THE SHILLONG TIMES / 2026-05-23, 7, Shillong
(The Conversation)
It may sound too bizarre to be true but the Amazon molly (Poecilia formosa), a fish that inhabits rivers, lakes and swamps in Mexico and Texas, exists over much of its range in populations that are 100% female.
In 1932, the Amazon molly became the first known verte brate to reproduce by cloning itself, producing all-female populations. A new genetic study has given scientists in sights into the longstand ing mystery about how and why this happens.
The proportion of fe males in the human popu lation is roughly 50%. A few countries such as Maldives (38% female) and Moldova (54% fe male) diverge from this, but these differences can largely be explained due to male immigration and emigration.
However, much more dramatic sex ratios are found in the animal king dom. Kentish plover bird populations, where males care for offspring, com prise only 14% female, and sea turtle popula tions, where sex is determined by temperature often exceed 75% female.
Most animal species repro duce sexually. This involves the fusion of two gametes, the sperm and egg, that develops into an embryo. A process, known as recombination, ran domly shuffles the genetic material from the mother and father.
This produces increased variability in the offspring, and new combinations of traits. The genetic diversity improves the chances of survival for the species if its environment changes.
But the Amazon molly re produces asexually, where there is no mixing of genetic material. This reduces genetic diversity, making populations vulnerable to extinction – if one Amazon molly is suscepti ble to a disease, they all are.
And there is another prob lem to being identical. Asexual species are more likely to ac cumulate harmful mutations.
This phenomenon, known as Muller’s ratchet, predicts that clones should go extinct within 10,000 years.
Yet, the Amazon molly, a hybrid that arose through sexual reproduction between a female Atlantic molly (P. mexi cana) and a male sailfin molly (P. latipinna) has survived for over 100,000 years.
So, what is the secret to their sustained existence? Gene conversion is a process where one version of a gene is replaced by another. In most species, such as humans, it is used to repair damaged DNA. However, in the Amazon molly, gene conversion has slowed Muller’s ratchet.
The new study found that gene conversion appears to play the same role as recombi nation. This essentially enables the fish to purge harmful muta tions and preserve beneficial ones.
Indeed, despite reproducing asexually, the Amazon molly shows differences in body shape between populations, demonstrating evolution in response to its local environ ment.
The Amazon molly repro duces via a process called parthenogenesis, also known as “virgin birth”, where young are produced from an unfertilised gamete.
This allows rapid growth of successful genotypes, the ge netic blueprints of organisms, as all of the Amazon mollies can reproduce without finding a mate.
As such, animals created via virgin births can colonise habitats quickly.
Parthenogenesis can be ob ligative, like in the Amazon molly, where it is the only means of reproduction. But, it can also be facultative, where species can switch between sexual and asexual reproduc tion.
For example, the marbled crayfish, reproduce sexually in their native range but rapidly establish themselves in new habitats asexually, often from a single female.
The Amazon molly has a type of parthenogenesis known as gynogenesis where sperm is required to stimulate develop ment of the unfertilised egg.
So, the Amazon molly still needs to “mate” each time she reproduces, but the sperm is of their own species see the males with an Amazon molly, they are more likely to mate with them.
Parthenogenesis is common in invertebrate animals, includ ing ants, bees and wasps. It is less common in vertebrates but has been found in other fish, amphibians, reptiles includ ing the Komodo dragon, birds such as Californian condors and sharks for example ham merheads.
Other all-female parthe nogenic vertebrates include the whiptail lizards, where almost a third of species are comprised solely of females. The New Mexico whiptail liz ard has even become a queer icon.
Unlike the Amazon molly, these “lesbian lizards” do not need sperm from a male to stimulate egg development. They just need to engage in mating behaviour to stimulate ovulation, bypassing males completely.
Some blue-spotted salaman ders have reproduced sexu ally for several million years. Although the all-female popu lations of the salamanders re produce in a similar way to the molly fish, requiring sperm to stimulate development, they are kleptogenic.
This means that they replace a portion of the mother’s DNA with a portion of DNA from the male’s sperm, incorporat ing a small amount of his genetic material into the offspring.
This facilitates the ge netic diversity that has enabled the salamanders to survive for so long.
Like the Amazon mol ly, the Brahminy blind snake, also known as the flowerpot snake due to its habit of burrowing in potted plants, is the only other known vertebrate that breeds exclusively via parthenogenesis.
The snakes have three copies of each chromosome, rather than the usual two, probably due to an error in cell division at some point in the evolutionary history of the species.
Increased numbers of chro mosomes have been found in many species, including salmon with four copies, and sturgeon fish with eight cop ies.
Increased numbers of chro mosomes generates increased genetic diversity, which prob ably explains how the blind snake clones have survived for so long.
There could be more all female animals out there yet to be discovered. After all, up until a few years ago we didn’t know that female snakes have two clitorises.
