Crickets in two places fall silent to survive

To hide themselves from deadly flies, crickets on two Hawaiian islands have evolved an inability to sing.

Ten years ago, two years apart, males appeared on Kauai and Oahu with altered wings, which they would normally rub together to chirp and attract females.

New findings published in the journal Current Biology show that the wing changes are physically different and arose from separate mutations.

This makes the silent crickets a brand new example of "convergent evolution".

The killer flies have an unusual ability to pinpoint a cricket's location using sound.

After finding its victim - a male cricket, singing to attract a mate - a pregnant fly will spray baby maggots onto the cricket's back, which burrow in, feed, and emerge a week later leaving the husk of the hapless cricket behind.

The presence of these North American flies placed the crickets, themselves relatively recent arrivals from Australia, under pressure to adapt.

In less than 20 generations, a mutation that leaves males unable to sing spread to over 90% of the crickets on the island of Kauai.

Because they are mute, these "flatwing" male crickets are hidden from the parasitoid flies and escape being eaten by maggots. That triumph comes at a cost, however, since finding a mate is tricky without a voice. The silent types loiter near the few males still singing away, and intercept females for themselves.

Two years after the Kauai discovery in 2003, flatwing crickets were also found over 100km away on Oahu.

Researchers first assumed that the silent crickets had simply travelled the distance - with some help.

"An egg laid by a female in some soil could hitch-hike on someone's boot," said Dr Nathan Bailey, whose group at the University of St Andrews led the new study.

The idea that the trait had evolved twice, at almost the same time, seemed far-fetched. "It still seems amazing to me," Dr Bailey told BBC News.

The first clue was an observation that the mutant, silenced wings on the two islands had two different shapes.

Next, by doing crossing experiments with the mutant crickets, the researchers confirmed that both types of flatwing arose from a single gene on the X chromosome.

Finally, comparing a raft of other genetic markers between the two groups yielded convincing evidence that the two mutations had occurred independently.

"Up until my post-doc Sonia showed me the diagram of nearly non-overlapping genetic markers from each island... I was unconvinced either way about what we were dealing with," Dr Bailey said.

When the same feature evolves separately in two genetically distinct populations, scientists describe the process as convergent evolution. Another example is the "projectile tongue" of certain salamanders, which appears to have evolved independently on more than one occasion.

Dr Bailey said this is an unusual example, because it has happened "in what appears to be the blink of an eye in evolutionary time" and researchers can now watch as the story unfolds further.

"This is an exciting opportunity to detect genomic evolution in real time in a wild system, which has usually been quite a challenge, owing to the long timescales over which evolution acts."