Population structure of Brachidontes pharaonis (P. Fisher, 1870) (Bivalvia, Mytilidae) in the Mediterranean Sea, and evolution of a novel mtDNA polymorphism

Abstract Brachidontes pharaonis (Fisher P, 1870) isan Indo-Pacific mussel that has colonized the MediterraneanSea via the Suez Canal. Mussels may havemigrated by natural dispersal of pelagic larvae, or theymay have been transported on the hulls of ships, or inballast water, or by some combination of these. MitochondrialCOI sequences (618 bp) from 101 musselsfrom six localities in the central and eastern MediterraneanSea and from one site in the Red Sea were usedto describe population structure. Analysis of molecularvariance indicated that frequencies differed amongpopulations, and that 92% of the variation residedwithin populations. The majority of haplotypes wereprivate alleles. No simple pattern of longitudinal variationwas detected for haplotype frequencies, haplotypediversity, or nucleotide diversity. A variety oftree-building algorithms (NJ, MP, ME) gave consistentresults, showing two sister groups. One clade had aleucine (L-form) and the other one a methionine(M-form) at the 30th amino acid. These sympatricforms were detected within all localities and they hadsimilar frequencies in males and females. The M- andL-form were separated by a 7.3% sequence divergence,indicating that this is an ancient polymorphism. Closely related species of mussels had exclusively the L-form,so we infer that the M-form evolved from the L-formlineage of B. pharaonis, probably in the northernportion of the Red Sea. Neutrality tests detected anexcess of rare mutations, a pattern of variation expectedin expanding populations. Patterns of variationdetected by Tajima’s D suggest that some form ofbalancing selection has maintained this polymorphism.