one fish, two fish, cloning sawfish

Let’s talk about sex, baby—but maybe not how Salt-N-Pepa sang about it. We’re talkin’ parthenogenesis, a form of asexual reproduction in which offspring are produced from an unfertilized egg.

Reproduction is one of the defining characteristics of living things. The goal of any species—whether it is a single-celled bacterium, a flowering plant, or a complex mammal—is survival. We do this by passing on our genetic material to our offspring, with the hope that the next generation will inherit the strongest genetic traits for survival in a given ecosystem.

Vertebrate species typically reproduce sexually, with genetic material being passed on from the male’s sperm and the female’s egg. Yet, there have been cases of “virgin births” in several species of birds, reptiles, and sharks in captivity. While these species typically require sexual reproduction to produce offspring, they have switched to a method of asexual reproduction called facultative parthenogenesis. It is hypothesized that this occurs when a viable male is not present. The females are biologically turning to a “last resort” mechanism to preserve the survival of their species. Cue Dr. Malcolm from Jurassic Park:

“Life finds a way.”

When egg cells form, a primary cell undergoes meiosis to reduce the chromosome number in half. This is to ensure that when sexual reproduction occurs, the offspring receives half the chromosomes from the egg and half from the sperm. In this division process, one copy becomes a mature egg cell, while three inactive copies become polar bodies. Polar bodies will usually die and disintegrate, while the mature egg cell becomes available to be fertilized. However, in the case of facultative parthenogenesis, it is thought that a polar body fuses with the egg cell, functioning as a sperm stand-in. The resulting offspring will be a therefore be a genetic half-clone of the mother.oogenesis

As mentioned above, this reproductive method has been observed by several species in captivity. Yet, a recent study conducted by geneticists at Stony Brook University revealed that a population of smalltooth sawfish show evidence of facultative parthenogenesis in the wild.

Historically, smalltooth sawfish inhabited the coasts of southern United States from Texas up to North Carolina. Modern development along the coast likely contributed to the destruction of the sawfish’s natural habitat—in addition to the threat of being unintentionally caught by fishermen’s nets—leaving the smalltooth sawfish an endangered species.

Andrew Fields and his colleagues at Stony Brook were interested in studying the genetic diversity of the sawfish, assuming that the small endangered population must be inbreeding due to the limited number of males. They measured their data on an internal relatedness (IR) scale from zero to one; zero meaning no relation between an individual’s parents, 0.5 meaning the parents were siblings, and one meaning completely identical genetics between parent and offspring. Nearly 4% of the 190 individuals had IR values between 0.84 and 1.0. These outliers were all female, suggesting that they are half-clones of their mother, born out of facultative parthenogenesis.

It is still unknown how commonly this phenomenon occurs in the wild. Since the population of an endangered species is greatly reduced, it is possible that other at-risk vertebrate species may also turn to facultative parthenogenesis in the wild in an attempt to survive. However, this method of reproduction is not necessarily sustainable for a species that normally benefits from the diversity of genetic traits via sexual reproduction. Half-clone offspring will always be female, increasing the number of females in comparison to the dwindling number of males. Unless these offspring are able to reproduces asexually as well, they will only be adding to the competition to find a mate. Additionally, facultative parthenogenesis greatly reduces the gene pool (as does inbreeding). This increases the risk of infertility, disease, and other problems associated with continued fitness and survival of a species.

While it is amazing to observe how the natural world adapts to change, factors such as poaching, deforestation, pollution, and climate change continue to contribute to a rapid decline in flora and fauna—all factors which are driven by human impact. However, we are also a species gifted with intellect and compassion, who can use the knowledge we gain from scientific studies to make a change for the better.

Featured image (public domain) from Ichthyologie (1796) by Marc Éliéser Bloch. Obtained from the Biodiversity Heritage Library, digitized by Harvard University, Museum of Comparative Zoology, Ernst Mayr Library.
Egg cell image (public domain) adapted from 
Anatomy of the Human Body (1918) by Henry Grey & Henry Vandyke Carter.

Published by

Mariel Mohns

By day, Mariel is a science writer at a research institute. By moonlight, she is a freelance science writer for various web-based outlets. She has a habit of collecting hobbies in her spare time, but mostly enjoys movies, TV binges, video games, and knitting.

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