MITOCHONDRIAL GENE VARIATION IN MERCENARIA CLAM SIBLING SPECIES REVEALS A RELICT SECONDARY CONTACT ZONE IN THE WESTERN GULF-OF-MEXICO

We investigated phylogeographic relationships among American Mercenari a taxa by assessing variation in a 444 nucleotide fragment of the mito chondrial 16S ribosomal gene in clams sampled from four representative sites in January to November 1994. Three of these sites were in the G ulf of Mexico. one was on the Atlantic coast in South Carolina. Direct sequencing of this amplified gene fragment in 85 individuals revealed 21 haplotypes. Phylogenetic analyses consistently resolved this varia tion into three well supported clades, and within-clade genetic diverg ence levels were markedly lower than among-clade values. One of the cl ades, A, was taxon-specific, in that it solely and exclusively contain ed specimens of M. mercenaria (Linnaeus, 1758) sampled in South Caroli na. The other two clades, B and C, were the most divergent and both en compassed specimens of M. campechiensis (Gmelin, 1791) and of M. campe chiensis texana (Dall, 1902), sampled from the three Gulf of Mexico si tes. Clade B was found at high frequencies at all three Gulf sites, wh ereas Clade C occurred at low frequencies at two western Gulf sites. W e interpret this pattern as resulting from the secondary contact and i ntrogression of two allopatrically differentiated Mercenaria taxa in t he western Gulf of Mexico. Clade C haplotypes may represent relict mit ochondrial lineages from original Gulf Mercenaria spp. populations tha t predate massive mitochondrial introgression by M. campechiensis. We further propose that the M. campechiensis texana nuclear genome is a m osaic, heavily weighted toward M. campechiensis, but containing some r elict alleles inherited from the precontact population, especially tho se governing shell characteristics, which may be adaptive in cohesive sediments of bays and estuaries in the northwestern Gulf of Mexico.