Inside knowledge of the enemy's weaknesses can help yeast to kill their enemy—other yeast cells—report Reiter et al. on page 353. They show that yeast activate the self-destruct mechanism in their yeast foes.

Self-destruction, or apoptosis, is thought to be a common outcome in yeast cells exposed to environmental stresses, such as peroxide or UV light, or internal stresses, such as aging. Suicide of a unicellular organism is thought to benefit the healthier, surviving members of the population by conserving limited nutrients.

But some members of the yeast populace are far less altruistic—they harbor toxin-producing viral sequences that eliminate other yeast in two ways. At high concentrations, the toxins bring about necrotic death in other cells via several strategies: some make holes in the plasma membrane; others inhibit DNA synthesis. In natural environments, however, the toxins rarely accumulate to such high levels.

The new results show that low toxin concentrations are still deadly because they trigger apoptosis in the enemy cells. As with other stresses, toxins required a caspase-like enzyme and reactive oxygen species to induce apoptosis. Because the killer cells are resistant to the toxins, these cells would probably dominate over time in the wild.