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Single Cells Evolve Large Multicellular Forms in Just Two Years

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It’s difficult to deny that evolution happens when it is demonstrated — difficult, but certainly not impossible as Ken Ham (no relation! at all!) will tell you. But it’s interesting to see the big step taken in a laboratory setting. Veronique Greenwood writes in Quanta:

To human eyes, the dominant form of life on Earth is multicellular. These cathedrals of flesh, cellulose or chitin usually take shape by following a sophisticated, endlessly iterated program of development: A single microscopic cell divides, then divides again, and again and again, with each cell taking its place in the emerging tissues, until there is an elephant or a redwood where there was none before.

At least 20 times in life’s history — and possibly several times as often — single-celled organisms have made the leap to multicellularity, evolving to make forms larger than those of their ancestors. In a handful of those instances, multicellularity has gone into overdrive, producing the elaborate organisms known as plants, animals, fungi and some forms of algae. In these life forms, cells have shaped themselves into tissues with different functions — cells of the heart muscle and cells of the bloodstream, cells that hold up the stalk of a wheat plant, cells that photosynthesize. Some cells pass their genes on to the next generation, the germline cells like eggs and sperm, and then there are all the rest, the somatic cells that support the germline in its quest to propagate itself.

But compared to the highly successful simplicity of single-celled life, with its mantra of “eat, divide, repeat,” multicellularity seems convoluted and full of perilous commitments. Questions about what circumstances could have enticed organisms to take this fork in the road millions of years ago on Earth — not once but many times — therefore tantalize scientists from game theorists and paleontologists to biologists tending single-celled organisms in the lab.

Now, the biologist William Ratcliff at the Georgia Institute of Technology and his colleagues report that over the course of nearly two years of evolution, they have induced unicellular yeasts to grow into multicellular clusters of immense size, going from microscopic to branching structures visible to the naked eye. The findings illustrate how such a transition can happen, and they imply intriguing future experiments to see whether these structures develop differentiation — whether cells start to play specialized roles in the drama of life lived together.

Incentives to Be Snowflakes

Nearly a decade ago, scientists who study multicellularity were set abuzz by an experiment performed by Ratcliff, Michael Travisano, and their colleagues at the University of Minnesota. Ratcliff, who was doing his doctoral thesis on cooperation and symbiosis in yeasts, had been chatting with Travisano about multicellularity, and they wondered whether it might be possible to evolve yeast into something multicellular. On a whim, they took tubes of yeast growing in culture, shook them, and selected the ones that settled to the bottom fastest to seed a new culture, over and over again for 60 days.

This simple procedure, as they later described in the Proceedings of the National Academy of Sciences, rapidly caused the evolution of tiny clumps — yeasts that had evolved to stay attached to each other, the better to survive the selection pressure exerted by the scientists. The researchers subsequently determined that because of a single mutation in ACE2, a transcription factor, the cells did not break apart after they divided, which made them heavier and able to sink faster.

This change in the cells emerged quickly and repeatedly. In less than 30 transfers, one of the tubes exhibited this clumping; within 60 transfers, all of the tubes were doing it. The researchers dubbed the cells snowflake yeast, after the ramifying shapes they saw under the microscope.

Snowflake yeast started out as a side project, but it looked like a promising avenue to explore. “That’s been my life for 10 years since then,” Ratcliff said. The work garnered him collaborators like Eric Libby, a mathematical biologist at Umeå University in Sweden, and Matthew Herron, a research scientist at Georgia Tech, where Ratcliff is now a professor. He had joined the varied ecosystem of researchers trying to understand how multicellular life came about.

It’s easy for us, as the vast architectures of cells that we are, to take it for granted that multicellularity is an unqualified advantage. But as far as we can tell from fossils, life seems to have been cheerfully unicellular for its first billion years. And even today, there are far more unicellular organisms than multicellular ones on the planet. Staying together has serious downsides: A cell’s fate becomes tied to those of the cells around it, so if they die, it may die too. And if a cell does become part of a multicellular collective, it may end up as a somatic cell instead of a germ cell, meaning that it sacrifices the opportunity to pass on its genes directly through reproduction.

There are also questions of competition. “Cells of the same species tend to compete for resources,” said Guy Cooper, a theorist at the University of Oxford. “When you stick a bunch of them together, that competition for resources becomes even stronger. That’s a big cost … so you need a benefit that’s equal or greater on the far side for multicellularity to evolve.” . . .

Continue reading. There’s more, including videos.

Written by Leisureguy

22 September 2021 at 5:20 pm

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