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In this new paper "Experimental evolution of multicellularity" found via Ars Technica the researchers describe having developed multicellularity and apoptosis within 60 days from a unicellular yeast species.

Is it possible that what they have done is merely turn on an ability of that species that evolved previously and just lay dormant? Do we know that that yeast species and all its ancestors were never multicellular? The paper doesn't even mention the possiblity of this.

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Well, they put the cells in particular conditions that determined the multicellularity, so they have not specifically given the cells this ability per se. And I do not think they are claiming to have done so. –  nico Jan 18 '12 at 18:32
    
The conditions they put them under favored multicellularity, and the cells ended up being multicellular, and their abstract claims "These results show that key aspects of multicellular complexity, a subject of central importance to biology, can readily evolve from unicellular eukaryotes." –  Mark Probst Jan 18 '12 at 20:35
    
exactly my point. I don't read "we gave an unicellular organism the ability to become multicellular" in that sentence. I read "we put an unicellular organism in a situation in which it became multicellular". It's like people reprogramming stem cells to become this or that type of cell, they don't give them any ability, they just put them in the good conditions. –  nico Jan 19 '12 at 8:10
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up vote 11 down vote accepted

You're right:

Within the Fungi, simple linear multicellularity of hyphae occurs in all major clades (see below), but only Ascomycota and Basidomycota display more complex two- and three- dimensional multicellularity in the form of sexual spore- producing fruiting bodies. In both of these groups, reversals to unicellular lifeforms have occurred, for example, Saccharomyces and many other related yeasts in the Saccharomycotina (Ascomycota) or Cryptococcus albidus and related species in the hymenomycete clade of Basidiomycota (de Hoog et al. 2000, p. 130).

Medina, M., A. G. Collins, J. W. Taylor, J. W. Valentine, J. H. Lipps, L. A. Amaral Zettler and M. L. Sogin (2003). "Phylogeny of Opisthokonta and the evolution of multicellularity and complexity in Fungi and Metazoa." International Journal of Astrobiology 2(3): 203-211. doi:10.1017/S1473550403001551 (PDF)

Update: The authors responded to criticism like this on The Loom, here's an excerpt:

Our yeast are not utilizing ‘latent’ multicellular genes and reverting back to their wild state. The initial evolution of snowflake yeast is the result of mutations that break the normal mitotic reproductive process, preventing daughter cells from being released as they normally would when division is complete. Again, we know from knockout libraries that this phenotype can be a consequence of many different mutations. This is a loss of function, not a gain of function. You could probably evolve a similar phenotype in nearly any microbe (other than bacteria, binary fission is a fundamentally different process). We find that it is actually much harder to go back to unicellularity once snowflake yeast have evolved, because there are many more ways to break something via mutation than fix it.

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Just to reiterate (I am not a biologist): this means that we know of Fungi that reverted from multicellularity to unicellularity, so the same could be true of this particular yeast they used? –  Mark Probst Jan 18 '12 at 20:41
    
The fungus they used, Saccharomyces cerevisiae, is part of the fungi described above. So the part I quote actually refers to this particular yeast (Brewer's yeast). –  Michael Kuhn Jan 19 '12 at 5:47
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