The Colonization of Land by Crustaceans

甲壳类动物在陆地上的殖民

• 批准号: 0111001
• 负责人: Jonathan Wright
• 金额: $ 13.33万
• 依托单位: Pomona College
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-07-01 至 2005-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0111001&HistoricalAwards=false
• 关键词: Colonization; Land; Crustaceans

Although several groups of animals have colonized land, the evolutionary changes that have made this possible are only superficially understood. This is due in large part to difficulties in resolving the evolutionary relationships among terrestrial taxa within a particular group, and their relationships to aquatic counterparts, problems that leave the status of both the aquatic ancestors and transitional or 'prototypal' terrestrial species unresolved. In the case of the three most successful terrestrial taxa, the tetrapod vertebrates, the insects, and the arachnids, it is clear that living descendents of these ancestral groups no longer survive. This greatly limits our understanding of the adaptive transitions that have made terrestrial life possible. Terrestrial isopods, commonly known as woodlice or sowbugs, provide possibly the best 'model system' for studying the evolution of physiological systems accompanying the aquatic-terrestrial transition. In contrast to the other major terrestrial animals, they are represented by living groups that occupy diverse habitats from marine aquatic to 'transitional' intertidal and fully terrestrial. The evolutionary relationships of living species have recently been elucidated in considerable detail from comparative anatomy and molecular phylogenetic techniques, and these indicate that the aquatic and intertidal species are more basal, and thereby more closely related to other marine isopods, than the more 'derived' terrestrial species. Furthermore, they indicate at least three independent origins of a terrestrial habit. Taken together, these factors provide an excellent basis for comparative physiological studies. By reconstructing an evolutionary lineage from marine to fully terrestrial, and comparing specific physiological traits across species taken from that lineage, it is possible to understand the physiological transitions involved. Building a picture of the physiological changes in this way allows us not only to study specific changes that have accompanied terrestriality, but also to compare adaptive solutions among the independent terrestrial radiations. Studies will focus on five main physiological systems. Water vapor absorption (WVA) is known from only one group of terrestrial isopods and allows animals to recover water in ambient humidities exceeding approximately 87%. The condensation of water vapor involves the secretion of concentrated salt solution by the gills to generate reduced vapor pressure. A key question in explaining the evolution of WVA is to unravel the ancestral functions of salt secretion in intertidal or terrestrial species. Like WVA, both nitrogenous excretion and ion regulation are closely associated with water balance. Terrestrial isopods excrete nitrogenous waste as ammonia gas, and this presents an intriguing solution to water conservation by potentially eliminating simultaneous water losses. Ammonia volatilization does, however, cause acidification of the gill surface and buffering of accumulated acid is essential for the process to work. Understanding the buffering mechanism, and its possible preadaptive functions, is thus essential in explaining how ammonia volatilization evolved. Most terrestrial animals possess efficient mechanisms for regulating blood solute concentration, protecting the tissues against osmotic hydration or dehydration and resultant changes in cell volume. Aquatic isopods are capable of regulating blood salt concentration in both elevated (hyper-osmotic) and depressed (hypo-osmotic) external salinities. Recent studies have shown that intertidal species counteract increasing blood salinities imposed by dehydration by excreting salts across the gills, as in aquatic species. Fully terrestrial species cannot excrete salts, however, since dietary salts are impoverished. One group has been shown to down-regulate blood salts by sequestration in the hindgut during dehydration. Although regulation has been demonstrated in other groups, the mechanisms await investigation. At least one group appears to have lost the ancestral capacity for osmoregulation. As well as osmotic concentration imposed by dehydration, isopods may face osmotic dilution during rain showers, a danger exacerbated by their permeable cuticles. The mechanisms by which excess water is eliminated are essentially unstudied, though recent work indicates a role of the maxillary glands in producing dilute urine. The remaining areas of study will focus on reproductive physiology, specifically maternal regulation of the embryonic environment and ion regulation of periembryonic fluid by the eggs. The eggs and newly hatched juveniles (mancas) are brooded in a fluid-filled marsupium. Marine isopods flush this with seawater, but terrestrial species either fill it from external water sources or from the blood. Desiccation and thermal stresses on land demand either maternal regulation of the marsupial environment or high physiological tolerance in early developmental stages. Recent studies have shown extreme tolerance of osmolality, pH, temperature and ammonia extremes in embryos of terrestrial isopods, and pH and ion regulation appears to employ a greatly enlarged embryonic 'dorsal organ'. Studies will investigate ion transport mechanisms in this organ. Comparisons within lineages will reveal whether the enlargement of the dorsal organ is a unique terrestrial innovation and will test the hypothesis that its ancestral function was calcium uptake for mineralization of the cuticle.

