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

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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.