Mitochondrial DNA variation in populations of the mussel Mytilus trossulus from the Southern Baltic

The mussel Mytilus trossulus is an important component of the Baltic brackish water ecosystem. The genetic structure of mussel (M. trossulus) populations was studied in sites along the Polish coast, Southern Baltic for two segments of mitochondrial DNA (mtDNA). The mode of inheritance of Mytilus mtDNA is termed doubly uniparental; two genomes are passed independently down the female (the F genome) and male (the M genome) lines of descent. The M genome has not been detected at high frequency in M. trossulus, thus the present study focuses on the F genome. PCR and RFLP analysis was used to characterise haplotypes in the coding region ND2-COIII; PCR was used to detect length variants in a major noncoding region. Significant differentiation between populations was observed in the frequency of 24 coding region haplotypes and 14 different length variants. For the three most frequent coding region haplotypes, two (I and III) are associated with the length variants, whereas the third (II) is monomorphic for a single variant of short length. It is suggested that variant II is derived by introgression from a related species, M. edulis, and may be resistant to expansion in the noncoding region. In both regions studied, the Ewens–Watterson test reveals significant deviations from neutrality with an excess of rare variants. This might be due to selection against slightly deleterious variants and is consistent with previously published results for Mytilus taxa. The present study also points towards the potential utility of mtDNA length variation in studies of population differentiation of Mytilus.