Adaptation via Symbiosis: Recent Spread of a Drosophila Defensive Symbiont

Abstract

Offsetting the Cost of Parasitism: Fruit flies, like most animals, are vulnerable to infection by a range of organisms, which, in co-infections, can interact with sometimes surprising effects. Jaenike et al. (p. 212 ) discovered that a species of Spiroplasma bacterium that is sometimes found in flies, and that is transmitted from mother to offspring, protects its host from the effects of a nematode worm parasite, Howardula aoronymphium . The worm sterilizes the female flies and shortens their lives, but when flies were experimentally infected with Spiroplasma , their fertility was rescued. Similarly, in wild populations of fruit flies infected with worms, those also infected with Spiroplasma had more eggs in their ovaries. The bacterium inhibits the growth of the adult female worms, but such is the advantage of this bacterial infection in offsetting the burden of nematodes on reproductive fitness, Spiroplasma appears to be spreading rapidly through populations of fruit flies in North America.