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.
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.
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.
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.
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.
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.
To fully unravel the mysteries of reproduction, we’ll need more research—and probably a few more surprises like Ispera.
Design an infographic that explains the concept of parthenogenesis. Include key points such as how it works, examples of species that reproduce this way, and the evolutionary benefits and drawbacks. Use images, diagrams, and bullet points to make your infographic engaging and informative.
Split into two groups and hold a debate on the advantages and disadvantages of parthenogenesis versus sexual reproduction. One group will argue in favor of parthenogenesis, while the other will argue for sexual reproduction. Use evidence from the article to support your arguments.
Participate in a classroom simulation that demonstrates the effects of genetic diversity. Use colored beads to represent different genes and simulate several generations of both parthenogenetic and sexually reproducing populations. Observe and discuss the differences in genetic diversity over time.
Choose a species that reproduces through parthenogenesis and create a research project about it. Include information on its habitat, reproductive process, and any unique adaptations it has. Present your findings to the class through a poster or a digital presentation.
Write a short story from the perspective of an animal that reproduces through parthenogenesis. Describe its life, the challenges it faces, and how it benefits from this unique form of reproduction. Share your story with the class and discuss the biological concepts it illustrates.
smoothhound shark – A species of shark that has a streamlined body and smooth skin, found in coastal waters. – The smoothhound shark is known for its impressive swimming speed.
all-female lizard species – A group of lizards that consists only of females and reproduces without males. – The all-female lizard species have adapted unique reproductive strategies.
sex cells – Specialized cells involved in sexual reproduction, such as sperm and egg cells. – The fusion of sex cells leads to the creation of a new individual.
chromosomes – Thread-like structures in cells that contain genetic information. – Humans typically have 23 pairs of chromosomes in each cell.
viable embryo – A developing organism that has the potential to survive and grow. – The doctor confirmed that the pregnancy was progressing well and the embryo was viable.
egg cell – A reproductive cell produced by females that can be fertilized by a sperm cell. – The hen laid an egg containing a developing chick.
sperm cell – A reproductive cell produced by males that can fertilize an egg cell. – The sperm cells swim towards the egg in search of fertilization.
parthenogenesis – A form of asexual reproduction in which an unfertilized egg cell develops into an offspring. – Some reptiles can reproduce through parthenogenesis, without the need for a male.
asexual reproduction – Reproduction without the involvement of sex cells or genetic recombination. – Bacteria reproduce through asexual reproduction, dividing into two identical cells.
unfertilized egg cell – An egg cell that has not been fertilized by a sperm cell. – The unfertilized egg cell did not develop into an embryo.
chromosome count – The number of chromosomes present in a cell or organism. – Humans have a chromosome count of 46 in each cell.
animals – Living organisms that are multicellular and capable of movement. – Dogs, cats, and birds are examples of animals.
sexual vertebrate species – Vertebrate organisms that reproduce sexually. – Most mammals, reptiles, and birds are sexual vertebrate species.
Komodo dragons – The world’s largest lizard species, found in Indonesia. – Komodo dragons are known for their powerful bite and venomous saliva.
evolutionary benefits – Advantages that enhance the survival or reproductive success of an organism. – Camouflage provides evolutionary benefits to prey species, allowing them to avoid predators.
mating – The act of coming together to engage in sexual reproduction. – Birds perform elaborate mating dances to attract a mate.