虽然有几种动物已经在陆地上定居，但使这成为可能的进化变化只是表面上的理解。 这在很大程度上是由于在解决一个特定的组内的陆生类群之间的进化关系，以及它们与水生对应物的关系，留下的水生祖先和过渡或“原型”陆生物种的地位悬而未决的问题困难。 就三个最成功的陆生分类群而言，四足脊椎动物、昆虫和蛛形纲动物，很明显，这些祖先群体的现存后代不再存活。 这极大地限制了我们对使陆地生命成为可能的适应性转变的理解。 陆地等足类动物，通常称为木虱或草虫，可能为研究伴随水生-陆地过渡的生理系统的进化提供了最好的“模型系统”。 与其他主要的陆生动物不同，它们以生活在不同栖息地的生物群为代表，从海洋水生到“过渡”潮间带和完全陆生。 最近，从比较解剖学和分子系统发育技术中，已经相当详细地阐明了现存物种的进化关系，这些表明水生和潮间带物种比更多的“衍生”陆地物种更基础，因此与其他海洋等足类动物更密切相关。 此外，它们表明至少有三个独立的起源的陆地习惯。 总之，这些因素为比较生理学研究提供了良好的基础。 通过重建从海洋到完全陆地的进化谱系，并比较从该谱系中提取的物种的特定生理特征，有可能理解所涉及的生理转变。 以这种方式构建生理变化的图像，不仅使我们能够研究伴随着地球性的特定变化，而且还可以比较独立的地球辐射之间的适应性解决方案。 研究将集中在五个主要的生理系统。 水蒸气吸收（WVA）是已知的只有一组陆生等足类动物，并允许动物恢复水在环境湿度超过约87%。 水蒸气的冷凝涉及鳃分泌浓缩的盐溶液以产生降低的蒸气压。 解释WVA进化的一个关键问题是解开潮间带或陆生物种盐分泌的祖先功能。 像WVA一样，氮的排泄和离子调节都与水分平衡密切相关。 陆生等足类动物以氨气的形式排出含氮废物，这通过潜在地消除同时的水分损失而提出了一种有趣的水保护解决方案。 然而，氨挥发确实会导致鳃表面酸化，缓冲积累的酸对于该过程起作用至关重要。 因此，了解缓冲机制及其可能的适应前功能对于解释氨挥发是如何演变的至关重要。 大多数陆生动物具有调节血液溶质浓度的有效机制，保护组织免受渗透性水合或脱水以及由此引起的细胞体积变化。 水生等足类动物能够在升高的（高渗）和降低的（低渗）外部盐度下调节血盐浓度。 最近的研究表明，潮间带物种通过在鳃上排泄盐来抵消因脱水而造成的血液盐度增加，水生物种也是如此。 然而，完全陆生的物种不能排泄盐，因为食物中的盐是贫乏的。 一组已被证明在脱水期间通过后肠的隔离来下调血液盐。 虽然在其他群体中已经证明了管制，但其机制有待调查。 至少有一个群体似乎已经失去了祖先的神经调节能力。 除了脱水造成的渗透浓度外，等足类动物在阵雨期间可能会面临渗透稀释，这种危险因其渗透性的角质层而加剧。 虽然最近的研究表明上颌腺在产生稀尿中起作用，但消除多余水分的机制基本上还没有研究。 其余的研究领域将集中在生殖生理学，特别是母体对胚胎环境的调节和卵子对胚周液的离子调节。 卵和刚孵化的幼体（曼卡）在充满液体的有袋动物中孵化。 海洋等足类动物用海水冲洗它，但陆地物种要么从外部水源或血液中填充它。 干燥和热应力对土地的需求，无论是有袋动物的环境或早期发育阶段的高生理耐受性的母亲调节。 最近的研究表明，极端的耐受性渗透压，pH值，温度和氨的极端胚胎的陆生等足类动物，和pH值和离子调节似乎采用了大大扩大的胚胎“背器官”。 研究将调查该器官中的离子转运机制。 谱系内的比较将揭示背器官的扩大是否是一个独特的陆地创新，并将测试假设，其祖先的功能是吸收钙矿化的角质层。