Sex, flies and lateral gene transfer

Like all parasites, Wolbachia engages in an intimate relationship with its host. After all, getting up close and personal is what parasites do. Wolbachia, a bacterium, is small enough to live inside the reproductive cells of its host, where it causes havoc by interfering with sex determination, killing males and inducing parthenogenesis. (That's havoc for the host, of course. Wolbachia does just fine from the manipulation. It is passed on to subsequent generations in the eggs, which is why it biases host populations towards females. More females, more eggs, more Wolbachia.)

But the relationship between the bacterium and its host is even more intimate than that.

In 2003, a team at the University of Tokyo investigated what they thought were three strains of Wolbachia in adzuki bean beetles (Callosobruchus chinesis). They treated the beetles with antibiotics to rid them of all bacteria but found that only two of the three strains responded. Each time they screened the beetles for the presence of bacterial genes, the results came up positive for the third strain. But it wasn't because this lineage of Wolbachia had developed antibiotic resistance. It hadn't been in the beetles at all during the study. What the team detected was a fragment of Wolbachia genome, which had been incorporated into the X chromosome of its host.

Bacteria swap genes all the time but shifts between bacteria and multicelled organisms are thought to be very rare. Or were thought to be very rare.

Following this discovery, scientists at the University of Rochester and the J. Craig Venter Institute decided to screen host genomes for Wolbachia genes. They weren't quite prepared for what they found. The genome of the fruit fly Drosophila amanassae doesn't simply include the odd foreign gene. It contains what appears to be the whole bacterial genome. At some point, the parasite's genome has become integrated with that of its host.

Wolbachia genes transferring to host's DNA
(Nicolle Rager Fuller, National Science)
Larger image.

Professor Jack Werren (University of Rochester) puts the discovery into evolutionary perspective:

In our very own cells and those of nearly all plants and animals are mitochondria, special structures responsible for generating most of our cells' supply of chemical energy. These were once bacteria that lived inside cells, much like Wolbachia does today. Mitochondria still retain their own, albeit tiny, DNA, and most of the genes moved into the nucleus in the very distant past. Like Wolbachia, they have passively exchanged DNA with their host cells. It's possible Wolbachia may follow in the path of mitochondria, eventually becoming a necessary and useful part of a cell.

So what was once considered exceptional — lateral transfer of genes between bacteria and animals — may turn out to be a common occurrence. Wolbachia infects a wide range of insects, arachnids and crustaceans. How many of those host species have incorporated all or part of the bacterial genome into their genetic make up? And how many of those transferred genes — if any — have a role in their new location?

Read more about the story at the University of Rochester's news page and at news@nature.com.