Bacteria selectively kill males

By Victoria Gill
Science reporter, BBC News

The brood sac of an infected female (left) contains far fewer embryos

Bacteria that infect insects kill male embryos and themselves in "adaptive suicide", say researchers.

Wolbachia bacteria inhabit the cells of many small creatures, and females can transmit that infection to their offspring in their eggs.

Scientists now report in the Royal Society journal Proceedings B that the bacteria selectively kill unborn males, to increase the number of females born.

The infection also reduces the total number of offspring.

Wolbachia are cellular endosymbionts, meaning that they can only survive inside the cells of their hosts.

They are very successful and widespread. Researchers estimate that two thirds of insect species harbour the bacteria.

They have also been found in nematode worms, and non-insect groups including spiders, scorpions and pseudoscorpions.

Males are an evolutionary dead end for the bacteria Jeanne Zeh University of Nevada, Reno

The team, led by Jeanne Zeh at the University of Nevada, Reno, in the US, looked specifically at infected pseudoscorpions - Cordylochernes scorpioides.

They found that the infected females gave birth to an average of 20 daughters for every son.

To discover if this strongly female-biased brood was caused by the bacteria, the scientists cured some of their infected animals.

"The infection can be cured using (the) antibiotic tetracycline," explained Dr Zeh. "And this treatment restored the sex ratio to 1:1."

She told BBC News that Wolbachia "engage in adaptive suicide".

"Maternal resources originally allocated for developing male embryos are shunted to females when the males die, which increases the number of female offspring that can be born to infected hosts," said Dr Zeh.

Males cannot transmit infection to their offspring because, while egg cells are large and have lots of cytoplasm, sperm cells are comparatively tiny and have very little cytoplasm, she explained.

The combined weight of the embryos can exceed that of the mother

Dr Zeh continued: "Males are an evolutionary dead end for the bacteria. So they engage in gender manipulation to bias sex ratio in favour of females."

But this selective killing is very damaging for the host.

"Infected hosts give birth to only 62% as many nymphs (or offspring) as tetracycline-cured females," said Dr Zeh.

"The frequency of Wolbachia infection in the populations we have studied is currently pretty low - about 10%.

"If the Wolbachia frequency increased to a very high level, the host population could go extinct due to a shortage of males."