Molecular developmental analyses of animal larval development: searching for deep homology

动物如何进化和适应环境是进化生物学的一个重要主题。许多复杂特征（分节、体腔、眼睛、中枢神经系统等）在动物之间的不均匀分布可以用两种方式来解释：要么是由于适应原因导致相似特征的反复趋同进化，要么是原始特征，这些特征虽然在某些群体中保留下来，但在多个谱系中也丢失了。这一建议试图区分这两种情况下的两个重要的特征与一个令人费解的系统发育分布在后生动物门：通肠和纤毛幼虫。扁形蠕虫是一类重要的动物。有些是自由生活的，但另一些是重要的寄生虫，如绦虫和血吸虫（引起bilharziia的原因，影响全世界2亿多人）。它们的身体相对简单，没有贯通的肠道（即没有分开的嘴和肛门），也没有内部充满液体的腔，也没有许多其他动物群体（如环节动物（如蚯蚓）或软体动物）所见的分段身体结构。多年来，人们一直认为，它们的相对简单表明它们很早就从动物树的主要分支中分离出来了：数亿年前，在这些更高级的特征进化出来之前。最近已经证实，这种关于它们与其他动物群的关系的观点是不正确的，事实上，它们与一个被称为栉虫纲的大型动物群中的环节动物和软体动物有着相对密切的关系。这种关系意味着它们最初拥有在其亲属中看到的肠道、体腔和部分，但后来失去了它们。我们想通过在它们的幼虫中寻找这些特征的遗传痕迹来直接测试这个想法。扁虫的幼虫在某种程度上类似于环节动物和软体动物的幼虫（一种微小的有纤毛的游泳阶段，称为栉水母）。我们想测试，再次使用遗传标记，是否环节动物，软体动物和扁虫的幼虫实际上都继承自一个共同的祖先（Lophotrochozoa的祖先），也有一个幼虫。如果是这样，那么扁形虫的幼虫将与环节动物和软体动物的食虫幼虫共享特定的遗传特征，而如果幼虫是在环节动物和扁形虫中独立进化的，情况就不是这样了。这将告诉我们在生命周期中增加一个幼虫阶段的可能性——这是一个非常有争议的想法——同时也告诉我们存在于5亿年前的浮游动物共同祖先的特征。最后，我们想利用我们在项目中已经描述过的部分中产生的遗传数据来完善我们对扁虫和所有其他浮游动物群体之间进化关系的理解。对这些关系的理解将使我们能够更好地解释我们感兴趣的特征的演变。

How animals evolve and adapt to their environment is a major theme of evolutionary biology. The patchy distribution of many complex characters (segmentation, coelomic cavities, eyes, central nervous system, etc.) across the animals could be interpreted in two ways: either as resulting from repeated convergent evolution of similar characters for adaptive reasons or as primitive characters that, while maintained in certain groups, have also been lost in multiple lineages. This proposal seeks to differentiate between these two scenarios for two important characters with a puzzling phylogenetic distribution amongst metazoan phyla: a through-gut and ciliated larvae. The platyhelminthes or flatworms are an important group of animals. Some are free living but others are important parasites such as tapeworms and schistosomes (the cause of Bilharzia which affects over 200 million people worldwide). They have a relatively simple body without a through gut (i.e. no separate mouth and anus) nor do they have the internal fluid filled cavities or segmented body plans seen in many other animal groups such as annelid worms (e.g. earth worms) or molluscs. For many years there was an assumption that their relative simplicity was an indication that they separated early from the main branches of the animal tree: many hundreds of millions of years ago and before the evolution of these more advanced traits. Recently it has been established that this view of their relationships to other animal groups is incorrect and that they are in fact relatively closely related to the annelids and molluscs in a large assemblage of animals called the Lophotrochozoa. This relationship implies that they originally possessed the through gut, body cavities and segments seen in their relatives but have lost them secondarily. We want to test this idea directly by looking for genetic traces of these characteristics in their larvae. The larvae of flatworms resemble, to an extent, the larvae of annelids and molluscs (a tiny ciliated swimming stage called a trochophore). We want to test, using genetic markers again, whether annelid, mollusc and flatworm larvae are in fact all inherited from a common ancestor (the progenitor of the Lophotrochozoa) that also had a larva. If so flatworm larvae would be expected to share specific genetic characteristics with the trochophore larvae of annelids and molluscs whereas if the larvae have been evolved independently in annelids and flatworms this would not be the case. This will tell us about the possibility of adding a larval stage into your lifecycle - a very controversial idea - and also inform us about the characteristics of the common ancestor of the lophotrochozoan animals which existed half a billion years ago. Finally we want to use the genetic data we generate in the parts of the project already described to refine our understanding of the evolutionary relationships between flatworms and all the other lophotrochozoan groups. An understanding of these relationships will better enable us to interpret the evolution of the characteristics that interest us.