Cross-section of branching yeast (left) and simulated group growth (right) showing where the multicellular organism might cleave. Colors = growth branches from original parent cell (0); numbers = generation of each cell’s birth.

SFI Omidyar Fellow Eric Libby and co-author William Ratcliff explore how early multicellular life might have persisted amidst the evolutionary tug-of-war between single-celled and multi-celled living arrangements.

In their Perspectives piece in Science magazine, "Ratcheting the evolution of multicellularity," Libby and Ratcliff ask how evolved multicellular organisms might have managed conflicts between levels of selection, "as cell-level selection can easily overwhelm the generally slower process of group-level selection."

To explore the question, they examine contemporary organisms that straddle the boundary between single-celled and multi-celled life, such as bacteria that (on static growth media) cluster into bacterial mats, thereby gaining better access to oxygen. These multi-cellular mats, however, fall victim to single-celled "cheats" that do not produce the "glue" needed for mat formation -- and which tend to escape the mat's eventual collapse -- tipping the scales of evolutionary advantage toward these single-celled free riders.

"This experimental work reveals that a key problem in the transition to multicellularity is not how multicellular clusters evolve in the first place, but rather how multicellularity persists when the balance of selection tilts toward single cells," they write.

"One solution to stabilizing multicellularity is the evolution of traits that increase cell level fitness in a group context, but come at a cost to free-living fitness" -- such as elevated rates of programmed cell death, called apoptosis, that in yeast experiments produce weak links that allow large multi-cellular clusters to break apart into smaller, more sustainable multi-cellular formations.

"Accumulation of [such] traits would ratchet cells into a group lifestyle, ultimately preventing unicellular reversion," they suggest.

Read their article in Science (October 24, 2014, subscription required for full text)

Read the article in Astrobiology magazine (February 5, 2015)

Read the article on Space.com (February 11, 2015)

Read the article on Yahoo.com (February 11, 2015)

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