The transition to multicellularity enabled the evolution of large, complex organisms, but early steps in this transition remain poorly understood. Here we show that multicellular complexity, including development from a single cell, can evolve rapidly in a unicellular organism that has never had a multicellular ancestor. We subject the alga Chlamydomonas reinhardtii to conditions that favour multicellularity, resulting in the evolution of a multicellular life cycle in which clusters reproduce via motile unicellular propagules. While a single-cell genetic bottleneck during ontogeny is widely regarded as an adaptation to limit among-cell conflict, its appearance very early in this transition suggests that it did not evolve for this purpose. Instead, we find that unicellular propagules are adaptive even in the absence of intercellular conflict, maximizing cluster-level fecundity. These results demonstrate that the unicellular bottleneck, a trait essential for evolving multicellular complexity, can arise rapidly via co-option of the ancestral unicellular form.
Bibliographical noteFunding Information:
We would like to thank Matthew Laudon at the Chlamydomonas Resource Center for cell cultures and Tony Dean and Will Harcombe for helpful feedback. This work was supported by National Science Foundation grant DEB-1051115 (W.C.R., K.H., J.T.P. and M.T.), NASA grant NNX07AJ28G (F.R.), NASA EPSCoR/Montana Space Grant Consortium grant G149-13-4R1063 (M.D.H.) and a NASA Astrobiology Institute postdoctoral fellowship (M.D.H.).