Category Archives: Species

Multicellular yeast evolved from single cells?

Ratcliff, et al. demonstrated in their lab that the yeast, Saccharomyces cerevisiae, clustered in sixty days.1 They claim that these yeast clusters are an amazing example of rapid evolution of single cells evolving into multicellular organisms.

Ratcliff says, “we show that key steps in this transition could have occurred quickly.1 However, skeptics point out that latent gene expression does not suggest speedy start. Rather there was a speedy expression of genes that were already present. This is consistent with claims made by proponents of punctuated equilibrium.

Being in the kingdom Fungi, many yeast strains naturally form colonies and have been known to be multicellular.1 The appearance of multicellular features, then, is no surprise. The phenotype just needed to be coaxed from the genotype.

In response to critique, Ratcliff and his colleagues are planning to conduct similar experiments with Chlamydomonas, a single-celled alga that has no known multicellular ancestors. However, there is no mention of confirming the absence of colonial or multicellular genes in the control group.

Classic understanding of distinguishing unicellular colonies from multicellular organisms is the ability of the individual cells being capable of independent survival whether by themselves or by propagating a new colony. A colony is made of independent cells. Survival of cells from multicellular organisms depend on other cells in the organism for survival. Mushrooms are made of a colony of fungal cells that develop specialized features and functions, but the cells are still capable of independent survival.

However, Ratcliff claims that, “The key step in the evolution of multi-cellularity is a shift in the level of selection from unicells to groups. Once that occurs, you can consider the clumps to be primitive multicellular organisms.”

Primitive is the key word here because the descriptions of what they call multicellular are premature. For example, the clusters of yeast cells reproduced daughter clusters by separating from the larger clump of cells. They referred to the smaller daughter cluster as a juvenile phase. The parent cluster is called a multicellular propagule, and thus a ‘life-cycle’. And according to them, apoptosis of cells represents a division of labor.

How does the breaking off of clusters simulate multicellular reproduction? Is this something that should be expected because yeast cells separate as single cells too? If Ratcliff’s definition of multicellular is accepted, then how is it distinguished from a colony? Obviously, there are more questions needing to be answered to clarify the significance of these clustered yeast cells.

Ratcliffe’s research is of interest to creation scientists because it gives insight to the boundaries of adaptation among organisms. The creation science model proposes that archetype organisms were created, as indicated by the sudden appearance of body types in the Cambrian, from which the created genotypes allowed for adaptation and variation in successive generations. Defining the limits of genotype variation is an important area of study for creationists.

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1Ratcliff, W. C., Denison, R. F., Borrello, M. & Travisano, M. Proc. Natl Acad. Sci. USA advance online publication http://dx.doi.org/10.1073/pnas.1115323109 (2012).

2Blacketer, M.J. et al. 1993. Regulation of dimorphism in Saccharomyces cerevisiae: involvement of the novel protein kinase homolog Elm1p and protein phosphatase 2A. Mol Cell Biol. 13 no. 9:5567“5581. – See more at: http://www.answersingenesis.org/articles/2011/07/02/news-to-note-07022011#fnList_1_5.  [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC360278/]

3Supporting Information: http://www.pnas.org/content/suppl/2012/01/11/1115323109.DCSupplemental/pnas.201115323SI.pdf

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