Males are waste to Wolbachia. These tiny bacteria live inside the cells of many different kinds of insects, and as their hosts reproduce the Wolbachia can hitch a ride from generation to generation. But Wolbachia are only transmitted in the female’s egg cells; sperm are much too small to fit any bacteria inside. This means that for Wolbachia, males are a dead end, unable to transfer any bacteria on to the next generation. To maximize the number of eggs and spread themselves as far as possible, Wolbachia have evolved several creative strategies to eliminate males from a population. Their biological mission is to ensure the dominance of females, from the inside.
Bacteria that live inside other cells are called “endosymbionts” and they can have a very big impact despite their small size. Some endosymbionts are helpful to their hosts, exchanging nutrients for a cozy place to live. Other endosymbionts can be harmful: Chlamydia, for example, is a bacteria that can only survive inside human cells and causes disease. As they pass from person to person or from generation to generation, endosymbionts change and evolve with their hosts. Sometimes the host and endosymbiont evolve together into a new organism, a composite creature where each part is unable to live without the other.
Lynn Margulis was a microbiologist who promoted the idea that all plant and animal cells evolved as composites between bacteria and a bigger host cell. The chloroplasts and mitochondria that power plants and animals were once free-living bacteria that created new branches in the tree of life. Margulis’s great insight was that symbiosis and cooperation could be as important to evolution as competition and struggle. If the popular imagination of evolution is dominated by stories about selfish genes, rugged individualism, and the survival of the fittest, endosymbiosis suggests that there might be other ways to think about it.
But, back to Wolbachia:
Working from inside an insect’s sex cells, Wolbachia use a variety of techniques to ensure the birth of as few males as possible. One of these strategies for favoring females is what microbiologists term “male killing.” If a female ladybug infected with Wolbachia lays a batch of eggs, the bacteria inside the eggs will sense whether the embryo is male or female and then kill the males. When the female ladybugs eventually hatch, their dead brothers serve as their first meal. Over time, the Wolbachia can create a population that is 80-90% female, approaching the minimum number of males that can still maintain the species.
Male killing is not the only way to create a female-dominated species. Sex determination is complicated and can be affected by many different factors — genes, chromosomes, hormones, and the environment influence how an animal develops as male, female, both, or somewhere in between. With so many ways to produce males, Wolbachia need many ways to stop them.
In some crustaceans (which are more closely related to insects than you might like to think), developing embryos are female by default. If an embryo has particular chromosomes, it will produce a hormone that turns the embryo into a male. In these creatures, Wolbachia don’t have to kill males — they can just block the action of this hormone, keeping the embryo in its default female state. Biologists call this “feminization.”
Some insects like ants, bees, and wasps have a very different mechanism for deciding sex. In these bugs, all eggs — whether or not they are fertilized — will hatch. What fertilization does is determine the sex of the offspring. Fertilized eggs turn into females; unfertilized eggs turn into males. To prevent the birth of males, Wolbachia have figured out how to turn the unfertilized eggs directly into females. In some wasps, this means females can be born directly from other females, with no need for males at all. Wolbachia have made these species 100% female.
This female-only population can be thought of as a new species, unable to mate with other uninfected wasps and thus evolving as an independent lineage. Like Lynn Margulis’s insight that endosymbiosis was a major player in the evolution of all plant and animal cells, biologists are beginning to understand how Wolbachia play a role in the evolution of new species of insects. By controlling reproduction, Wolbachia dramatically change the behavior and population structure of many species, influencing where, when, how, and with whom they can mate and thus shaping the evolutionary trajectory of their hosts.
Wolbachia primarily act on insects and other shelled creatures, but they might also be able to have an effect on human societies — not by infecting our reproductive organs and eliminating males from the population, but through a kind of conceptual endosymbiosis. Scientific concepts evolve inside of societies, influenced by many social issues and influencing cultural norms in turn. In humans, cultural ideas about sex, gender, sexuality, and reproduction can be extremely powerful, having as much if not more of an effect on our sex lives as genes, chromosomes, and hormones. In particular, ideas from biology about what sorts of sex and sexualities are “natural” have frequently been used to discriminate against people who might deviate from the norm.
Likewise, human scientists have read human social norms about sex and sexuality into the microscopic behavior of cells and the reproductive behavior of plants and animals. In “The Egg and the Sperm,” the anthropologist Emily Martin shows how gender stereotypes about aggressive males and passive females have shaped the metaphors that biologists use to explain the science of fertilization, even though these stereotypes have nothing to do with the actual behavior of eggs and sperm. Implicit heterosexism has also affected the science of genetics; in “Mutant Sexuality: The Private Life of a Plant” the historian of biology Luis Campos writes about chromosomal behaviors that don’t match the binary, heterosexual norm and were thus written off as strange “unnatural” aberrations, preventing scientists from understanding the full diversity of nature. This amazing diversity is cataloged in Joan Roughgarden’s book Evolution’s Rainbow, which challenges the stereotypes about gender and sexuality that find their way into biology, giving many examples of animal behavior that can change what we think of as “natural.”
Cultural norms and scientific understandings about sex and evolution have changed dramatically over the past 150 years since Darwin first published On the Origin of Species and Mendel began crossing pea plants. Science and society evolved symbiotically, and continue to evolve as we learn more about how bacteria and animals interact, how sex and sexuality evolve and change, and how when it comes to sex, there’s no one “natural” way to do it. Wolbachia can help us to change our minds about sex and evolution without changing our gonads.