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Butterfly Evolves Leg Up on Male-Killing Parasite

Mason Inman
for National Geographic News
July 12, 2007
 
The continuing battle between a butterfly and the bacteria that nearly wiped out all the insect species' males has taken a sudden and unexpected turn.

In just a few years, the butterfly has evolved a way to evade the bacteria's tightly controlling grip.

The findings show that evolution can strike in a flash, even after long periods of time with little change, researchers say.

For at least a century, according to the experts, bacteria called Wolbachia had been playing puppet master with Hypolimnas bolina butterflies found on two Samoan islands (see an Oceania map).

The bacteria had been killing off nearly all the male larvae of the butterfly, also known as the eggfly or the blue moon butterfly.

But males made a comeback in 2006, the researchers found, with nearly as many of them as females.

"We thought this kind of thing was happening, but we didn't know we'd be lucky enough to see it," said Sylvain Charlat of the University of College London, one the researchers involved in the new study.

The shift happened in five years or less—just ten generations for the butterflies—according to the new study, which will appear tomorrow in the journal Science.

This is a "very, very fast evolutionary change, possibly the fastest ever monitored," Charlat said.

Bonanza of Females

In the early 20th century, some visitors to Polynesian islands noticed that, curiously, there were hardly any eggfly males.

Later, researchers discovered that Wolbachia, one of the world's most common insect parasites, was the culprit. Wolbachia invade reproductive systems, allowing the bacteria to control insect development. (Related: "Suicide Grasshoppers Brainwashed by Parasite Worms" [September 1, 2005].)

It makes evolutionary sense for Wolbachia to kill males, because they're a dead end for the bacteria. Wolbachia can only pass from generation to generation inside females' eggs—sperm are too small for the bacteria to hitch a ride inside.

In 2001 Gregory Hurst of University College London in England and his colleagues did a census of H. bolina and found that the population was at least 99 percent female.

Most animal species are about half male and half female.

But social insects like bees and ants are an exception, with hives and nests dominated by females. So are the insects, spiders, and other species infected with Wolbachia or other bacteria that can distort sex ratios.

For decades, it seems that eggfly males were the lucky few. Either their mothers were among the few that did not harbor Wolbachia, or they escaped being killed by chance.

Either way, those males "would have a bonanza of females" to mate with, said John Werren of the University of Rochester in New York, who was not involved in the new study.

So any mutation that helped males survive would give those butterflies a huge advantage and could therefore spread quickly, Werren said.

"Warp Speed"

In 2005 Hurst, Charlat and their colleagues did a less formal count, and didn't find any males at all.

But when the team returned in 2006, they found almost as many as females as males. A single male may have developed a mutation that allowed it and its male offspring to evade Wolbachia's hold and pass on their genes, the researchers argue.

It's not yet clear how the butterfly does this or how long its evasive maneuver might work. It's possible that Wolbachia could soon evolve to regain its male-killing ability.

But for now, the mutant male butterflies apparently have a huge advantage over their ancestors.

In this way, the mutation "could really spread at warp speed" through the population, said John Jaenike of the University of Rochester, who was not involved in the study.

The work shows that "evolution doesn't have to take eons," Jaenike said. "It can take place in a couple of years."

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