barriers to gene flow in the marine environment, resulting in the isolation of populations on either side. The Turkish Straits System, comprising two straits (the Dardanelles and the Bosphorus Strait) and the Sea of Marmara forms the only connection between the Black Sea and the Mediterranean, and is a unique ecosystem with a well-defined two-layered stratification and current regime. The role of Turkish Straits System on gene flow, as a barrier and/or corridor has been proposed, but not extensively tested using genetics. Here, using three regions of mitochondrial DNA (CO1, COIII and 16S) and five microsatellite markers I tried to understand the effect of the system on gene flow in populations of the Mediterranean Mussel, Mytilus galloprovincialis (Lamarck, 1819), the common European prawn, Palaemon elegans Rathke, 1837 and the marbled crab, Pachygrapsus marmoratus (Fabricius, 1787). With this study, individuals belonging to three species were collected from 42 sampling sites, encompassing the Black Sea, the Turkish Straits System and the Mediterranean Sea. The results of the mtDNA analyses of Mytilus galloprovincialis showed that the Black Sea populations were isolated and differentiated from those in the Aegean during the last ice age and subsequently were able to colonize the Sea of Marmara and Aegean twice, with larval transport via the surface currents of the Turkish Straits System. However, individuals from the Aegean population were not able to migrate into the Turkish Straits System and the Black Sea in the reverse direction due to the lower-layer currents of the system. Microsatellite analyses did not support the mtDNA differentiation observed among the Black Sea and the Aegean mussel populations, suggesting that they did not correspond to different species. Two different haplogroups were detected in Palaemon elegans as a result of the mtDNA analyses, though with a lower degree of differentiation than previously recorded in the literature. The results of CO1 analysis for Pachygrapsus marmoratus also indicated a weak restriction of gene flow from the Mediterranean to the Black Sea. For all three species, the Turkish Straits System played a semi-permeable barrier role to gene flow and dispersal. This semi-permeable characteristic of the Turkish Straits System, simultaneously acting as a barrier and corridor to gene flow is relatively uncommon, and has been documented in the Turkish Straits System for the first time, using genetics methods.