A bacterium in an insect can turn males into females

Wolbachia is passed only through eggs. Males are a dead end for it. In different hosts it kills male embryos, lets females reproduce without mating, or converts genetic males into females.

Wolbachia lives inside the cells of a large fraction of insects and of many other arthropods, and it is inherited the way mitochondria are: in the cytoplasm of the egg. A son that carries it does not hand it on. The bacterium has evolved a set of answers to that problem, and which answer you see depends on the pair of species. It can kill male embryos so the infected sisters eat better. It can make an unmated female produce daughters. It can sabotage a mating between an infected male and an uninfected female, so infected mothers outcompete the rest. And in woodlice and some insects it can make a genetic male develop as a female, ovaries included, who then lays eggs full of Wolbachia.

The tricks are real and they are specific. There is no single Wolbachia that does all of them in every host, and a great many infections sit quietly. What the quiet ones and the loud ones share is the direction of the interest. A parasite that can only leave through the egg is not a neutral passenger in the question of sex. It has been rearranging that question, species by species, for a very long time. Laboratory workers now use one strain to crash mosquito populations. The method is ours. The invention was the bacterium's.