Genes caught skipping from bacteria to beetle

Tokyo team claims first direct evidence of horizontal gene transfer.
27 September 2002

KENDALL POWELL

Wolbachia lives inside many insects' cells © Richard Stouthamer

Researchers think they have caught a set of bacterial genes that jumped ship and relocated to the genome of a Japanese beetle.

They could be the first to witness natural horizontal gene transfer between a bacterium and an animal. Although many researchers suspect this sort of gene movement occurs, no one had stumbled across evidence as direct as this before.

The event might help clarify whether similar integration has ever occurred in the human genome - and if so, when. It could also shed light on whether such exchanges are likely between genetically modified foods and the bacteria in our guts. But many sceptics remain convinced that both these scenarios are highly unlikely.

The team, based at the University of Tokyo, say they have found 11 or so bacterial genes inserted into a chromosome of the adzuki bean beetle Callosobruchus chinensis. The genes are from the parasitic bacterium Wolbachia, which lives inside the beetle's cells and those of many other insects.

"Many researchers have been anticipating this kind of data between Wolbachia and its hosts, but so far nobody has found as much data as we have," says Natsuko Kondo, the graduate student who discovered the migrated genes.

While other cases of possible gene transfer "are open to different interpretations, this one is fairly clear cut," says James Cook, an evolutionary biologist who studies Wolbachia at Imperial College in London. "They know where it came from and where it went to."

Pass notes

Kondo first noticed that she could not rid the beetles of their Wolbachia 'infection' even after several rounds of antibiotics. Normally, Wolbachia passes from one generation to the next through egg cells and is therefore inherited only from the mother. But Kondo also noticed Wolbachia 'infection' inherited from the father.

This led the researchers to think that perhaps the beetles weren't infected with bacteria at all, but instead just carried some bacterial genes in their chromosomes. A test that found only a few Wolbachia genes rather than the whole genome backed this up. And of those genes, one appeared to have insect DNA as its next-door neighbour.

Cook says the transfer could have taken place within the lifetime of the beetle, or roughly in the past million years. Even this latter would be recent enough - evolutionarily speaking - to observe with lab techniques because the transferred genes are mostly still intact.

But Jonathan Eisen, who runs the Wolbachia genome project at The Institute for Genomic Research in Rockville, Maryland, is not convinced. "There's no smoking gun," he says. Transfer probably has occurred, says Eisen, but the evidence is circumstantial.

If this case does hold up, it could fuel the debate about whether bacterial genes inserted into food crops could creep into another organisms. "People raising alarms will seize upon this and say this illustrates the danger," says Michael Syvanen, an expert on gene transfer at University of California, Davis. On the other hand, he says, those in favour of genetic engineering "will point to it and say this is a natural phenomenon."

These arguments could be a bit premature either way, Cook points out. "There's no indication that the transferred genes are actually working," he says. "It's very likely that they just become a bit of junk even if they do get in."