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SINGLE-celled organisms are capable of altruism, says a German
microbiologist. He has found that individual yeast cells commit suicide so that
their starving neighbours can cannibalise their nutrients.

In multicellular animals, dangerously mutated or infected cells frequently
kill themselves for the greater good of the organism. When these cells die,
through programmed cell death or “apoptosis”, they are careful not to harm
nearby cells. The suicidal cells reduce themselves to neat packages of
membrane-bound detritus containing chopped-up pieces of DNA and other cell
components. This ensures that neighbouring cells aren’t exposed to damaging
enzymes.

In the case of single-celled organisms, however, there is no obvious greater
good to be served by suicide, says Kai-Uwe Fröhlich of the University of
Tübingen in Germany. Yet when researchers put mammalian genes that activate
apoptosis into brewer’s yeast, Saccharomyces cerevisiae, the yeast
died, apparently through apoptosis. Yeast genes seemed to be playing an active
part in this, and the process could be turned off by anti-apoptosis genes from
mammals. “The basic mechanism was there, without all the controls that have
evolved in animals,” says Fröhlich.

Similar events have been seen in other single-celled organisms, including the
parasites Trypanosoma and Leishmania (NewScientist, 18
November 1995, p 22), the dinoflagellate Peridinium, the ciliate
Tetrahymena, and the slime mould Dictyostelium. “The idea of
programmed cell death in unicellular organisms is so new, and seems so
non-constructive, that people haven’t really thought about why they do it,” says
Martin Zörnig of the University of Frankfurt.

Fröhlich now thinks he has the answer. He argues that yeast cells commit
suicide for altruistic reasons, specifically to prevent others from starving.
“If a cell becomes damaged, it must kill itself quickly. Otherwise it could take
a long time to die, depriving its neighbours of nutrients,” he says. This is
understandable because the yeast’s neighbours are likely to be its
clones—so it is, in effect, helping them to promote its own genes.

And in work to be published shortly, Fröhlich believes he has found the
clincher: wild strains of yeast that commit mass suicide when nutrients are in
short supply. “In normal yeast environments, say on a grape, nitrogen is often
lacking,” he says. “The yeast will benefit if many die but some live, and eat
the dead yeast.” Apoptosis chops potentially dangerous enzymes in dying yeast
into usable nutrients, and because it is under genetic control, can remain
inactive in certain cells. Depending on the strain of yeast, says Fröhlich,
mass suicide can allow between 1 in 10 or 2 in a million cells to survive a
famine.

Such altruism may go even farther back in evolution, to bacteria, says
Zörnig. “The good thing about finding this in yeast is that we can study
it, and find drugs that control it,” he says. Failure of normal apoptosis in
humans causes some tumours, while diseases such as Alzheimer’s and AIDS happen
when apoptosis is unleashed in the wrong cells.