Evolving-multicellularity lab exercises
If you're a teacher looking for a couple of great lab exercises on the evolution of multicellularity, see this website.
Why did it take so long for unicellular organisms to evolve multicellularity? Is evolving even simple multicellularity (groups of cells staying together, with little or no division of labor) so difficult, perhaps requiring several simultaneous mutations? Or were conditions such that there was little or no advantage to simple multicellularity?
My colleague Mike Travisano and his postdoc Will Ratcliff (my former PhD student) reasoned that selection for rapid settling through liquid would favor clumps over individual cells. Sure enough, they were able to evolve simple multicellular clusters from unicellular yeast, in less than a month. Mark Borrello and I participated in the discussions and were coathors on an open-access paper describing this work. This paper generated a lot of interest, as I've discussed previously.
If simple multicellularity can evolve so easily, why did it take so long? One explanation is that rapid settling in liquid wouldn't usually have been beneficial. What factor(s) would give simple clusters of cells a reproduction or survival advantage over single cells? One possible advantage would be resistance to predation by larger single-celled organisms. A brilliant pioneering paper by Boraas et al. (1998) showed that, faced with predation, unicellular algae evolve simple multicellular clumps that are too big for the predators to eat. An earlier post links to a movie Will Ratcliff made, showing a unicell-gobbling rotifer "flinching" when it encounters a multicellular yeast cluster.
Last summer, high-school teachers Tami Limberg and Nicholas Beerman worked with Will to develop a lab exercise showing how predation can favor multicellularity. Will's snowflakeyeast.com website has all the information you need to 1) evolve multicellular yeast, or 2) explore the effects of predation on multicellular versus unicellular yeast.
Will is starting a faculty position at Georgia Tech. Ryoko Oono, my other recent PhD student, is starting a faculty position at UC Santa Barbara. The only other person to earn a PhD with me so far is Toby Kiers, who has just accepted a University Research Chair at Vrije Universiteit Amsterdam. Past results do not guarantee future results.