Almost everybody will be familiar with the chirp of male crickets as they attempt to woo a prospective mate, but scientists have stumbled upon two species of cricket that have stopped “singing” altogether, providing a remarkable example of adaptation and evolution in action.
Male crickets usually rub the top of one of their wings along a line of small teeth like protrusions on the bottom of their other wing to create that unmistakable chirping song. This is a call to females in the area that the male wants some romance, but there is one main disadvantage to this: if there are predators about, this cricket song will put the male in danger of being discovered and killed.
In the past, evolution seems to have turned a blind eye to this danger because the risk of predators is relatively low compared to the overall number of males, which has been so abundant that even when individuals are lost, the larger gene pool continues to thrive.
But what if a predator emerged that was particularly dangerous and abundant? A parasitoid fly (Ormia ochracea), which is attracted by the very song the crickets use as part of their mating ritual, fill that roll well. The fly’s larvae burrow into the cricket and then grow inside the insect until they emerge a few weeks later, killing the cricket in the process. This presents a problem: how do the cricket’s genes survive when the very act of passing on those genes has become so very dangerous?
Almost a decade ago, male crickets appeared on the Hawaiian islands of Kauai and Oahu that, suddenly, were eerily silent. Upon closer inspection by University of St Andrews researchers, it turned out that the crickets’ wings had been altered by subtle genetic variances that meant they were now flat and, while still allowing the crickets to fly, didn’t produce sound.
Publishing in the Cell Press journal Current Biology, the researchers detail how they were able to track this change. Using DNA samples, they employed a slicing technique that gives them the ability to look at small fragments of DNA and detect and analyze genetic markers and distinct regions of the genome. Lab experiments then showed that the silent wings could be narrowed down to single genes. This is where something even more remarkable cropped up.
The scientists had first assumed that the silent crickets evolved on one island and somehow migrated to the next, for instance by hitching a ride with tourists. Yet, what they found was that the so-called “flatwing” trait actually occurred through similar but different changes to the two species on the two islands, with no intermingling having occurred. To stress that: the crickets evolved the silent “flatwing” trait entirely separately but at roughly the same time over a very rapid 20 year period. A key clue that these were different changes was that the physiological differences in the males’ wings are very slightly different depending on which island they come from.
“There is more than one way to silence a cricket,” researcher Nathan Bailey is quoted as saying. “Evolution by natural selection has produced similar adaptations from different genetic starting points in what appears to be the blink of an eye in evolutionary time.”
This, the researchers say, is an excellent example of what is known as convergent evolution: where different mutations produce functionally nearly identical results. The fact that this happened so quickly and so closely has excited the researchers because it gives them a chance to study this kind of rapid evolution in superb detail.
You might be wondering one important thing, though. How do the silent crickets manage to attract a female if they can’t sing? Well, they can’t attract females so they have to be devious. On both islands there are still crickets without the mutations that give them silent wings. As a result, the silent crickets have to position themselves near to the still singing males. They then wait for a female to turn up and try to intercept. This might be slightly more circumspect and cumbersome than the song method, but because it means they do not attract the fly larvae, it’s a great deal safer.
Now, scientists will have the opportunity to see how how the flatwings continue to develop and whether their adaptation will give them an evolutionary advantage or if this is ultimately a neat but evolutionary dead end.
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