Genetic War Against Male-Killing Bacteria

Parasites are not nice guys and many times they can even kill. But what about a parasite killing only males? All of them or not? And they talk about sexual discrimination...

This is the case of a butterfly, Hypolimnas bolina (eggfly, blue moon butterfly), found in two Samoan Islands (Polynesia) and the Wolbachia bacteria that nearly wiped out all the species' males. But recently, in just a few years, the butterfly has developed a way to get rid of the bacteria's sex control.

For over a century, the bacteria had been killing almost all the male larvae of the butterfly, but males made a comeback in 2006, being almost as many as females and this change took place in five years or less (10 butterfly generations).
This is a "very, very fast evolutionary change, possibly the fastest ever monitored," said co-researcher Sylvain Charlat of the University of College London.

One century ago, there were almost no eggfly males in the Polynesian Islands. Later, Wolbachia, one of the world's most common parasites of the insects, was found to cause this. The bacteria invade the reproductive systems, controlling the insect's development. But these bacteria were not interested in maintaining males, as they can only be transmitted from generation to generation inside females' eggs, as sperm is unable to accomplish this task.

In 2001, a team led by Gregory Hurst of University College London did a census of H. bolina and discovered that the population was over 99 % female.

Most animal species have a sex ratio male: female of about 1:1, except social insects like bees and ants, with hives and nests dominated by females (termites' workers are equally males and females). But the same situation is found in some insects and spiders infected with Wolbachia or other sex controlling bacteria.

The very few lucky eggfly males might have come from non-infected mothers or escaped death by chance.

"Those males would have a bonanza of females to mate with. So any mutation that helped males survive would give those butterflies a huge advantage and could therefore spread quickly", said John Werren of the University of Rochester in New York, not involved in the new study.

A 2005 census did by Hurst's team didn't encounter any males at all, but in 2006, the researchers discovered almost as many 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 believe.

It is not clear how this happened and Wolbachia still could soon regain through its own mutations the male-killing ability.
But the current mutation of the butterflies "could really spread at warp speed. Evolution doesn't have to take eons. It can take place in a couple of years," said John Jaenike of the University of Rochester, not involved in the study.