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

太平洋-大西洋系统地理学的平静属Lasaea：跨洋扩散假说的测试直接发展的克隆分类在海洋生物学的一个流行的范例是，一个扩展的远洋幼虫阶段是一个先决条件，以长距离的定居底栖无脊椎动物类群的扩散。 三种假说已被提出来解释如何类群缺乏扩大远洋幼虫的发展，影响了跨洋范围的扩展。 假设一个属性的关键作用，祖先的传播，通过deletotrophic幼虫的进化丧失这种发展模式，和长距离的传播能力，在后代人口。 假设B指出，跨洋地理范围的起源是最近的，源于历史时期内没有记录的人为运输。 当代大规模的人为跨洋交换正在全球浅水动物群中发生，主要是通过远洋船只的压载水。 假设C假设，自发漂流事件，涉及被动运输的定居或固着生活史阶段的漂流物体，已作为主要的传播机制的物种的生活史特征，提高漂流能力和殖民能力。 这些特征是许多具有扩展地理范围的克隆分类群的特征，包括海绵、海鞘、石珊瑚、唇口苔藓虫和水螅。 然而，这些相互竞争的假设还没有得到检验。 拟议的研究将调查直接发展的人口之间的系统发育关系的主要无性，多倍体蛤属Lasaea沿着一个跨洋分散的途径：北大西洋湾流系统。 Lasaea的生物地理学是值得注意的，因为它与基于扩展的中上层幼虫发育和增强的地理范围的假设联系的预测模式完全相反。 具有营养型幼虫的物种仅限于西太平洋的大陆边缘（澳大利亚和日本），而遗传上不同的直接开发者共同实现了真正的全球分布，包括大量的海洋岛屿。 无性的，直接发展的Lasaea适合一个有效的漂流殖民者的假定配置文件和拟议的工作将测试三个竞争的扩散假设北大西洋大陆边缘和海洋岛屿人口完全由直接开发商。 一个独特的系统发育树拓扑结构已被预测为每个假设，试图解释如何直接发展Lasaea已成功地殖民百慕大和亚速尔群岛从假定的大陆源人口。 这些预测的系统发育差异源于进化上的显着差异的时间参数的竞争假说，再加上北大西洋表面环流模式所施加的扩散极性。 除了通过细胞流式细胞术进行等位酶分析和倍性测定外，还将使用线粒体基因序列评估研究人群中的遗传变异。 结果将揭示北大西洋研究人群之间的进化关系和基因流模式，并将测试衍生的系统发育树拓扑结构与每个竞争扩散假设预测的一致性。 初步结果证明了研究系统的技术和科学可行性，并为假设B和C提供了临时支持。

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.