Study: Parasitic Bacteria May Combat Sterility

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Bacteria have long been associated with infection, irritation, illness and death, but UC Berkeley scientists recently discovered that certain bacteria also promote reproductive health.

Bacterial infection by the parasite Wolbachia pipientis can make sterile fruit flies fertile, according to a study conducted by Thomas Cline, professor of molecular and cell biology, and graduate student Diana Starr.

Cline and Starr found that Wolbachia reverses the effects of a mutation in Sex-lethal, a gene found in the fruit fly Drosophila melanogaster, which determines the sex of an embryo and the development of the egg. Certain mutations in Sex-lethal cause sterility, yet the presence of the bacteria overcomes this effect.

"This is the first spectacular effect of this phenotype in Drosophila melanogaster," Cline said. "We have the chance to find a mechanism."

It is unknown how the parasite overrides the mutation and the phenomenon is isolated to fruit flies-a similar relationship has been observed between Wolbachia and the wasp Asobara tabida. Since fruit flies are understood better than wasps, it is hoped the newest discovery will lead to an understanding of how the insects overcome sterility.

"We can link (our work) to all the

resources developed over the years, like the (fruit fly) genome project," Starr said.

The parasite also influences the success rate of mating between flies.

Infected females can only reproduce with infected males. When Wolbachia resides in the cytoplasm, (the area outside of the nucleus of developing fruit fly eggs), the sperm of the male fly-infected with the same strain of bacteria-must fertilize the eggs .

The possibility of "cytoplasmic incompatibility," when an infected female mates with a non-infected male or vice versa, puts infected females at an advantage by giving them a better chance to procreate because the bacteria is transferred maternally.

The infected females have a reproductive advantage because uninfected females will not be able to reproduce with infected males.

"The Wolbachia are creating a new selective pressure (in order to survive) because more infected females means more surviving Wolbachia," Starr said.

Uninfected flies are still suspectible to the Sex-lethal mutation, whereas infected flies are not.

This discovery has implications for the field of evolutionary biology.

"Cytoplasmic incompatibility sets up a reproductive barrier that could lead to the evolution of a new species," Cline said.

The idea that host-parasite interactions influence the evolution of a new species has placed members of the scientific community at odds with each other.

"(This) is speculation with very little empirical support," said Michael Turelli, chairman of the Section of Evolution and Ecology at UC Davis. "No genetic divergence was seen in fruit flies when (a Wolbachia) infection spread through California to the Canadian border."

Cline remained enthusiastic about the discovery, but added there is still much to be researched on the topic.

"What is exciting about this research is that it shows just how little we appreciate of what is possible in nature," Cline said. "That is why understanding these mechanisms is so important."


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