Posted by: Lister | October 28, 2007

Bacteria genome in Fruit Flies

The entire DNA of a parasite called wolbachia has been found in the DNA of their host. From Rochester University:

“It didn’t seem possible at first,” says Werren, professor of biology at the University of Rochester and a world-leading authority on the parasite, called wolbachia. “This parasite has implanted itself inside the cells of 70 percent 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.”

[…] Since wolbachia typically live within the reproductive organs of their hosts, Werren reasoned that gene exchanges between the two would frequently pass on to subsequent generations. Based on this and an earlier discovery of a wolbachia gene in a beetle by the Fukatsu team at the University of Tokyo, Japan, the researchers in Werren’s lab and collaborators at J. Craig Venter Institute (JCVI) decided to systematically screen 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 to look into the mystery. To isolate the fly’s genome from the parasite’s, Clark fed the flies a simple antibiotic, killing the Wolbachia. To confirm the ananassae flies were indeed 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.”

Clark had cured the fly of the parasite, but a copy of the parasite’s genome was still present in the fly’s genome. Clark was able to see that wolbachia genes were present on the second chromosome of the insect.

[…] Before this study, geneticists knew of examples where genes from a parasite had crossed into the host, but such an event was considered a rare anomaly except in very simple organisms. Bacterial DNA is very conspicuous in its structure, so if scientists sequencing a nematode genome, for example, come across bacterial DNA, they would likely discard it, reasonably assuming that it was merely contamination—perhaps a bit of bacteria in the gut of the animal, or on its skin.

But those genes may not be contamination. They may very well be in the host’s own genome. This is exactly what happened with the original sequencing of the genome of the anannassae fruitfly—the huge wolbachia insert was discarded from the final assembly, despite the fact that it is part of the fly’s genome.




  1. I found this interesting, from Mick at EVCForums (via Dr Adequate at JREF)

    Wolbachia is a bacterium that infects arthropods such as insects, spiders, isopods, etc. The infection has a variety of interesting effects on its carriers (including such things as biasing the sex ratio of offspring, transforming individuals into hermaphrodites, and other strange things varying from species to species) but as far as speciation is concerned there are two common effects of wolbachia infection that are of interest.

    First, an infected male is unable to fertilize an uninfected female. Second, if both male and female are infected, they must be infected with the same strain of the bacterium if fertilization is to be successful.

    Imagine a single species of insect, with two neighboring population A and B. If A gets infected and B does not, the result will be severely diminished gene flow between the two populations, because while males of population B can mate with members of either population, males of population A can only mate with members of their own population. The exact amount of gene flow reduction will depend upon patterns of migration between the two populations. Now if population B gets infected with a different strain of wolbachia, reproductive isolation will be complete, and in the long term speciation could follow.

    The exact mechanism whereby wolbachia effects these changes to mating success are currently unknown. We know that the bacterium does not cause direct changes to the DNA of its host because the effects can be removed by treatment with antibiotics. An overview of some possible mechanisms can be found at this article (pdf).

    The bacterium has been demonstrated to be the sole source of reproductive isolation between two species of parasitic wasp, Nasonia giraulti and Nasonia longicornis (see this article). So it does really happen in nature.

    As far as I know, cytoplasmic incompatibility caused by wolbachia is the only known source of reproductive isolation that does not depend on mutations to the species’ genome itself. Whether it qualifies as “speciation without mutation” is a different matter. Obviously the phenomenon requires different strains of wolbachia to exist if it is to generate complete reproductive isolation, and these different strains could only arise through mutation of wolbachia. So you might get speciation in an insect, driven by mutation of its wolbachia parasite rather than by mutation of the insect itself. Mutation is still at the bottom of it all.

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