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Wednesday, April 11, 2012

Probing the Origin of Multicellularity in a Test Tube

Contributed by: Jianzhi Zhang

The evolution of multicellularity is undoubtedly one of the most important events in the history of the Earth. Although multicellularity originated multiple times, all occurred long ago and no transitional form exists today. Thus, there is little information about the key steps that led to these transformative evolutionary events. While the origin of multicellularity may sound very complex, a recent study showed that primitive forms of multicellularity can emerge from unicellularity within a matter of weeks in test tubes, when the “environment” is right.

Ratcliff et al. (1) used gravity to select yeast cells that stick together and thus settle more quickly than separated cells. After just 60 rounds of selection, the yeast population is dominated by snowflake-like phenotypes, with a new life history characterized by reproduction via multicellular propagules, a juvenile phase, and determinate growth. However, the genetic changes responsible for the new phenotype are unknown. Given the ease of sequencing yeast genomes, it would be of great interest to identify the causal mutations, which will allow a molecular understanding of such primitive multicellularity.

            In an earlier study that is somewhat less dramatic but more elegant, Koschwanez et al. (2) studied the condition under which multicellularity is favored over unicellularity, by creating an environment in which cells need to cooperate to be able to survive and reproduce. To use sucrose as the sole carbon source, yeast expresses an enzyme called invertase in the cell wall to degrade sucrose into glucose and fructose. These monosaccharides unfortunately diffuse into the environment and need to be re-absorbed by yeast cells. One can imagine that under a low sucrose concentration, a single cell in a test tube would not be able to survive because most of the monosaccharides produced are diffused and wasted. When the cell density increases, the monosaccharide concentration in the environment rises, which may be able to support yeast growth. This was indeed demonstrated experimentally. Furthermore, the authors found mutant yeasts whose cells fail to separate after mitosis to be fitter than the wild-type yeast under certain parameters of cell density and sucrose concentration. Thus, a single mutation may initiate the path to multicellularity.   

Molecular dissections of microevolutionary changes are becoming routine, thanks to rapid progress in molecular genetics and genomics. But big, transformative changes seen in macroevolution remain largely unexplained in terms of their molecular genetic basis. The very micro evolutionary experiments in test tubes may prove instrumental in unlocking the mysteries of macroevolution.

1. Ratcliff WC, Denison RF, Borrello M, Travisano M. 2012. Experimental evolution of multicellularity. Proc Natl Acad Sci U S A. 109:1595-600.
2. Koschwanez JH, Foster KR, Murray AW. 2011. Sucrose utilization in budding yeast as a model for the origin of undifferentiated multicellularity. PLoS Biol. 9:e1001122.

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