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

Brachidontes pharaonis (Fisher P, 1870) is an Indo-Pacific mussel that has colonized the Mediterranean Sea via the Suez Canal. Mussels may have migrated by natural dispersal of pelagic larvae, or they may have been transported on the hulls of ships, or in ballast water, or by some combination of these. Mitochondrial COI sequences (618 bp) from 101 mussels from six localities in the central and eastern Mediterranean Sea and from one site in the Red Sea were used to describe population structure. Analysis of molecular variance indicated that frequencies differed among populations, and that 92% of the variation resided within populations. The majority of haplotypes were private alleles. No simple pattern of longitudinal variation was detected for haplotype frequencies, haplotype diversity, or nucleotide diversity. A variety of tree-building algorithms (NJ, MP, ME) gave consistent results, showing two sister groups. One clade had a leucine (L-form) and the other one a methionine (M-form) at the 30th amino acid. These sympatric forms were detected within all localities and they had similar frequencies in males and females. The M- and L-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-form lineage of B. pharaonis, probably in the northern portion of the Red Sea. Neutrality tests detected an excess of rare mutations, a pattern of variation expected in expanding populations. Patterns of variation detected by Tajima’s D suggest that some form of balancing selection has maintained this polymorphism.