When bacterial infection is a form of protection

Last week the Guardian reported on a relationship evolving between a bacterium and a fruit fly in the US. The relationship is highly beneficial as it protects against a parasitic nematode.

Parasitic nematode worms Howardula aoronymphium infect female larvae of the fruit fly Drosophila neotestacea, where they prevent the eggs from developing. The worms also reduce mating success and survival in male flies.

"The mother worm gives birth to literally thousands of live young in the abdominal cavity of the fly," said John Jaenike of the University of Rochester in New York, who led the research. "From there they work their way into the ovaries. The female fly thinks she's laying eggs, but she's actually laying nematodes," he said.

But the flies have a trick up their sleeve. For years researchers have suspected that Spiroplasma, a bacterium that is passed directly from mother to larva through the egg, is protecting the flies.

They found that parasitised females became more fertile after exposure to Spiroplasma. The nematode-carrying flies that had not been infected with bacteria became sterile.

Spiroplasma seems to protect the flies from the ill effects of worm infection in a textbook example of symbiosis. Both the fly and the bacterium benefit from the situation: the fly gets protection from worms, and the bacterium gets somewhere to live.

The symbiotic relationship between fly and bacterium seems to have been spreading across the US over the past three decades. "The effect in the wild is absolutely whopping," said Jaenike. "Back in the 1980s every nematode-infected fly was completely sterile ... The situation is different today. They were getting clobbered in the 1980s but now they have quasi-normal nematode infection levels."

In the 1980s, only 10% of wild flies were infected with Spiroplasma in the eastern US. That figure had jumped to 80% when a survey was conducted in 2008. Wild females infected with the bacterium had more than 10 times as many eggs in their ovaries as uninfected females.

The bacterium appears to be spreading from east to west across the country, so Jaenike wants to repeat the survey this year to find out if Spiroplasma has reached the west coast.

"The speed of the spread is interesting, but what I personally find astounding is that we can see it," said Jaenike. "Something happening in 20 years is a split second on an evolutionary timescale."

He compared the way the bacterium is passed from generation to generation of flies to how mitochondria are passed from parent to child in humans. "What we are seeing are the early stages of a beneficial relationship. Who knows where it will lead?"

Well there you have it I could go into a rant about the heresy of evolution but it wouldn’t really be that convincing now!