Male-killer makes female butterflies promiscuous

17:50 05 February 2007
NewScientist.com news service
Roxanne Khamsi

Female butterflies become sexually promiscuous in the presence of a bacterium that fatally targets the male offspring of their species, a new study shows. However, the few males that survive become fatigued by the increased sexual demands of the females, and so release fewer sperm in each mating.

The unexpected findings could shed light on how the insect species can survive when there are only a few males available, the researchers say.

Greg Hurst at University College London in the UK, and colleagues, captured and studied Hypolimnas bolina butterflies from various islands in South Asia and the Pacific.

On some of the islands these butterflies suffer from a type of Wolbachia bacteria that specifically kills their male embryos. Other islands, meanwhile, remain free of the bacteria.

Sperm balls

About 25 female butterflies were captured from each of the various islands, which were dissected in search of empty “sperm packages”. Female butterflies receive a tiny ball of sperm each time they mate and store its empty casing inside them for the rest of their life.

By counting the empty sperm packages inside the insects, Hurst’s team knew how many males each female had mated with.

The researchers found that on the islands with abundant males, female butterflies mated only once between the time they hatched from their cocoon and died one month later. By comparison, the females living on islands infested by the killer microbes mated about five times before dying.

Lucky few

The fact that female butterflies could even find mates in the presence of male-killing Wolbachia stunned scientists. In some islands there was only one male for every 40 females. One would expect this type of sex imbalance to leave females deprived of a mate, explains Hurst.

“To our knowledge we’ve never heard of female promiscuity being caused by fewer males,” he says.

The finding is important, he adds, because it explains how these insect populations can survive even with hardly any males: “This means that the bacteria will not cause butterfly populations to crash – you can live with just a few males.”

But even though females on islands with male-killing bacteria mate often, the sperm packages they receive are smaller than usual. This is because the male butterflies on the islands become fatigued: “The size of the packages can go down by 50% in volume if the males are really tired,” says Hurst.

Journal reference: Current Biology (DOI: 10.1016/j.cub.2006.11.068)

Story

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."

Story

World's smallest butterfly confirmed in Shaanxi

A butterfly named Tongeia minima Shou et Yuan, discovered in Shaanxi Province in 2002, was recently confirmed as the smallest butterfly in China and the world, Monday, Aug. 27, 2007. (Photo: wenhui.news365.com.cn) Chinese butterfly export Shou Jianxin, who discovered Tongeia minima Shou et Yuan, shows off his specimen collections of small and large butterflies on Wednesday, August 29, 2007. (Photo: xawb.com) Story