Evolution

Getting from One to Two

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Science  10 Feb 2012:
Vol. 335, Issue 6069, pp. 637
DOI: 10.1126/science.335.6069.637-a

Life on Earth was initially unicellular, but at some point these morphologically simple organisms started forming cooperative clusters of cells that allowed them to overcome the limited size and complexity of their single-celled brethren and thus colonize new environments.

To explore how this transition might have occurred, Ratcliff et al. subjected the unicellular yeast Saccharomyces cerevisiae to a gravity selection protocol. Heavy (and therefore more rapidly sinking) “snowflake” clusters of cells arose through dividing cells remaining stuck together. The close genetic relatedness of the cells in the clusters reduced the potential evolutionary conflict. Further selection resulted in larger snowflake clusters, which nonetheless grew to a specific size limit and reproduced through the production of smaller, but still multicellular, daughter snowflakes. The asymmetric divisions needed to generate the daughter snowflakes were driven by the formation of specialized apoptotic (self-killing) cells that allowed fragmentation of the interconnected cluster of parent snowflake cells. Higher rates of apoptosis correlated with smaller, more numerous daughter snowflakes, which, given their relatively fast growth rates, probably increased the chance of cluster survival through the next selection step. These results suggest that multicellularity may have arisen early in the evolution of life, and indeed there is evidence of filamentous and mat-like organisms dating back to over 3 billion years ago.

Proc. Natl. Acad. Sci. U.S.A. 109, 10910.1073/pnas.1115323109 (2012).

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