Systematics and Biogeography of Lampreys ("Agnatha":Petromyzontiformes) and the Evolution of Parasitic and Non-Parasitic Life Cycles

[89507] mayden和Roe根据化石记录，无颌脊椎动物（agnathans）多样性最大的时期出现在志留纪上第三纪和早、中泥盆世地质时期，大约在4.2亿至3.8亿年前。大多数agnathans在泥盆纪末期（3.6亿年前）灭绝，有一个或多个谱系幸存下来，产生了现代无颌脊椎动物。七鳃鳗代表了两个幸存的agnathans群体之一（盲鳗是另一个群体）。现存七鳃鳗属于岩纹异形目，现存有40种。七鳃鳗的地理分布是值得注意的，它们在北半球和南半球都有发现，但在热带地区几乎完全没有。据推测，七鳃鳗的这种分布是超大陆盘古大陆和后盘古大陆劳亚和冈瓦纳大陆分裂的结果。除了它们有趣的地理分布之外，七鳃鳗还因以下几个原因而引人注目：1)它们代表了无脊椎脊索动物和有颌的脊椎动物之间的过渡阶段，因此可以检验关于脊椎动物特有特征进化的假设；2)它们具有复杂的生活史，包括从静坐、滤食的幼虫到有性繁殖的成虫的蜕变；3)根据物种是寄生还是非寄生可分为两类。寄生七鳃鳗既包括溯河产卵的种类，也包括局限于溪流和河流的种类。具有非寄生生命周期的物种与具有寄生生命周期的物种相似，只是成虫不取食。从历史上看，寄生和非寄生物种之间的形态相似性被解释为现存的非寄生物种在不同的寄生物种中独立进化的证据。这些假设的兄弟物种被称为“配对物种”或“卫星物种”。尽管七鳃鳗研究人员对卫星物种假说的有效性达成了一致意见，但没有进行明确的系统发育分析来支持这种关系。Drs。阿拉巴马大学的理查德·梅登（Richard Mayden）和凯文·罗（Kevin Roe）正试图验证这些假设的关系，从而验证脊椎动物中最引人注目的多重趋同现象之一。该研究将利用形态和分子（DNA序列）数据集对所有七鳃鳗物种进行系统发育假设。具体目标是验证以下假设：(1)寄生和非寄生生命史的进化起源；(2)岩足类中不同物种、属和科之间的关系；(3)岩足类的生物地理历史与地球历史和其他具有相似分布的分类群之间的关系；(4)七鳃鳗与颚脊椎动物之间提出的关系。该项目是国际范围的，包括在几大洲（北美、南美、欧洲和澳大利亚）收集标本，并与三位公认的七鳃鳗生物学和系统学专家合作。澳大利亚珀斯默多克大学的I. Potter和H. Gill以及安大略省渥太华加拿大自然博物馆的C. Renaud博士，他们主要负责收集额外的标本（对形态特征和分子分析很重要）和形态数据集的开发。这些数据将与基于几个线粒体基因的完整DNA序列的分子数据集相结合。

0089507Mayden and Roe Based on the fossil record, the greatest diversity of jawless vertebrates (agnathans) occurred during the upper Silurian and Early and Middle Devonian geological periods, approximately 420 to 380 million years ago. The majority of agnathans became extinct by the end of the Devonian (360 million years ago) with one or more lineages surviving to give rise to modern jawless vertebrates. Lampreys represent one of the two surviving groups of agnathans (hagfishes are the other group). Living lampreys are classified in the order Petromyzontiformes, which includes 40 extant species. The geographic distribution of lampreys is notable in that they are found in both Northern and Southern hemispheres, but are almost entirely absent from tropical regions. It has been hypothesized that this distribution of lampreys is the result of the breakup of the supercontinent Pangea and the post-Pangean continents of Laurasia and Gondwana. In addition to their interesting geographic distribution lampreys are also remarkable for several reasons: 1) they represent a transitional stage between invertebrate chordates and vertebrates that possess jaws, and as such allow the testing of hypotheses concerning the evolution of characters considered to be unique to vertebrates; 2) they possess a complex life history that involves metamorphosis from a sedentary, filter feeding larval form termed an ammocete, to a sexually reproducing adult; and 3) they can be divided into two groups based on whether the species are parasitic or non-parasitic. Parasitic lampreys include both anadromous species and those restricted to streams and rivers. Species with non-parasitic life-cycles are similar to those with a parasitic life-cycle except that the adults do not feed. Historically, morphological similarities between parasitic and non-parasitic species have been interpreted as evidence that extant non-parasitic species evolved independently on several occasions from different parasitic species. These hypothesized sibling species have been termed "paired species" or "satellite species." Despite agreement among lamprey researchers as to the validity of the satellite species hypothesis, no explicit phylogenetic analysis has been conducted to support such relationships. Drs. Richard Mayden and Kevin Roe at the University of Alabama are seeking to test these hypothesized relationships, and thereby test one of the most remarkable cases of multiple convergence ever suspected in vertebrates. The study will produce phylogenetic hypotheses for all lamprey species using both morphological and molecular (DNA sequences) data sets. Specific objectives are to test hypotheses concerning (1) the evolutionary origins of the parasitic and non-parasitic life histories, (2) the relationships between the various species and genera and families within Petromyzontiformes, (3) the biogeographic history of Petromyzontiformes with reference to the history of the earth and other taxa having similar distributions, and (4) the proposed relationship between lampreys and jawed vertebrates. The project is international in scope, and includes the collection of specimens on several continents (North America, South America, Europe and Australia), and involves collaboration with three acknowledged experts in lamprey biology and systematics: Drs. I. Potter and H. Gill of Murdoch University, Perth, Australia and Dr. C. Renaud from the Canadian Museum of Nature, Ottawa, Ontario, who have primary responsibility for the collection of additional specimens (important for morphological characters and molecular analyses) and the development of morphological data sets. These data will be combined with the molecular data sets based upon the complete DNA sequences of several mitochondrial genes.