Any parasite that helps reproduction is likely to spread among the population.
A mother Howardula worm would swell to massive size inside a host fly, laying thousands of eggs. After they hatched, larvae coursed through the host’s body. The flies didn’t die, but they no longer had the strength to reproduce. In the eastern United States, the worms infested nearly one in four D. neotestacea flies, and sterilization seemed absolute.
The pattern fits with what’s predicted by traditional evolutionary theory: A beneficial mutation arises, confers a reproductive advantage, and over time spreads through a population — except that the adaptation isn’t genetic, but bacterial. Microbes can be passed from mother flies to offspring, but also carried by mites between flies, and even between species.
This kind of evolution “allows an adaptation in one species to be moved to another species,” said Yale University evolutionary biologist Nancy Moran, who was not involved in the study.
But, Jaenike found, in some flies, Howardula didn’t lead to sterilization. Instead it was the worms who were sickly, and their offspring few. And the only difference between fertile and infertile Howardula-infected flies was the presence of a bacteria called Spiroplasma.
Other examples of insect endosymbioses include Moran’s specialty, a microbe that protects aphids from parasitic wasps. Another bacteria called Wolbachia causes some male insects to turn female — a relationship that doesn’t make obvious evolutionary sense. In one species of fruit fly, Wolbachia can also kill all male offspring, but only if their mother alone is infected. If both parents are infected, the offspring are fine