Lucy Cooke on the clever bacterium that can change a woodlouse’s sex
As a kid I had a huge affection for the humble woodlouse – a curious garden invertebrate with a love of damp, dark hiding places and a cute habit of curling up into a ball when threatened. What I never knew about these defensive little bugs is that they’re in an evolutionary war with a bacterium that’s intent on turning them all into females.
Why? Well, Wolbachia, the bacterium in question, needs female woodlice in order to proliferate. It infects the cells of woodlice and only passes down the female line. Mothers can transmit the bacterium to their young but daddy woodlice are dead ends. So Wolbachia has evolved to do something very devious indeed: turn useless male embryos into useful female ones by interfering with the development of hormone-producing glands. So any infected male embryonic woodlice develop into females instead, regardless of their sex chromosomes.
Woodlice have a different sex determining system to the XX/XY system found in humans. Males have ZZ sex chromosomes, and females are ZW. Unless they are infected by Wolbachia, in which case ZZ males become females. In some populations affected by Wolbachia the W chromosome has disappeared altogether, leaving the bacterium alone in charge of determining the sex of each woodlouse by its presence or absence.
That a bacterium could take complete control of determining the sex of its host is an astonishing twist of evolutionary fate. But the Wolbachia versus the woodlouse story gets twistier still.
In 1984, scientists studying the woodlouse found some populations with ZZ females that were uninfected by Wolbachia. How could it be possible for genetic males to become functioning females without
