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

平静属Lasaea的两栖大西洋系统地理学：对直接发育的克隆类群的跨洋扩散假说的检验海洋生物学中流行的一种范式是，延长的远洋幼虫阶段是栖息性底栖无脊椎动物类群远距离扩散的先决条件。已经提出了三个假设来解释缺乏扩展的中上层幼体发育的类群如何影响跨大洋范围的扩展。假说A将关键作用归因于祖先通过浮游性营养幼虫的传播，随后在后代种群中这种发育模式和远距离传播能力的进化丧失。假设B指出，跨洋地理范围的起源是非常近的，起源于历史时间框架内未经记录的人类中介运输。当代大规模的人类跨洋交换正在全球浅水动物群之间发生，主要是通过远洋船只的压舱水。假设C认为，自发漂流事件，包括漂流物体上静止或静止的生活史阶段的被动运输，是具有生活史特征的物种的主要扩散机制，这些特征既增强了漂流能力，也增强了定居能力。这些特征是许多具有广泛地理范围的克隆分类群的特征，包括海绵、海鞘动物、硬指目珊瑚、唇口苔藓动物和水生动物。然而，还没有对这些相互竞争的假设进行测试。这项拟议的研究将调查主要是无性、多倍体的文蛤属直接发育种群之间的系统发育关系，这些种群沿着一条跨越大洋的弥散路径：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远洋幼虫发育和扩大的地理范围之间的假设联系的预测模式完全相反。具有浮游营养型幼虫的物种仅限于西太平洋大陆边缘(澳大利亚和日本)，而遗传上不同的直接开发商共同获得了真正的全球分布，包括大量的海洋岛屿。无性、直接发育的Lasaea符合有效漂流殖民者的假设轮廓，拟议的工作将检验北大西洋大陆边缘和海洋岛屿种群的三个相互竞争的扩散假说，这些种群完全由直接开发者组成。对于每个假说，都预测了一个独特的系统发育树状拓扑结构，试图解释直接发育的Lasaea如何从假定的大陆来源种群成功地殖民百慕大和亚速尔群岛。这些预测的系统发育差异源于相互竞争的假说的时间参数在进化上的显著差异，以及北大西洋表面环流模式施加的扩散极性。除了等位酶分析和细胞流式细胞术的倍体测定外，还将使用线粒体基因序列来评估研究群体之间的遗传变异。结果将揭示北大西洋研究种群之间的进化关系和基因流动模式，并将测试得出的系统发育树拓扑结构是否与每个相互竞争的扩散假说预测的一致。初步结果证明了该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.