Putting the science in fiction - Dan Koboldt, Chuck Wendig 2018
Gender determination in animals
From zero to sixty (legs, that is)
By Robinne Weiss
Hordes of mutant insects, alien creatures, dragons, unicorns, and all the other weird and wonderful creatures that appear in fiction have to reproduce. Otherwise they wouldn’t exist, right? It’s tempting to ignore the reproduction of our fantastical creatures, though—they hatch, or land on Earth, or whatever, and then the excitement starts.
But the world of reproductive biology is full of juicy facts, ripe for exploitation by fiction writers. Take the matter of gender, for instance. For many animals, gender is a relatively simple matter determined by genetics, but in other animals, gender is far more complex.
Why does it take two to tango?
Gender determination has evolved many times in different animals, but it has almost always led to two genders. Why two? Why not three or four? To understand, we have to look back to the evolution of sex cells (gametes). When sexual reproduction evolved, the first gametes were all the same size—there were no sperm or eggs. But selective pressures pushed gametes into two directions. Small gametes were better at moving around and finding other gametes to fertilize. Larger gametes were better provisioned and better able to develop into an embryo once fertilized. These competing pressures led to small, mobile sperm (and the males to produce them) and large, immobile eggs (and the females to produce them).
Some like it hot
Some species of reptile have temperature-determined genders. The incubation temperature of the egg determines whether the embryo develops as female or male. Unlike warm-blooded birds that sit on their eggs to keep them warm, reptiles are largely at the mercy of their environment when it comes to incubation. They generally lay their eggs in clutches, buried in sand or soil in a sunny location, and hope for the best.
In many turtles, eggs incubated at lower temperatures develop as males, and those incubated at higher temperatures become females. In American alligators, extreme temperatures (high or low) produce females and moderate ones produce males. With this system of gender determination, entire clutches of eggs may emerge as one gender or the other. A storm that buries eggs more deeply in sand or soil or exposes eggs more directly to the sun can change the gender of a whole clutch of eggs.
Some fish are also affected by incubation temperature, with extreme temperatures leading to more males, though studies suggest that fish also have a genetic component to gender determination and temperature only affects gender at extreme conditions. The same is true for some insects, where cool temperatures cause the chromosomes to behave differently during meiosis, resulting in more male offspring.
Marie Brennan used this idea beautifully in Voyage of the Basilisk: A Memoir by Lady Trent (Tor, 2015). In the story, a nation tries to breed dragons to defend their borders, but the dragons all develop as small males, useless for defense. The intrepid naturalist, Lady Trent, conjectures it is because incubation conditions were wrong for the production of the much larger females.
Going both ways
Some fish and snails actually switch genders throughout their lives. The switches may be triggered by social or environmental cues. Clown anemonefish (think Nemo) all begin life as males. In a group, only the two largest fish are sexually mature—one female and one male. If the group’s dominant female dies, the dominant male becomes female and takes her place.
Indo-Pacific cleaner wrasses do it the opposite, with groups of one male and many smaller females. When the male dies, the largest female becomes male and takes his place.
Parrotfish and hawkfish can begin life as either gender, and switch throughout their lives.
Imagine how Finding Nemo could have been wildly different with this little bit of biology included, considering that Nemo’s mother is killed, and he and his father are the only two clownfish left …
Have your cake and eat it, too
Some animals don’t really have different genders because each individual possesses both male and female sex organs. We call these animals hermaphrodites, after Hermaphroditus, the son of Hermes and Aphrodite who became both male and female after being united with a water nymph. Hermaphroditism is a useful adaptation for slow-moving animals that may have difficulty finding a member of the opposite sex. If all members of the species are both male and female, any individual will do as a mate. Earthworms are an example of hermaphroditic animals. Every earthworm has testes near the rear of its body and ovaries near the head. During mating, earthworms lie head to tail, and each worm acts as male and female, passing sperm to the other worm. Many species of snail and slug are also hermaphroditic and practice the same sort of mutual fertilization that worms do.
Sea squirts and other tunicates are also hermaphrodites. As adults, these animals are sessile—they are attached to a surface and can’t move around to find mates. They release sperm and/or eggs into the water to be fertilized. Many of these organisms are self-sterile, meaning that if sperm and eggs from the same individual should encounter each other, the sperm cannot fertilize the egg.
All this may sound familiar because plants do exactly the same thing. Most plants are hermaphrodites—within each flower are male parts (anthers, producing pollen, the equivalent of sperm) and female parts (ovaries, producing ovules, the equivalent of eggs). Only rarely are plants gendered in a genetic system similar to our own, with X and Y chromosomes. Ginkgo is one example. Male ginkgo trees are prized for urban landscaping, but the females are reviled for their foul-smelling fruit.
He’s half the woman she is
In many bees, wasps, and ants, males develop from unfertilized eggs (haploid—with genes from the mother only), and females from fertilized eggs (diploid—with a set of genes from each parent). Usually the adult female controls the process, choosing when to produce males and when to produce females by controlling the release of sperm she has stored from mating. Sometimes, as in some scale insects, the males originate from fertilized eggs in which the father’s genome is destroyed during development, becoming haploid.
Invasion of the gender snatchers
The strangest (and perhaps most interesting for fiction) gender determination oddity is that found in many insects and other arthropods.
A wide variety of insects produce only females or produce males only rarely. There are insect species for which no males have ever been found—females simply clone themselves or lay unfertilized eggs.
The elimination of the male gender appears to be caused almost entirely by strains of bacteria in the genus Wolbachia, which infects arthropods and some nematodes and is considered a reproductive parasite. The relationships between Wolbachia and its hosts range from parasitic to mutualistic, but almost always involve a modification of genders and reproduction.
Infected females pass Wolbachia to their offspring but males don’t, so it is in the bacteria’s interest to eliminate males. Wolbachia accomplishes this in one of several ways (different among Wolbachia strains and hosts): by killing males before maturity, causing males to develop as females, giving female hosts the ability to reproduce parthenogenically (without males), or causing cytoplasmic incompatibility between infected males and uninfected females so only infected females can reproduce.
The relationship between Wolbachia and arthropods is a long one and has shaped the evolution of both hosts and parasites. In some cases, the relationship has evolved into an obligate one—the host cannot survive or reproduce at all without the bacteria. In some cases, insect species under the influence of the bacteria have evolved to be single-gendered, so even if the bacteria are removed, only females are produced.
Whether obligate or not, Wolbachia gives many infected insects a reproductive advantage over uninfected ones. It also often gives the insect a survival advantage, providing the host protection against pathogens and, in some cases, improved nutrient uptake.
So, infection by bacteria produces all-female races of superbugs … now there’s some cool science fiction!
Because Wolbachia does more than simply change insects’ gender, its presence and its many and varied strains raise interesting opportunities for us as humans. Mosquitoes that transmit diseases like malaria, chikungunya, West Nile virus, dengue, and Zika virus are less susceptible to infection by these pathogens when they are infected with certain strains of Wolbachia. Scientists have been studying the feasibility of using Wolbachia in the fight against arthropod-borne diseases, both to decrease their ability to carry the diseases and as a way to limit reproduction (by infecting males with incompatible strains of Wolbachia, thus preventing viable offspring).
Gender in other life forms
Of course, this is just animals. Gender determination becomes even more weird and wonderful when it comes to other organisms. Fungi, for instance, have genetically based gender determination, but may have thousands of genders (mating types). This diversity can make it hard for a fungus to find a compatible mate, so many have evolved the ability to change some of their cells to a compatible mating type and mate with themselves.