ABSTRACT

The underlying causality of the Cambrian explosion still remains one of the great paleontological mysteries. The sudden explosion of life ~530 Ma ago from an almost static Precambrian world suggests that the tempo and/or the mode of evolution changed at the end of the Precambrian, possibly driven by some momentous event. There is, in fact, a dramatic change that occurred during the Ediacaran that could potentially account for much of what is called the Cambrian explosion: the evolution of mobile multicellular heterotrophs, i.e., eumetazoans. Here, the PI proposes to test this hypothesis by exploring the evolution of the gut. The first hypothesis to be tested is the idea that the eumetazoans, and hence the eumetazoan gut, evolved from a sponge-like ancestor. Previous NSF-funded research suggests that sponges are paraphyletic such that homoscleromorphs and calcisponges are more closely related to eumetazoans than they are to demosponges. If Porifera is indeed paraphyletic, then the last common ancestor of metazoans and the last common ancestor of calcisponges + eumetazoans was constructed like a modern sponge. The paleoecological importance of this observation is that because the water-canal system is designed to feed primarily on dissolved organic matter, a benthos dominated by sponges would have had no ecological impact on the world's eukaryotic realm. However, with the invention of the gut, eukaryotes could now feed on other eukaryotes, setting up co-evolutionary arms races and establishing complex ecological food webs. Monophyly vs. paraphyly of Porifera will be assessed by phylogenetic analysis of housekeeping genes from new sponge taxa. Poriferan paraphyly would root eumetazoan origins and the origin of the eumetazoan gut within the context of sponge evolution. PI also proposes to isolate gut-specific genes in the sea urchin S. purpuratus, and test for phylogenetic conservation by examining the expression profiles of their orthologues in other eumetazoans. Conserved gut-specific genes are then candidates for genes necessary for the evolution of the gut. Once identified, orthologues can then be searched for in the genome of the marine demosponge Amphimedon. Once it is known what genes were co-opted into gut development, vs. what genes were newly invented, gut evolution can be then tackled by a series of hypothesis-driven questions aimed at understanding how the gut gene-regulatory-network evolved. These studies will not only give a better understanding of the causality of the Cambrian explosion, but they will serve as the training ground for several undergraduate student's intellectual foray into one of the most interesting events in the history of life.

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