Origin of the antitropical distribution pattern in marine mussels (Mytilusspp.): routes and timing of transequatorial migration

Many marine species, including mussels in the Mytilus edulis species group (i.e. M. edulis L., M. galloprovincialis Lamarck, and M. trossulus could), have an antitropical distribution pattern, with closely related taxa occurr ing in high latitudes of the northern and southern hemispheres but being ab sent from the tropics. We tested four hypotheses to explain the timing and route of transequatorial migration by species with antitropical distributio ns. These hypotheses yield different predictions for the phylogenetic relat ionship of southern hemisphere taxa relative to their northern counter-part s. The three Mytilus species were used to test these hypotheses since they exhibit a typical antitropical distribution and representative taxa occur i n both the Pacific and Atlantic. Two types of mtDNA lineages were found amo ng populations of mussels collected from the southern hemisphere between 19 88 and 1996; over 90% of the mtDNA lineages formed a distinct subclade whic h, on average, had 1.4% divergence from haplotypes found exclusively in nor thern Atlantic populations of M. galloprovincialis. These data indicate tha t southern hemisphere mussels arose from a migration event from the norther n 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 norther n hemisphere, suggesting a second, more recent migration to the southern he misphere. There was no evidence that southern hemisphere mussels arose from Pacific populations of mussels.