Choanoflagellates: Perspective on the Origin of Animal Multicellularity

For more than a century and half choanoflagellates, the closest living relatives of animals, have fascinated evolutionary biologists. By characterizing the similarities and differences between choanoflagellates and animals, biologists have gained perspective on the biology of their last common ancestor, the “Urchoanimal”, as well as the evolutionary foundations of multicellularity and the origin of animals. The best-studied colonial choanoflagellate, Salpingoeca rosetta, forms colonies by cell division and not by cell aggregation. The observation that cytoplasmic bridges connect cells in S. rosetta colonies and other colonial choanoflagellates, as well as cells in sponges, suggests that this mechanism of colony formation may be ancestral within the choanoflagellate lineage and may have been present in the Urchoanimal as well. The comparison of choanoflagellate gene content and gene function with animal gene content and gene function has revealed that many of the basic mechanisms of cell adhesion, signaling, and differentiation that were previously thought to be unique to animals are also present in choanoflagellates, indicating that these genes were present prior to the evolution of animals. These insights refine our understanding of genes that emerged on the stem lineage leading to the last common ancestor of all animals, the “Urmetazoan”. Taken together the data from choanoflagellates have provided deep insights into the biology of the Urchoanimal and the evolution of animal multicellularity.