Genomes within genomes

Bacterial parasite's DNA discovered in fruitfly host.

Invader: Wolbachia bacteria (yellow) inside a developing fruitfly egg (red). Credit: SCIENCE

Another team of genome researchers at the J. Craig Venter Institute in Rockville, Maryland, which has been investigating the DNA of a rather less salubrious organism, this week reports a surprise discovery: the DNA of fruitfly Drosophila ananassae contains the entire genome of a parasitic bacterium of the Wolbachia genus. Smaller parts of the parasite's genetic material also turned up in worms and wasps.

Bacteria commonly swap DNA with each other. But transfer of bacterial genes into animals was thought to be rare. The new work, published in Science (J. C. Dunning Hotopp et al. Science doi:10.1126/science.1142490; 2007), suggests that gene flow from bacteria to animal hosts happens on a larger scale and more commonly than suspected. And it hints that the bacterial genome may have provided some sort of evolutionary advantage to its host. “You're talking about a significant portion of [the fruitfly] DNA that is now from Wolbachia,” says Julie Dunning Hotopp, who led the study. “There has to be some sort of selection to carry around that much extra DNA.”

But Dunning Hotopp's former colleague Jonathan Eisen of the University of California, Davis, contests this. “One cannot conclude that some DNA is advantageous simply because it is there,” he says.

Up to 75% of insect species are plagued by Wolbachia, which lives inside testes and ovaries and passes from one female generation to another through infected ova. To ensure its spread, Wolbachia can skew insect birth ratios towards females and even prevent infected males from successfully mating with disease-free females. The bacterium's close association with egg cells means there's ample opportunity for bacterial DNA to get permanently sewn into a host's nuclear genome, says Dunning Hotopp.

When Dunning Hotopp and her colleagues analysed the DNA of D. ananassae uninfected by Wolbachia, they found 44 of the 45 Wolbachia genes they searched for. Because these selected genes are so widely spread throughout Wolbachia DNA, this suggests that the rest of its more than 1-million-base-pair genome is also likely to be found in fruitflies.

Many of the Wolbachia genes were infiltrated by strands of insect DNA that jump around the genome, and so are unlikely to be functional. But the researchers showed that at least 28 of the bacteria's 1,206 genes are active in the flies. More genes that have seeped from bacteria into animals are certain to be found, the researchers say, particularly in reptiles and amphibians. But finding bacterial genes in mammals is unlikely because no bacteria are known to infect their sperm and egg cells.

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Callaway, E. Genomes within genomes. Nature 449, 6 (2007). https://doi.org/10.1038/449006b

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