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Single Cells Evolve Large Multicellular Forms in Just Two Years [quantamagazine.org]:

One incentive might be that larger groups of cells can be harder for predators to eat. Independent work by Roberta Fisher [cbmr.ku.dk] at VU University Amsterdam in 2015 [doi.org] and Stefania Kapsetaki [asu.edu] at Oxford in 2019 [doi.org] showed that algae and bacteria responded to predatory protozoa by forming groups. Herron and his colleagues showed in 2019 [nature.com] that this adaptive multicellularity in algae did not depend on the reappearance of some buried ancestral trait: It was a fully original, evolved adaptation.

Another possible incentive for multicellularity could be that organisms move better or forage better as a group under certain conditions. If that’s the case, Cooper explained, “that leads to a viability-fecundity trade-off, in the sense that you increase your survival at the cost of being less reproductive, because you’re competing for the resources.”

Some algae can switch between multicellular groups and single cells when their environments change. Choanoflagellates, the closest single-celled relatives to animals, can also opt to take actions that make them look curiously multicellular. Thibaut Brunet [brunetlab.com], an evolutionary biologist at the Pasteur Institute, recalls a workshop in Curaçao where he and colleagues collected water near the shore to check for choanoflagellates and noticed late at night, after dinner, that there was something moving in their sample. It was a new species of choanoflagellate [doi.org] that had joined together to form a cup shape, which was flipping itself inside out to move. “It was mesmerizing to see this thing just deform. … It had this complex collective behavior that made it almost animal-like,” Brunet said. “You could almost feel that transition from the microbial world to the animal world.”

But for the cells of most multicellular creatures, there is no choice — it’s multicellularity or death. “It somehow becomes a one-way road,” said Cooper. “And division of labor is predicted to be a big player in that transition.” Once some cells start to perform a new role, giving up their own reproductive success to increase that of their neighbors, computational models suggest that living in a group must provide efficiency benefits for the lifestyle to stand a chance of surviving. The parameters required for success must have been met in the past, but how exactly?

When Ratcliff began his long-term experimental evolution work, he combined a theorist’s interest in myriad possible scenarios with a biologist’s curiosity about what a real, living organism would do when pressed to the limit. He was also thinking about one of the most famous evolution experiments [quantamagazine.org], started by Richard Lenski [msu.edu] more than 30 years ago: 12 E. coli colonies in Lenski’s lab have been maintained since 1988. They’ve morphed over the years in surprising ways: For instance, in 2003, Lenski and his colleagues found that one population had evolved the ability to digest citrate, which E. coli had never been known to do before.

Ratcliff wondered what would happen to snowflake yeast grown that long — would they eventually achieve large size? Would that lead to differentiation?

The snowflake yeast achieved multicellularity readily, but their clumps remained microscopic, no matter what Ratcliff tried. For years he failed to make progress, and he credits Ozan Bozdağ [gatech.edu], a research scientist at Georgia Tech who was a postdoc in Ratcliff’s lab, with breaking through the wall.

Living Large Without Oxygen

The crucial ingredient turned out to be oxygen. Or rather, a lack of it.

Oxygen can be very helpful for living things, because cells can use it to break down sugars for massive energy payouts. When oxygen isn’t present, cells must ferment sugars instead, for a smaller usable yield. All along, Ratcliff had been growing yeast with oxygen. Bozdağ suggested growing some cultures without it.

Bozdağ began the selection experiments with three different groups of snowflake yeasts, two that could use oxygen and one that, because of a mutation, could not. Each group consisted of five genetically identical tubes, and Bozdağ mounted them in a shaking machine. Around the clock, the yeast were shaken at 225 revolutions per minute. Once a day, he let them settle on the counter for three minutes, then used the contents of the bottom of the tube to start fresh cultures. Then, back in the shaker they went. Every day in 2020 and early 2021, even during the lab closures of the COVID-19 pandemic, Bozdağ was there, with a special exemption granted by the university, exerting selection on the yeast.

Journal Reference:
William C. Ratcliff, R. Ford Denison, Mark Borrello, et al. Experimental evolution of multicellularity [open], Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.1115323109 [doi.org])
Ratcliff, William C., Fankhauser, Johnathon D., Rogers, David W., et al. Origins of multicellular evolvability in snowflake yeast [open], Nature Communications (DOI: 10.1038/ncomms7102 [doi.org])
Dryad Data -- Multicellular group formation in response to predators in the alga Chlorella vulgaris, (DOI: 10.5061/dryad.c5902 [doi.org])
Stefania E. Kapsetaki, Stuart A. West. The costs and benefits of multicellular group formation in algae*, Evolution (DOI: 10.1111/evo.13712 [doi.org])
Light-regulated collective contractility in a multicellular choanoflagellate, Science (DOI: 10.1126/science.aay2346 [doi.org])
J. T. Bonner. PERSPECTIVE: THE SIZE‐COMPLEXITY RULE, Evolution (DOI: 10.1111/j.0014-3820.2004.tb00476.x [doi.org])
Hammerschmidt, Katrin, Rose, Caroline J., Kerr, Benjamin, et al. Life cycles, fitness decoupling and the evolution of multicellularity, Nature (DOI: 10.1038/nature13884 [doi.org])

Original Submission