Selfish responses by clone invaders

Abstract

One's first encounter with aphids is likely to have been as a green velvety mat of tiny bodies placidly sucking the sap out of rosebushes with their tiny soda-straw stylets. The impression of gentleness is further enhanced if one knows that aphids are also the cows of the ant world, tended and defended for the honeydew that they excrete nearly constantly as they process enough sap for their protein needs. However, recent research has revealed that a fraction of aphids—perhaps 1% of the over 4,000 species (1)—are less like cows and more like killer bees. Evolution has endowed some of them with bulked-up Schwarzenegger arms (six of them, of course) and turned their soda straws into lethal weapons. These fierce soldiers can repel and even kill predators many times their size. Now, as Abbot et al. show in this issue (2), some of these social aphids demand a further revision of their aphid-as-placid-cow reputation. Not only are they fierce, but they are also sneaky and clever. They exhibit a complex and variable suite of responses to their specific social situation and are able to freeload on the protection offered by their unrelated neighbors. As such, they illustrate an important issue in the evolution of complex life.