You might say the benefit of staying alive is an actual no-brainer: even brainless lifeforms do their best not to die. For the most part, anyway. When they're under stress, single-celled organisms may opt to cut up their DNA and neatly implode. A new study hints that by committing suicide in this way, an organism helps its nearby relatives to stay alive—and hurts its rivals at the same time. In animals with many cells like us, cellular suicide happens all the time, and it helps keep the whole organism in tip-top shape. As embryos, for example, the cells that form our little paws kill themselves off to make fingers. We're born with a brain that's too densely connected, and as we grow the superfluous brain cells die to get things in order. Even as adults, our bodies' regular upkeep includes constantly adding new cells and commanding older ones to die. If your entire body consists of one cell, the benefit of killing it is less obvious. Yet various single-celled organisms—from fungi to parasites to bacteria—have been shown to off themselves under stress. Pierre Durand, an evolutionary biologist at the University of the Witwatersrand in Johannesburg, has been trying to figure out why. In an earlier study with a single-celled algae called Chlamydomonas reinhardtii, Durand grew cells in the liquid where other cells had previous killed themselves (in response to heat stress). The algae grew faster than usual in the suicide liquid. But liquid where cells had been killed from the outside (the researchers tore them apart with sound waves) was harmful to living cells. A cell that dies suddenly leaks toxic contents into its surroundings, but cells that commit suicide apparently don't—and even leave behind something healthy for other cells to eat. Cells that have killed themselves on purpose leave a clear biochemical signature, Durand says. They cut up their DNA in an orderly way, for example, and alter their outer membranes. In a new study, he and his coauthors again stressed out Chlamydomonas reinhardtii cells by heating them. Once more, they looked for the signs of cellular suicide. Then they removed some of the liquid in which the algae had killed themselves. For comparison, they'd also removed some liquid before the cells died. They fed both liquids to new batches of C. reinhardtii cells, as well as two other species of Chlamydomonas. As before, the cells growing in suicide liquid grew more quickly than controls. But more surprisingly, the two other algae species fared much worse in this liquid. After a few days, their growth tanked, compared to cells grown in pre-suicide liquid. Durand isn't sure what materials a suicidal cell could dump into its surroundings that would help its relatives while hurting others. "There are some hints as to how it may work," he says. The dying cell might release certain resources that the various algae species use differently, or the secret may be in signaling molecules it sends out. Research by other scientists has also shown that a cell's suicide can help neighboring cells, Durand says. But not everyone agrees that this is intentional. "Explanations for the origin of programmed cell death are controversial," he says. Any benefits a cell gets from being near a suicidal cell might simply be an accident. If it's true that suicidal cells hurt their rivals, though, while only helping their relatives, then there may be no accident after all. A cell that kills itself under stress would give a boost to relatives that share its DNA. That would mean suicide is a way for a single-celled organism to keep its own genes alive and well—still a no-brainer.
Image: US Department of Energy
Durand, P., Choudhury, R., Rashidi, A., & Michod, R. (2014). Programmed death in a unicellular organism has species-specific fitness effects Biology Letters, 10 (2), 20131088-20131088 DOI: 10.1098/rsbl.2013.1088