A new player has emerged in the war of the sexes: a bizarre 'ultra-selfish' bacterial parasite that hijacks animal reproduction to promote their own existence by favouring female hosts over males, according to a British report.

Biologists increasingly recognise that many such symbiotic parasites - capable of being inherited - can selectively kill their male hosts, turn them into females or induce virgin births of all-female offspring, said geneticist Dr Michael Majeruss of Cambridge University.

"The proportion of insect species that harbour ultra-selfish inherited bacteria of one genus, Wolbachia, has been estimated to be between 15 and 20%," Majerus writes in Microbiology Today. "If other groups of parasitic microbes are included, it may be that the majority of invertebrates host such symbionts."

Apart from bacteria, a wide range of viruses and protists - a group of living things that includes algae, some fungi and protozoans - has also evolved the ability to invade the sex cells of their hosts and be transmitted to the next generation, he said.

These parasites live in the cytoplasm of animal cells and - because sperm cells have very little cytoplasm, and egg cells have a lot - they are inherited through the female line. To advance the parasite's interests, they employ strategies to favour females at the expense of males. This includes feminisation, inducing asexual reproduction and the 'assasination' of males.

In brine shrimps and woodlice, they can inhibit the development of glands that produce male hormones, which causes a host with male sex chromosomes to develop as female.

In some wasps, they can cause eggs to develop without fertilisation - resulting in virgin births of females only. Scientists have observed asexual wasp species, which have never been known to produce males, suddenly started doing so after being treated with antibiotics to kill the parasite.

Many male-killing inherited microbes have so far been found in a diverse range of insects, including ladybirds, butterflies, moths and milkweed bugs, Majerus said.

In ladybirds, males are killed early in their development and cannibalised by newborn females, he said: "Ladybird eggs are laid in clutches, and neonate female larvae consume their dead brothers, gaining an extra rich meal."

In one ladybird species a 'male-rescue' gene was found to have evolved to reduce the impact of that strategy, which Majerus said is one of many signs that female-promoting parasites can have significant effects on the evolution of their hosts.

Distorted male-female sex ratios, for example, have profoundly altered the social dynamics of some species. In some colonies of nymphalid butterflies, over 90% of females are infected with a male-killing bacterium, and less than 5% of the population is male.

"The result is that a large proportion of females, over 80% in some colonies, die virgin," he said. "This has led to complete sex-role reversal, with females aggregating and competing with one another at lekking [courtship display] sites, and males visiting these sites to find mates,"

Male-killing can also change the way sexually transmitted diseases (STDs) are spread, he said. The two-spot ladybird, for example, not only plays host to four male-killers but two STDs. The species is highly promiscuous, and because males - due to their rarity - are more likely to have many mates, they are both more likely to contract and to pass on STDs.

"Given the impacts that these microbes have on the evolution of their hosts, it is essential that we increase our understanding of their biology, and that we start viewing them as an integral and influential part of the heritable material of their hosts," Majerus said.

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