The Tangled Tree: A Radical New History of Life

by David Quammen

These transferred genes came from bacteria in the genus Wolbachia, a group of aggressive intracellular parasites that infect at least 20 percent of all insect species on Earth. Wolbachia bacteria target the germline cells of the animals they enter, especially ovaries and testes, and, once established, a Wolbachia infection is passed from mother to offspring within her infected eggs. It does not usually pass within infected sperm. Wolbachia work around that constraint, the absence of sperm-to-offspring transmission, and proliferate themselves by manipulating the reproductive outcomes of their hosts. They do it in four different ways: killing male offspring before they can hatch, turning males into females, triggering parthenogenesis (virgin females delivering more females), and sabotaging the viability of uninfected eggs when fertilized with Wolbachia-infected sperm. The net result of their interference is to change the male–female ratio, shifting whole populations of insects quickly toward more Wolbachia-infected females producing more Wolbachia-infected offspring.