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Algal cells that cheat are more vulnerable to stress. This trade-off could help explain the emergence of multicellular organisms, and what keeps cells in algae and cells in our bodies cooperating with one another.听

Whether among cells or in societies, there are always opportunities for individuals to cheat, reaping the benefits of a group without paying the costs of cooperation.

To shed light on the forces that keep cheaters in check, at the University of New Brunswick, Canada, and her colleagues turned to the multicellular green alga听Volvox carteri. This alga has two types of cells: reproductive cells that replicate and body cells that don鈥檛. These normally work together. But when a particular gene called regA is manipulated, the body cells cheat and start reproducing themselves.

Because body cells outnumber reproductive cells, they give rise to far more cells, each of which is capable of producing more cheaters. Cheater mutants would seem to have an evolutionary advantage, yet cells in V. carteri听ultimately cooperate.

Nedelcu and her team exposed regular and cheater mutant V. carteri to harsh conditions of light, darkness and heat. Half of the regular algae ended up with dead cells, whereas all of the cheater mutants had such cells.

This suggests that cheater mutants are more vulnerable to environmental conditions, says Nedelcu.听In听V. carteri听at least, it could be that the cost of cheating is steep enough to favour cooperation.

at the University of Washington in Seattle calls it a 鈥渇ail-safe mechanism鈥� that keeps cheaters in line. He says studies like this can reveal relationships between the collective and private good, adding to explanations for why cheaters don鈥檛 always win.

The same type of trade-off could also halt other kinds of cheaters. All single-celled organisms have genes听that impose a balance between reproduction and survival. Many of these genes, like regA, persist in the lineages of multicellular organisms, including plants and animals.

Not only might these genes help keep multicellular organisms healthy, says Nedelcu, they might also have played an important role in the evolution of multicellularity听hundreds of millions of years ago.