How does all the female species reproduce?

In 2021, workers at a Sardinian aquarium were stunned by the birth of a smooth hound shark, who they called Ispera. What shocked them was that, for the last decade, Ispera’s mother had been living only with other females. But it’s actually entirely possible that Ispera had no father— and the reason why that is also explains other biological curiosities, like the existence of an all-female lizard species. Usually, sexual species have sex cells that contain half the number of chromosomes required to create a viable embryo. So, an egg cell must be fertilized by a sperm cell to form two full sets of chromosomes. But some species that have sex cells can undergo a type of asexual reproduction called parthenogenesis— meaning “virgin origin” in Greek. In parthenogenesis, an embryo develops from an unfertilized egg cell that doubles its own chromosome count. In fact, some animals only ever undergo parthenogenesis, while others can reproduce both sexually and parthenogenetically.

It's actually more common than previously thought. More than 80 different sexual vertebrate species— including Komodo dragons and certain kinds of turkeys, pythons, and sharks— have surprised us by occasionally reproducing this way. These discoveries were usually made when females unexpectedly gave birth in captivity. Ispera’s birth, for one, may have been the first account of parthenogenesis in smooth hound sharks. Scientists also confirmed that parthenogenesis was taking place in some wild snake populations. But just how many fatherless creatures are running, slithering, and swimming around out there is unknown: it’s a tough thing to track without population-wide genetic analyses. So, why is it happening at all? Scientists think parthenogenesis could be evolutionarily beneficial in some contexts because, well, sex can be a drag.

Mating and its associated demands and rituals can be time- and energy-intensive, leave individuals vulnerable to predators, and even be fatal. Parthenogenesis, meanwhile, requires only one parent. Mayflies can sometimes default to parthenogenesis if there are no males available, which is especially handy because they’ve only got a day or so to reproduce before dying. It can also help rapidly expand a population. In the summer, when food is abundant, pea aphids can rely on parthenogenesis, allowing their population to explode under favorable conditions. And in the autumn, they switch back to sex. But some aphids, katydids, lizards, geckos, and snakes only ever reproduce via parthenogenesis. So, why do other animals bother with sex? Scientists hypothesize that sex makes up for its shortcomings with long-term gains. It allows individuals to mix their genes, leading to greater genetic diversity. That way, when the going gets tough, beneficial mutations can be selected and harmful ones can be removed without ending the entire population. In a parthenogenetic population, on the other hand, individuals can only reproduce using their own genetic material.

According to a theory called Muller’s ratchet, that’s not good. The theory predicts that parthenogenetic lineages will accumulate harmful mutations over time and eventually, after thousands of generations, will reach a point of so-called mutational meltdown. At this stage, individuals will be so compromised that they can't reproduce, so the population will nosedive, leading to extinction. We haven’t yet seen this entire process unfold in nature. But scientists have observed an accumulation of harmful mutations in parthenogenetic stick insects that are absent in their sexual relatives. Only time will tell whether this will cause their extinction. Otherwise, some parthenogenetic species appear to have ways of circumventing a mutational meltdown.

New Mexico whiptail lizards came about when two different lizard species hybridized, creating this new all-female species. As hybrids, their genome is a combination of the different sets of chromosomes from their two parent species. This gives them a high level of genetic diversity, which may allow them to survive long into the future. Bdelloid rotifers, meanwhile, have been reproducing parthenogenetically for 60 million years. They might have managed this by taking in foreign genetic material. Indeed, about 10% of their genes comes from other organisms, like fungi, bacteria, and algae. How exactly they do this is unclear, but whatever the trick is, it seems to be working. To totally untangle the mysteries of reproduction, we’ll need more research— and probably a few more surprises like Ispera.