How Does This All-Female Species Reproduce

The Birth of Ispera: A Biological Curiosity

In 2021, workers at a Sardinian aquarium were left astounded by the birth of a smoothhound shark, whom they named Ispera. What made this event so shocking was that Ispera’s mother had been living solely with other females for the last decade. This led to the possibility that Ispera had no father, a concept that also sheds light on other biological curiosities, such as the existence of an all-female lizard species.

Understanding Parthenogenesis

Typically, sexual species have sex cells that contain half the number of chromosomes required to create a viable embryo. This means an egg cell must be fertilized by a sperm cell to form two full sets of chromosomes. However, some species with sex cells can undergo a type of asexual reproduction called parthenogenesis, a Greek term meaning “virgin origin”. In parthenogenesis, an embryo develops from an unfertilized egg cell that doubles its own chromosome count. Some animals only ever undergo parthenogenesis, while others can reproduce both sexually and parthenogenetically. This phenomenon is more common than previously thought, with over 80 different sexual vertebrate species, including Komodo dragons, certain kinds of turkeys, pythons, and sharks, occasionally reproducing this way.

Parthenogenesis in Captivity and the Wild

Most discoveries of parthenogenesis have been made when females unexpectedly gave birth in captivity. For instance, Ispera’s birth may have been the first account of parthenogenesis in smoothhound sharks. Scientists have also confirmed that parthenogenesis was taking place in some wild snake populations. However, the exact number of fatherless creatures in the world is unknown, as it’s challenging to track without population-wide genetic analyses.

The Evolutionary Benefits of Parthenogenesis

Scientists believe parthenogenesis could be evolutionarily beneficial in some contexts. Mating and its associated demands and rituals can be time- and energy-intensive, leave individuals vulnerable to predators, and even be fatal. Parthenogenesis, on the other hand, requires only one parent. For example, 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. However, some aphids, katydids, lizards, geckos, and snakes only ever reproduce via parthenogenesis.

The Long-Term Gains of Sexual Reproduction

Despite the benefits of parthenogenesis, 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. This means that 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, this is 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.

Surviving the Mutational Meltdown

While we haven’t yet seen this entire process unfold in nature, 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. However, some parthenogenetic species appear to have ways of circumventing a mutational meltdown. For instance, 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 come 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.

The Future of Reproduction Research

To fully unravel the mysteries of reproduction, we’ll need more research—and probably a few more surprises like Ispera.