Bacterial cells are now understood to live in close-knit communities where they can experience complete interdependence, as well as fierce competition with their neighbors. In extreme cases, cells can commit suicide for the benefit of the surrounding bacteria. In contrast, cells have also evolved a complex array of different ways to kill one another. Evolutionary theory can successfully make predictions about when cells should be nice to their neighbors and when they should be nasty. A key prediction is that cooperative behavior is more strongly favored when it is directed at cells that share genes in common, and that antagonistic behavior should be directed toward nonrelatives. An article in PNAS, however, shows how the same behavior can be nice or nasty, depending on the identity of the cell toward which it is targeted.
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