Contamination Acts as a Genotype-Dependent Barrier to Gene Flow, Causing Genetic Erosion and Fine-Grained Population Subdivision in Mussels from the Strait of Istanbul

Abstract

Abstract This study provides evidence of fine-grained genetic structuring in Mediterranean mussels (Mytilus galloprovencialis) from the Strait of Istanbul, caused by barriers to gene flow via contaminant-mediated selection. In this study, mitochondrial D-loop sequences were analyzed in mussels from 8 localities, all less than 30 kilometers apart, with differing contaminant loads. The results were: 1) Intra-population genetic differentiation (ΦST) between sites with high and low contaminant loads was high (up to 0.459), even at distances of only a few kilometers. 2) Genetic diversity was negatively correlated with the contaminant load (“genetic erosion”). 3) There was evidence of selection, based on haplotype frequencies and neutrality tests (Tajima’s D), with purifying selection at the most contaminated site and balancing selection at the least contaminated. 4) Genetic distance was not correlated with geographic distance (no isolation-by-distance), but was correlated with contaminant load at each site. 5) Population dendrograms and Bayesian estimators of migration indicated that gene flow between sites was affected by contamination. For the dendrograms of the sampling sites, the clades clustered according to contaminant load more than geographic distance. Overall, the conclusions are that 1) contamination serves as a genotype-dependent dispersal barrier (i.e., selection-mediated gene flow) for pelagic mussel larvae, leading strong population differentiation over short distances, and 2) this selection-mediated gene flow also lead to genetic erosion within contaminated sites. These effects may be more pronounced in the Strait of Istanbul than in other locations because of the riverine nature and strong, uni-directional current of the strait.