A Comparative Genomics Perspective on the Origin of Multicellularity and Early Animal Evolution

Abstract

The genetic basis of the origin of animal multicellularity and the subsequent diversification of complex animal forms has been a long-standing question in biology. In the past decade, the genomes of species representing early-branching animal lineages and close unicellular relatives of animals were sequenced, providing an unprecedented wealth of data from these understudied phyla. This chapter focuses on comparative genomic analyses of four animal lineages, cnidarians, ctenophores, placozoans, and sponges, and of two unicellular lineages, choanoflagellates and filastereans. These studies revealed striking conservation of gene structure and genomic organization among animals, and uncovered deep evolutionary origins of the genetic circuits underlying biological processes essential for animal biology, including cell cycle control, cell growth, programmed cell death, and specialized cell types. Genomic analyses therefore allow us to infer that all extant animals have descended from an ancestor with a complex genome that encoded a vast majority of the gene content responsible for biological processes in vertebrates. Strikingly, genomes of animals that lack specialized cell types such as muscles and neurons encode the molecular machinery required for the function of these cell types. Thus, the genomic events by which the ancestral animal genome gave rise to gross differences in animal morphology remain unknown. The genome sequences described here will enable future functional genetic studies of anciently-evolved genes in early-branching animal lineages and their unicellular relatives to decipher the evolution of animal body plans.