No abstract

Many marine species, including mussels in the Mytilus edulis species group (i.e. M. edulis L., M. galloprovincialis Lamarck, and M. trossulus Gould), have an antitropical distribution pattern, with closely related taxa occurring in high latitudes of the northern and southern hemispheres but being absent from the tropics. We tested four hypotheses to explain the timing and route of transequatorial migration by species with antitropical distributions. These hypotheses yield dierent predictions for the phylogenetic relationship of southern hemisphere taxa relative to their northern counter-parts. The three Mytilus species were used to test these hypotheses since they exhibit a typical antitropical distribution and representative taxa occur in both the Paci®c and Atlantic. Two types of mtDNA lineages were found among populations of mussels collected from the southern hemisphere between 1988 and 1996; over 90% of the mtDNA lineages formed a distinct subclade which, on average, had 1.4% divergence from haplotypes found exclusively in northern Atlantic populations of M. galloprovincialis. These data indicate that southern hemisphere mussels arose from a migration event from the northern hemisphere during the Pleistocene via an Atlantic route. The remainder of the southern hemisphere lineages (<10%) were very closely related to mtDNA haplotypes found in both M. edulis and M. galloprovincialis in the northern hemisphere, suggesting a second, more recent migration to the southern hemisphere. There was no evidence that southern hemisphere mussels arose from Paci®c populations of mussels.

The product of the Lap locus in the marine bivalve Mytilus edulis is a neutral, membrane-associated aminopeptidase that is primarily localized on intestinal microvilli and in digestive cell lysosomes. Natural populations are genetically differentiated at the Lap locus between areas of differing salinity. A steep (0.55-0.15) allele frequency dine connects differentiated populations between the Atlantic Ocean and Long Island Sound. We demonstrate an annual gene flow/ mortality cycle in dine populations whereby gene frequencies after mortality are correlated with salinity and enzyme activity. The dine is spatially and temporally unstable in immigrants, but stable in residents after mortality. Mortality is nonrandom with regard to the Lap locus; genotype-dependent properties of the aminopeptidase enzyme apparently lead to a differential rate of the utilization of nutrient reserves because selected genotypes exhibited an increased rate of tissue weight loss. Aminopeptidase genotypes are differentially adapted to different temperatures and salinities, which provides a mechanism for the relationship among biochemical, physiological, and population phenotypes.

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