A Story of the Bacterium and the Fly
Poke deep inside an insect cell, and you may be in for a shock. At least we were startled to learn that bacteria live inside many insects, including the fruit fly, one of the workhorses of biology.
Today, we hear how bacteria of the genus Wolbachia boost egg production in certain fruit flies. The mechanism, says Horacio Frydman, an assistant professor of biology at Boston University, involves a two-step: first the fly makes more egg cells, and then it blocks a process that would normally prune away extra eggs.
Insects, like other animals, are frequently “married” to bacteria in a relationship that benefits one or both parties. This is common: Bacteria in the cow’s rumen break down cellulose eaten by the cow. Bacteria in the human gut form vitamin K, necessary for blood clotting.
And bacteria in aphids synthesize essential amino acids that the aphids cannot make by themselves.
Wolbachia are not essential to the fruit flies, but their presence can quadruple egg production.
Egg development in the fruit fly Drosophila mauritiana
Producing four times as many offspring “is a powerful driver of infection,” Frydman says. “Wolbachia manipulate their host reproduction to favor their own spread in nature,” noting that in less than 20 years after Wolbachia was detected in fruit flies in southern California, the infection had spread as far as Canada. “It’s considered one of the largest pandemics in the recent evolution of life. Because Wolbachia influence their host reproduction, they also impact the evolutionary history of innumerable hosts.”
Wolbachia have been linked with a wide variety of effects in the insect realm. Wolbachia “lives in at least 20 percent of the world’s arthropods, including insects, spiders, mites, and crustaceans,” according to the Wolbachia project, making them an active area of investigation.
How could this symbiosis work to increase the number of offspring?
Using sophisticated microscopy, Frydman, Ph.D. student Eva Fast and colleagues tracked the location of Wolbachia in fruit flies. In D. mauritiana, a species native to the Mauritius Islands in the Indian Ocean, the bacteria congregate in the germline stem cell niche — a structure that supports stem cells that develop into eggs. In D. melanogaster, the bacteria accumulate in the niche that harbors a different type of stem cell, which produces the eggshell.
In the germline stem cell niche, the bacteria actually outnumber mitochondria, organelles involved in making energy for the fly.
Having the bacteria in the germline stem cell niche doubled the rate of division among those stem cells. Further investigation showed that the bacteria later also halved the rate of programmed cell death.
So the bottom line was a four-fold increase in egg production.
The virtue of pruning
“It’s remarkable that there are two mechanisms being manipulated by the bacteria, the rate of egg production and the rate of programmed cell death,” says Frydman.
Hitting both systems makes sense, Frydman adds, although the mechanisms remain unclear. “It is not surprising that Wolbachia would evolve to manipulate those two process, because they are key in controlling the rate of egg production, and therefore it has a profound impact in the reproductive success of the infected host and in spreading of bacteria in nature.”
Anything that increases the number of eggs and offspring is likely to be favored by natural selection, Frydman adds.
A healthy thing?
Beyond an insight into the fascinating biology of symbiosis, the finding could also have health implications. Parasitic worms that cause diseases like elephantiasis seem to benefit from Wolbachia infection.
And Wolbachia can affect insect immunity: Tests have shown that infected fruit flies are more resistant to some viruses, for example. And a recent paper in Nature found that mosquitoes in Australia could not transmit dengue fever if they carried a Wolbachia strain derived from Drosophila.
Mosquitoes also transmit malaria. Conceivably, better knowledge of the interaction between Wolbachia and insects might convert mosquitoes from a carrier of this ancient scourge into a defense against it.
– David J. Tenenbaum
Terry Devitt, editor; S.V. Medaris, designer/illustrator; Jenny Seifert, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive
- Wolbachia Enhance Drosophila Stem Cell Proliferation and Target the Germline Stem Cell Niche, Eva M. Fast et al, www.sciencexpress.org / 20 October 2011 / Page 1 / 10.1126/science.1209609 ↩
- Horacio Frydman. ↩
- Wolbachia biology. ↩
- A tale of sex and survival. ↩
- Wolbachia research database. ↩
- Wolbachia teaching resources. ↩
- Controlling dengue fever. ↩
- Malaria prevention? ↩
- Wolbachia makes widows. ↩
- It even creates new species! ↩
- River blindness culprit. ↩
- Can I borrow your genes? ↩