Amphi-Atlantic Phylogeography of the Clam Genus Lasaea: A Test of Transoceanic Dispersal Hypotheses for a Direct-Developing Clonal Taxon

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Amphi-Atlantic Phylogeography of the Calm genus Lasaea: a Test of Transoceanic Dispersal Hypotheses for a Direct-Developing Clonal Taxon A prevailing paradigm in marine biology is that an extended pelagic larval phase is a prerequisite to long-distance dispersal of sedentary benthic invertebrate taxa. Three hypotheses have been proposed to explain how taxa lacking extended pelagic larval development have effected transoceanic range extensions. Hypothesis A attributes a key role to ancestral dispersal via planktotrophic larvae followed by evolutionary loss of this developmental mode, and long distance dispersal capabilities, in descendent populations. Hypothesis B states that transoceanic geographic ranges are very recent in origin and stem from undocumented human-mediated transport within historical time frames. Contemporary anthropogenic transoceanic exchange on a massive scale is occurring among global shallow-water faunas, primarily via the ballast water of ocean going vessels. Hypothesis C assumes that spontaneous rafting events, which involve the passive transport of sedentary or sessile life-history stages on drifting objects, have acted as the primary dispersal mechanism for species having life-history traits that enhance both rafting ability and colonization ability. These traits are characteristic of many clonal taxa with extended geographic ranges, including sponges, ascidians, scleractinian corals, cheilostome bryozoans and hydroids. However, there has been no test of these competing hypotheses. The proposed research will investigate phylogenetic relationships among direct-developing populations of the primarily asexual, polyploid clam genus Lasaea along a transoceanic dispersive pathway: the North Atlantic Gulf Stream System. The biogeography of Lasaea is remarkable in that it is the exact opposite of predicted patterns based on the assumed linkage of extended pelagic larval development and enhanced geographic range. Species with planktotrophic larvae are restricted to the continental margins of the western Pacific (Australia and Japan) whereas genetically divergent direct developers have collectively attained a truly global distribution, including large numbers of oceanic islands. Asexual, direct-developing Lasaea fit the putative profile of an effective rafting colonizer and the proposed work will test the three competing dispersal hypotheses for North Atlantic continental margin and oceanic island populations which are exclusively composed of direct developers. A distinctive phylogenetic tree topology has been predicted for each hypothesis that seeks to explain how direct developing Lasaea have successfully colonized Bermuda and The Azores from putative continental source populations. These predicted phylogenetic distinctions stem from evolutionarily significant differences in the temporal parameters of the competing hypotheses, together with a dispersal polarity imposed by North Atlantic surface circulation patterns. Genetic variation among the study populations will be assessed using mitochondrial gene sequences, in addition to allozyme analyses and ploidy determination by cell flow cytometry. Results will reveal evolutionary relationships and gene flow patterns among the North Atlantic study populations and the derived phylogenetic tree topologies will be tested for congruence with those predicted by each of the competing dispersal hypotheses. Preliminary results demonstrate the technical and scientific feasibility of the study system and provide provisional support for both Hypotheses B and C.