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Patrick Guilfoile: Wasps, mating and microbes

A fundamental idea in biology is the concept of a species, groups of individuals that can actually or potentially interbreed and produce fertile offspring. Although this definition can be hard to apply with some critters, it often provides a useful framework for organizing similar plants and animals.

Several barriers can prevent different species from interbreeding. One of the most common barriers is genetic, and a well-understood example of such a species barrier is a mule, the offspring of a female horse and a male donkey. Mules are sterile because of incompatible genetic material, which can’t get properly shuffled to make viable sperm or eggs. In the case of matings between many — species, the result is often no viable offspring, the consequence of even greater genetic incompatibility than that between horses and donkeys.

Scientists at Vanderbilt University recently discovered an interesting twist on barriers that prevent different species from producing offspring. They studied parasitic jewel wasps- insects the size of a match head that inject their eggs into the bodies of immature flies. The young wasps emerge two weeks later, after devouring the fly from the inside, and are ready to start producing another generation. However, in crosses between two related species of Jewel wasps, 90 percent of second-generation males die, indicating a substantial barrier to inter-breeding between these species.

The researchers hypothesized that the high fatality rate of offspring from the two species was the result of incompatible gut bacteria. They found that males from this cross had a mixture of microbes that was unlike either parent, supporting their initial guess. Next, they compared the survival of 2nd generation males reared in sterile (microbe-free) conditions with those grown in filthy flies. The germ-free wasps showed a dramatically increased survival rate, implicating microbes as a cause of death of the hybrid offspring. This was confirmed when microbes taken from the original parents were added to the sterile environment of the offspring. Under these conditions, the hybrid offspring dropped like flies (so to speak), with a mortality rate essentially identical to growing in the unsanitary incubator of a baby fly.

To better understand how the microbes might contribute to lethality in the offspring, these researchers studied how the hybrids responded to their environments. The germ-free wasp offspring had a dialed-down immune system, as compared to the microbe-colonized offspring. Immunity is a two-edged sword; when it functions properly, it protects us from harmful infections. When it functions too vigorously, it can damage or destroy our own tissues. In the case of the wasps, in the presence of an odd collection of microbes in their guts, it appears their immune systems runs amok and contributes to their untimely demise.

This research provides another in a growing list of examples of the tight relationship of microbial companions and nearly every aspect of the biology of organisms.

More information is available in the article by Robert Brucker and Seth Bordenstein “The hologenomic basis of speciation: Gut bacteria cause hybrid lethality in the genus Nasonia.” in Science 341:667-669, Aug. 9, 2013.
