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.
Tags: bacteria, Christina Agapakis, misandry is real…real cool, science
This is so good that I couldn’t figure out whether it was real or a beautifully done satire.
QUESTION: if we’re taking the ladybugs as an example, how does wolbachia know when to pull up at that 80-90% mark, leaving enough males to maintain the species?
My guess: Perhaps strains of Wolbachia that become more effective than that at preventing males die out, while the strains that are in the 80-90% effective sweet spot thrive.
Bingo. It’s such an amazing aspect of evolution, that it doesn’t have to “know” anything. It’s either do or die.
Science! This was really great, Christina!!!
So the crustaceans that Wolbachia feminizes are eventually able to reproduce as females, despite having male chromosomes, or what?
Also, please tell us more about this male tear cheese; it sounds wonderfully salty.
Very interesting article! My 11-year-old was telling me about Wolbachia this weekend. Her science class is doing a unit on parasites, and it’s all very cool stuff. If only I could train her not to talk about the gory details of Wolbachia et. al. at meal time…
Ugh this weekend and today at work have been FULL of wacked-out men yelling about cocks and killing women PLUS entitled white guys (always, invariably) who want us to sell their self published books and get really fuckin’ pissed when we don’t take them. I am perfectly OK with Wolbachia taking these men out.
Promise me if you get comments about how this means scientists were wrong about evolution and isn’t it POSSIBLE that God created the world in six days you’ll post them? Pretty please? I need some entertainment.
Ooh, this is weird and wonderful and the most interesting article about bacteria I have ever read.
Also, that picture reminded me that I spent quite a lot of today wondering whether I would have been a suffragist or a suffragette if I had been born 100 years earlier. Probably a suffragist due to extreme confrontation-avoidance, but who knows how strongly they would have felt about something in another lifetime! (Anyone else wanna speculate about their past selves?)
Oh mah gawd. Emily Martin reference! I did a whole survey of the gender subjectivity of “hard science” in college and it was bloody goddamn great. About once a month when I pull out the old misandrist soapbox, I get on about the cultural construction of hard science and how male scientists have built gender differences into how they study people, etc.
The other anthropologist of science that I loved was Sharon Traweek, Beamtimes and Lifetimes: The World of High Energy Physicists about the different ways that culture and views of masculinity can influence even the “purest” hard math science of physics. The ways they deal with the machines, set up experiments, measure radiation dosage and put the badges differ between Japan and the US, and that in turn helps illustrate how your social constructs can also influence your “pure research” design, safety parameters, and view of something theoretically objective. etc.
She is the absolute coolest! I may have forced a male scientist I dated to read a the egg and the sperm essay.
Early histories of sexual difference and gynecology are also amazing. For instance, pre-enlightenment gender was a spectrum with men at the apex and vaginas were drawn as inverted penises, despite the fact that they were drawing from life. The upside is that you get a bunch of folk tales about women who leapt a little too widely and became men. And that one time male scientists assumed periods were exactly the same as heats, because dogs women what’s the difference am I right?
There’s this whole discussion of the way that colonialism, racism, sexism and science starts to go brutally hand in hand around 1700 or so, with scientists opining that women’s brains and skulls are the same as children’s, mental patients and people who live basically anywhere that isn’t europe, while the skulls of a white man are basically the only ones that aren’t “deformed/limited”. Aside from faking their measurements a whole bunch, this point in time is when things sort of lock up and cement the centuries of racism and sexism to come.
Especially interesting in reading this was a part that I recall about the role the hostesses of the French Salons had in bringing together scientists and intellectuals and the fact that nearly led to a model where women would have been more involved in science creation, but they lost out to the British coffeehouse/university model which was men only. It would be interesting if today the salon model came back. I wonder what we’d get out of it. Probably a lot of research into the benefits of a juice diet…
This is absolutely fascinating and wonderful. Is Christina our Gal Scientist? Please say Christina is our Gal Scientist.
Can we keep her, Mallory? Please please please?
It was submitted outside the Gal Science series, but I will ALWAYS be happy to have her back!
This is amazing. Both the existence of wolbachia and the musings on theory.
Why, hello, sperm-hunting bacteria. Any chance you’d like work as human birth control? Or are we forever stuck with the chemical-laden, blood-clot-producing birth control we have?
this is so great!! Behavior changing parasites are awesome in general but WOLBACHIA, YOU ARE A QUEEN OF PARASITES.
Late to the party, but this is so cool. What a well-written article! :>
This was excellent. I have a lot of reservations about “Evolution’s Rainbow.” She represents her views as revolutionary, but almost all the “exceptions” to classic sexual selection theory she catalogues had been noted and extensively researched elsewhere (for example), and many are in no way contradictory to the theory she claims to discredit. It pretty grossly misrepresents or glosses over the work of a lot of people working in animal mating systems.
Very interesting article! My 11-year-old was telling me about Wolbachia this weekend. Her science class is doing a unit on parasites, and it’s all very cool stuff. If only I could train her not to talk about the gory details of Wolbachia et. al. at meal time…