Bacteria that live inside parasitic wasps eliminate all the male wasps in the population. Now, scientists have discovered one way they do it.Long ago, the bacteria borrowed sex-determining genes from other insects, and now use them to crank out proteins that turn nearly all the male eggs female, a study published in March in the journal Genome Biology found.”When we first identified this female-factor in the bacterial genome, we were really thrilled: this protein explained so many aspects of how the bacteria were able to ensure all female wasps,” study senior author Amelia Lindsey, an entomologist at the University of Minnesota, told Live Science in an email.The bacterium, Wolbachia, lives inside the cells of insects and can only be transmitted directly from parent to offspring via female eggs and not via sperm. As such, they benefit from a mostly female population. While scientists discovered Wolbachia living inside insect tissues 100 years ago, and found they were capable of skewing insect sex ratios around 30 years ago, they are only now beginning to understand how the bacteria manipulate insect hosts to promote transmission.Related: Newly discovered fungi turn flies into zombies and devour them from the inside outThis manipulation can have dramatic consequences. For example, when researchers in Samoa looked at a population of infected Hypolimnas bolina butterflies, they found 100 females for every male, all thanks to this microscopic hitchhiker.Over the years, scientists have found that Wolbachia rely on a large arsenal of tricks to increase the ratio of females in the population, including killing male embryos and preventing the development of eggs that are not carrying Wolbachia, and making genetic males develop female traits instead.Get the world’s most fascinating discoveries delivered straight to your inbox.”Microbes have evolved highly complex mechanisms to manipulate their hosts,” Hiroshi Arai, who studies insect evolution at the University of Liverpool in the U.K. but was not directly involved in the research, told Live Science in an email.However, one of the key ways Wolbachia ensures an all-female population is by manipulating host reproduction to produce only female offspring without the need for male fertilization.Wasps of different sexes carry a different number of chromosomes. Males have only one set of chromosomes, and therefore, one copy of each gene. They develop from unfertilized eggs. Females, meanwhile, have two copies of each gene and develop from fertilized eggs.Normally during reproduction, chromosomes are duplicated, then split into separate cells, leaving daughter cells with half the number of chromosomes of parent cells. Using mechanisms scientists still don’t fully understand, Wolbachia somehow prevents that last step, causing females to develop from unfertilized eggs that would otherwise have developed into males. As a result, all the wasps will carry and transmit the bacteria to their offspring.”Wolbachia is less manipulating the host sex determination, but rather becoming an integral part of it,” Tobias Engl, a project leader in insect symbiosis at the Max Planck Institute for Chemical Biology in Germany, told Live Science. Engl was not involved in either study.But exactly how Wolbachia forced sex determination remained a mystery.The new research answers part of that question.A second study, published in June in the journal Current Biology found that hidden inside the DNA of Wolbachia is a gene that seems to have originated in insects, not bacteria. This gene codes for a protein that seems to play a key role in sex determination.”It was nice to see that work from Dr. Luan’s group and mine converged on the same protein responsible for the insects developing as females,” Lindsey told Live Science.Both studies found that the proteins made by two of these genes are very similar to sex determination genes found in other insects.This suggests that the genes Wolbachia uses to eliminate males are borrowed from another insect it infected deep in its evolutionary history.Wolbachia are already being used to reduce the number of insects that carry diseases such as malaria, dengue fever and Zika fever. And the parasitic wasp, Encarsia formosa, is used to eliminate whitefly, an agricultural pest. Research that provides greater understanding of how Wolbachia control insect populations can further improve the development of these projects, Arai told Live Science.