Sunday, February 5, 2012

Beware the Yeast of the Naturalist

A couple of weeks ago a paper was published in the Proceedings of the National Academy of Sciences that claimed to illuminate how single celled organisms began their evolutionary trek towards multicellularity. They applied strong selective pressure for weight on a culture of brewer's yeast cells for 60 days and found that the yeast formed clusters of cells and that some of the cells within a cluster exhibited "apoptosis," that is, they would die, making room for other parts of the cluster to grow. The hype surrounding the article is that it shows the "ease" with which the jump to multicellularity can occur.

All of this is predicated on the assumption that such a change did occur in the distant past. By conventional evolutionary reckoning, such a change occurred roughly 25 times independently in the past. The authors wonder why it didn't happen even more often given how easily they were able to provoke it in their laboratory. Hmm. A clue that perhaps their presumption is off?

Apart from the interpretation, the artificial selection of yeast generations showed some interesting yeast behavior, but certainly not something I would describe as a multicellular organism except by the most trivial of definitions. There were groups of cells that stuck together (as yeast cells commonly do anyway, just not often in such large clumps). Admittedly the authors would not claim they showed the full transformation either, but they do little to caution readers against wild extrapolation. Their claim of apoptosis is premature, since they have not yet determined the mechanism behind the cell death. There may have been a genetic mechanism involved, but there might also be a more mundane reason, say mechanical or chemical stress. Even if it were shown to be true apoptosis, it is doubtful that it came from something that was absent from the yeast at the outset.

So there is still a big question about just how "multicellular" these yeast really are. There is an even bigger question about how much further the yeast could be pushed toward multicellularity. An interesting follow-on experiment would be to remove the selective pressure and see what happens to the yeast globs. Do they persist or do they revert back to their less glommy former state? And what if there were a few of the "loner" yeasties still around? What would the neighborhood look like in a few generations? Would there be a predominance of yeast hotels, or would there be more single cell dwellings?

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