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主要主题。许多复杂特征的斑块分布(分段、体腔、眼睛、中枢神经系统等)动物之间的差异可以用两种方式解释：一种是相似角色出于适应原因反复趋同进化的结果，另一种是原始角色，虽然保留在特定的群体中，但也在多个谱系中消失了。这项建议试图区分这两种情况，两个重要的特征在后生动物门中具有令人费解的系统发育分布：穿透肠道和有纤毛的幼虫。扁虫或扁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(影响全世界2亿多人的黑热病的起因)。它们的身体相对简单，没有直肠(即没有分开的嘴和肛门)，也没有许多其他动物群体中看到的内部充满液体的腔或分段的身体平面，如环节动物蠕虫(例如蚯蚓)或软体动物。多年来，有一种假设认为，它们的相对简单表明它们很早就从动物树的主要分枝中分离出来：数亿年前，在这些更高级的特征进化之前。最近已经证实，这种关于它们与其他动物群体的关系的观点是不正确的，而且它们实际上与被称为Lophotrochozoa的大型动物群体中的环节动物和软体动物关系相对较近。这种关系意味着他们最初拥有亲人身上的直肠、体腔和节段，但后来失去了。我们希望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一想法。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(一个微小的有纤毛的游泳期，称为轮虫)。我们想再次使用遗传标记来测试环节动物、软体动物和扁虫的幼虫是否实际上都是从同样有幼虫的共同祖先(线虫的祖先)那里遗传过来的。如果是这样的话，扁虫幼虫有望与环节动物和软体动物的担轮动物幼虫共享特定的遗传特征，而如果幼虫是在环节动物和扁虫中独立进化的，情况就不是这样了。这将告诉我们在你的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--也告诉我们5亿年前存在的食肉动物的共同祖先的特征。最后，我们想要使用我们在已经描述的项目部分中产生的遗传数据来完善我们对扁虫和所有其他线虫种群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.