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Curr Biol. Author manuscript; available in PMC 2011 Oct 26.
Published in final edited form as:
Curr Biol. 2010 Oct 26; 20(20): R875–R876.
doi:  10.1016/j.cub.2010.09.014
PMCID: PMC2978077

Multicellular Development in a Choanoflagellate


Little is known about how the first animals evolved from their single celled ancestors. Over 120 years ago, Haeckel proposed that animals evolved through "repeated self-division of [a] primary cell,"[1] an idea supported by the observation that all animals develop from a single cell (the zygote) through successive rounds of cell division [2]. Nonetheless, there are multiple alternative hypotheses [3], including the formal possibility that multicellularity in the progenitor of animals occurred through cell aggregation, with embryogenesis by cell division being secondarily derived. The closest known relatives of animals, choanoflagellates, are emerging as a model system for testing specific hypotheses about animal origins [47]. Studying colony formation in choanoflagellates may provide a context for reconstructing the evolution of animal multicellularity. We find that the transition from single cells to multicelled colonies in the choanoflagellate Salpingoeca rosetta occurs by cell division, with sister cells remaining stably attached.

While the life cycles of all choanoflagellates feature a prominent single-celled phase, many species are also capable of forming colonies of morphologically similar cells [810]. Phylogenetics and the reconstruction of ancestral character states within the choanoflagellate group indicate that colony formation either evolved before the diversification of two of the three major choanoflagellate clades, or that it evolved multiple times independently [6]. It is also possible that the last common ancestor of animals and choanoflagellates was capable of forming multicelled colonies [6]. Thus, studies of the colony-forming choanoflagellate S. rosetta offer a unique opportunity to test hypotheses about the cell biology of colony formation and its potential relevance to the evolution of animal multicellularity.

S. rosetta can exist as either single cells or rosette-shaped colonies that contain between 4 and ~50 cells arranged in closely packed spheres (Fig. 1A). To determine how colonies form, cultures of solitary S. rosetta cells were induced to form colonies by co-cultivation with the prey bacterium Algoriphagus sp. and monitored for at least 12 hours by time-lapse microscopy (see Supplemental Information). S. rosetta colonies were consistently observed to form through cell division and never by aggregation (Fig. 1A, Movie S1). Cell division during colony formation was asynchronous, suggesting that the cell cycle is not coordinated between sister cells in colonies (Fig. 1B).

Figure 1
Salpingoeca rosetta colonies develop through cell division, not aggregation

Although direct observation demonstrated the centrality of cell division in colony formation and provided no evidence for cell aggregation, it is formally possible that S. rosetta colonies might form by aggregation at low frequency or under conditions that do not favor cell proliferation. In this case, colony formation through aggregation might be observed in cultures in which cell division is blocked. Therefore, we tested whether the cell cycle inhibitor, aphidicolin [11], can block cell proliferation in S. rosetta and thereby block colony formation. In the presence of aphidicolin, S. rosetta cells fail to divide, yet continue to increase in size and otherwise appear to behave normally (Fig. 1C); upon removal of the drug, cell division resumes. To test whether colonies can form in the absence of cell division, S. rosetta cells were treated with either aphidicolin or DMSO (as a negative control) prior to induction of colony formation (Fig. 1D). DMSO-treated cultures developed colonies within 24 hours after induction, while cultures incubated with aphidicolin failed to form colonies, even after 96 hours of induction. Removal of aphidicolin from induced cultures after 36 hours of treatment permitted the development of colonies, demonstrating that the drug’s effect was reversible and that the formation of colonies is dependent upon cell proliferation. Taken together, these findings demonstrate that rosette colonies form by cell division and not by cell aggregation.

Our finding that S. rosetta colonies develop through repeated cell division, coupled with the fact that development from a single cell is ubiquitous in animals, is consistent with the hypothesis that the last common ancestor of animals and choanoflagellates was capable of simple multicellularity. An important test of this hypothesis will be to determine whether colony formation is, indeed, ancestral within choanoflagellates and whether S. rosetta colony development is representative of an ancestral strategy for multicellular development. If so, the study of colony development in S. rosetta may provide mechanistic insights into early stages in the evolution of animal multicellularity and reveal the premetazoan function of developmental genes and their regulation.

Supplementary Material



This work was supported by funding from the NIGMS/NIH (grant number GM089977), the American Cancer Society and the Gordon and Betty Moore Foundation Marine Microbiology Initiative, and NIH Training Grant T32 HG 00047. N.K. is a Scholar in the Integrated Microbial Biodiversity Program of the Canadian Institute for Advanced Research.


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