DNA transfer between unrelated species more common than thought

Scientists at the University of Rochester and the J. Craig Venter Institute have discovered a copy of the entire genome of the Wolbachia bacterium lurking within the genetic code of fruit flies, suggesting that lateral gene transfer-the movement of genes between unrelated species-may happen much more frequently between bacteria and multi-cellular organisms than previously believed.

Washington, Aug. 31 : Scientists at the University of Rochester and the J. Craig Venter Institute have discovered a copy of the entire genome of the Wolbachia bacterium lurking within the genetic code of fruit flies, suggesting that lateral gene transfer-the movement of genes between unrelated species-may happen much more frequently between bacteria and multi-cellular organisms than previously believed.

Principle investigator Jack Werren, professor of biology at the University of Rochester, said that the findings posed dramatic implications for evolution, for such large-scale heritable gene transfers might allow species to acquire new genes and functions extremely quickly.

Earlier, it seemed impossible that fruit fly ovaries could show wolbachia infection, he said.

"It didn't seem possible at first. This parasite has implanted itself inside the cells of 70 per cent of the world's invertebrates, coevolving with them. And now, we've found at least one species where the parasite's entire or nearly entire genome has been absorbed and integrated into the host's. The host's genes actually hold the coding information for a completely separate species," says Werren.

Working with collaborators at J. Craig Venter Institute (JCVI), researchers in Warren's lab systematically screened invertebrates. Julie Dunning-Hotopp at JCVI found evidence that some of the Wolbachia genes seemed to be fused to the genes of the fruitfly, Drosophila ananassae, as if they were part of the same genome.

Michael Clark, a research associate at Rochester, then brought a colony of ananassae into Werren's lab, and fed the flies a simple antibiotic that killed the Wolbachia.

In order to confirm that the ananassae flies were cured of the wolbachia, Clark tested a few samples of DNA for the presence of several Wolbachia genes. To his dismay, he found them.

"For several months, I thought I was just failing," says Clark, "I kept administering antibiotics, but every single Wolbachia gene I tested for was still there. I started thinking maybe the strain had grown antibiotic resistance. After months of this I finally went back and looked at the tissue again, and there was no Wolbachia there at all."

Although Clark had cured the fly of the parasite, a copy of the parasite's genome was still present in the fly's genome. He was able to see that Wolbachia genes were present on the second chromosome of the insect.

The researchers confirmed that the Wolbachia genes were inherited like "normal" insect genes in the chromosomes, and that some of the genes were "transcribed" in uninfected flies, meaning that copies of the gene sequence were made in cells that could be used to make Wolbachia proteins.

They are now looking further into the huge insert found in the fruit fly, and whether it is providing a benefit.

"The chance that a chunk of DNA of this magnitude is totally neutral, I think, is pretty small, so the implication is that it has imparted of some selective advantage to the host. The question is, are these foreign genes providing new functions for the host. This is something we need to figure out," says Werren.

ANI