Recently, a research group generated a great deal of interest in their claim that we are much closer to understanding multicellularity because of a recent experiment with yeast.
At Wired’s story, the first commenter wanted to know. “Now can we sterilize the creationists?” (55 likes) See also, for example, Nature, National Science Foundation, and The Scientist, where Jef Akst reports (January 16, 2012),
Using an artificial selection paradigm, researchers watch as unicellular yeast evolve into snowflake-like clusters with distinct multicellular characteristics.
In as little as 100 generations, yeast selected to settle more quickly through a test tube evolved into multicellular, snowflake-like clusters, according to a paper published today (January 16) in Proceedings of the National Academy of Sciences. Over the course of the experiment, the clusters evolved to be larger, produce multicellular progeny, and even show differentiation of the cells within the cluster—all key characteristics of multicellular organisms.
But this is just one experiment under admittedly contrived conditions. “What remains to be seen for me is how relevant is it to actual transitions to multicellularity,” said Srivastava.
Indeed, the authors of the PNAS study admit that selecting for yeast cells or clusters that settled most quickly isn’t exactly a “natural” selection pressure. But there could be some important lessons here, Ratcliff insisted. “If we really understand the way that multicellularity can evolve, then that gives us a lot of insight to how this could have occurred in the past,” he said.
Mike Behe offers a different perspective on the yeast in “More Darwinian Degradation: Much Ado about Yeast” (Evolution News & Views, January 23, 2012):
The authors repeated three steps multiple times: 1) they grew single-celled yeast in a flask; 2) briefly centrifuged it; and 3) took a small amount from the bottom of the flask to seed a new culture. This selected for cells that sedimented faster than most. After a number of rounds of selection the cells sedimented much faster than the beginning cells. Examination showed that the fast-sedimenting cells formed clusters due to incomplete separation of replicating mother-daughter cells.
The cell clusters also were 10% less fit (that’s quite an amount) than the beginning cells in the absence of the sedimentation selection. After further selection it was seen that some cells in clusters would “commit suicide” (apoptosis), which apparently made the clusters more brittle and allowed chunks to break off and form new clusters. (The beginning cells already had the ability to undergo apoptosis.)
So, in his view, it is a general loss of fitness that caused the cells to cling together or break apart.
Incidentally, yeast cells are known to act as colonies at times.
A difficulty that occurs to some laypeople is this: The fact that generally unicellular life forms may sometimes benefit from acting as a colony or a combine of some kind does not get to the heart of multicellularity.
What most of us mean by multicellularity is more like the cat on the sofa than the yeast colony or the amoebic slime mold.
Every cell of the cat – neuron, heart cell, kidney cell – is not only specific to a cat but specific to that particular cat. Apart from human veterinary heroics, most of those cells might not even survive in another cat, let alone if they were ejected from the cat onto the floor. Now that’s multicellularity. The feline cells live and die in, by, and for one specific cat.
And how many cats are there in the world?
That’s the level of organized complexity for whose origin we need to account if we want to understand the origin of multicellularity. Not the – usually dispersable – associations of unicellular fungi and amoebas.