There is a certain genre of post-apocalyptic sci-fi which considers the following senario: something catastrophic happens and all the males in the world are killed off. It sounds like a feminist fairy tale or a purient male dream (that is, if you’re the last man left alive). But is this just a wild, impossible fantasy or is there any basis in reality?
It all starts out with the humble fruit fly, Drosophila, known to biology students as the workhorse for genetics studies. One species of the fly, Drosophila willistoni, is found in a variety of tropical places in the new world. Due to this species’ adaptability and possession of the greatest chromosomal polymorphism among Drosophila, it is an ideal candidate for studying population genetics and evolution. In the 1950s, some female D. willistoni caught in the wild exhibited a very strange trait–all of these flies’ progeny were female. Virtually no male flies were born.
Donald F. Poulson, an important Drosophila embryologist since the 1930s, worked with these flies that had progeny with skewed sex ratios. It was first thought that this strange trait was hereditary. However, Poulson showed in the late 1950s that this wasn’t the case–a skewed sex ratio was due to an infectious agent and not a genetic trait. When material from dying eggs that originated from flies with the skewed sex ratio condition was introduced to female flies without the condition, those female flies were converted to that condition. In 1960, Bungo Sakaguchi in collaboration with Poulson observed that the haemolymph of D. willistoni females producing only female progeny was teeming with microorganisms that were identified as spirochetes. Later, the bacteria was correctly identified as a member of the spiroplasmas.
Spiroplasmas are helical microbes originally discovered as plant pathogens, particularly for corn and citrus plants. For the periwinkle plant, it is known that spiroplasmas are transmitted through an insect vector, the leafhopper. Many spiroplasmas reside harmlessly in the insect gut, but some are outright insect pathogens. S. melliferum and S. apis are honeybee killers. The spiroplasmas found in D. willistoni, later named Spiroplasma poulsonii after the original discoverer, are also killers–although they kill in an extremely peculiar way.
But why would male-killing bacteria, well, kill only males? For these bacteria, it would be a great opportunity for them to propagate themselves by also infecting the host’s offspring. They can easily do this by simply “hitching a ride” in the egg–so a mother can infect a daughter who can in turn infect her daughter and so on down the line. But what happens with the male? He’s a dead end. Sperm, compared to eggs, contain very little cytoplasm so the bacteria cannot hitch a ride to the next generation. In this case, the bacteria kills the male so he becomes extra fodder for his female siblings that can infect future progeny.
The male-killing bacteria aren’t just confined to S. poulsonii. Rickettsia are male-killers in ladybird beetles. More famously are the male-killing Wolbachia bacteria which are distributed among many insect species. In some cases, the bacteria’s effect on the host is so extreme that the insect cannot reproduce sexually any more. In the imagination, this has led to fantasies of female-dominated worlds as mentioned in a recent New York Times article, but for humans, this may as well be fiction. It may be normal for 5%-50% of female insect hosts to be infected with male-killing bacteria, but these bacteria have not been found in other animals.
And why not? You might not realize it, but insects have a primitive immune system and most of the time, it’s quite effective at clearing bacterial infections. However, male-killing bacteria try to exploit any weakness of the insect immune system. Wolbachia, for example, simply evades the immune system by staying in an intracellular niche which the immune system has no access to. Spiroplasma, however, can infect all parts of the host with impunity. The insect immune system works by detecting components unique to a bacterial cell wall, but Spiroplasma lacks those components, thus rendering them invisible to the immune system.
One question that has been largely unanswered is, how do the bacteria know they’re in a male embryo or a female embryo? A recent Science paper by Veneti et al. tries to answer it. In Drosophila, the sex of the fly is determined by the ratio of X chromosomes to autosomes. There are several “signals” or proteins which help count the chromosomes (thus leading to correct sexual development) by binding to the X chromosome. The researchers decided to generate fly strains with mutations in the genes for these proteins, infect them with Spiroplasma, and count the surviving male progeny. In mutants for the dosage compensation complex (DCC), there was a significant increase in male survival. This means that Spiroplasma must be targeting males via this DCC protein.
So should we start wearing t-shirts with the slogan, “Save the Males”? Well, maybe–if you’re an entomologist.