Abstract

Contractile cell types are universally present in animals and fundamental to animal life. Contractions of individual, scattered cells underlies amoeboid cell migration, which is prevalent in both adult organisms and embryos, while tissue-scale collective cell contractility underlies both embryonic morphogenesis and adult motricity. However, the origin of animal contractile cell types remains obscure. As the sister group of animals, choanoflagellates hold the promise of illuminating the evolutionary origins of animal cell biology. Intriguingly, choanoflagellate genomes encode an extensive complement of homologs to animal contractility genes, suggesting the involvement of (yet unidentified) contractile processes in their life history. Here, I report on the recent discovery of both individual and collective cell contractility in choanoflagellates. Under confinement, the model choanoflagellate Salpingoeca rosetta rapidly undergoes a phenotypic switch from a flagellate to an amoeboid cell phenotype that resemble animal migratory cells in both structure and function. This represents an unexpected expansion of the known phenotypic repertoire of choanoflagellates and suggests an ancient origin for animal crawling cells, in line with the temporal-to-spatial transition hypothesis for the origin of animal cell types. Finally, collective cell contractility has also been recently discovered in a newly discovered colonial choanoflagellate isolated from a Carribean island, that undergoes rapid and reversible whole-colony inversion in response to external photic and mechanical stimuli.