No Males Needed: These Poecilia Species Astonish Scientists with Their Reproductive Mystery
Image credit, Manfred Schartl
A species of fish found in rivers of Mexico and southern Texas exists in a way that seems almost impossible.
This fish swims alongside its female counterparts, with nearby silver-colored males belonging to related species, yet it does not reproduce through typical sexual means. While this may appear to be a hybrid between male and female fish, male genes do not influence the offspring of the female fish in an evolutionary context.
This process, known in English as ‘gynogenesis,’ involves the male’s sperm activating the female’s egg for development, but the male DNA is discarded. Therefore, these females produce only daughters who in turn also give birth to daughters.
This fish is called the Amazon molly, a name inspired by the female warrior tribe from ancient Greek mythology. This species has remained a scientific enigma for nearly a century.
According to evolutionary theory, asexual species should face rapid extinction because reproduction without sexual activity leads to accumulation of harmful mutations over time. However, this fish species has persisted in this state for 100,000 years.
So how has the Amazon molly survived for so long?
Why is Sexual Reproduction Important?
Co-author of new research on the Amazon molly and computational biologist at Ludwig Maximilian University in Munich, Edward Reissmeier, explains, “Sexual reproduction is costly.”
Animals must find mates and reproduce with them, and in doing so, each parent only passes on half of their DNA. Most females invest considerable time and resources in raising their offspring.
From this perspective, asexual reproduction seems like a convenient alternative—no need to find or compete for mates and 100% of the genes are passed on to offspring.
However, sexual reproduction is essential for the life cycle.
Evolutionary biologist Dave Speijer from the University of Amsterdam says, “From a broader perspective, 99.9% of reproduction occurs sexually.”
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Sexual reproduction promotes genetic recombination between male and female DNA, producing unique gene combinations in offspring.
This leads to higher genetic diversity in sexual species, which supports their survival.
Sexual reproduction also protects species from the threat called Muller’s ratchet.
Speijer explains, “There are always errors when copying DNA.”
In sexual species, recombination helps remove these errors, but in clonal species, such errors can be passed down through generations.
Consequently, harmful mutations accumulate over time, damaging the genome and increasing the risk of extinction.
Long-Term Survival Without Sexual Reproduction
According to this principle, asexual species should have shorter lifespans. Yet, species like the Amazon molly not only survive but also flourish.
Speijer notes, “There may be some misconceptions regarding how such species survive.”
He argues that it’s not only about sexual reproduction but about understanding all life’s mysteries. Some mechanisms are essential for managing genetic errors, with sex being one of them.
In this light, long-surviving asexual species appear to have found alternative strategies to survive without violating evolutionary rules.
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There are asexual animals that have persisted in nature for extremely long periods.
The Amazon molly is one such species. Consequently, scientific debates continue regarding species that endure for long durations without sexual reproduction.
The ‘Copy-Paste’ Mechanism
Reissmeier states that the new study has uncovered a novel aspect of “gene conversion.”
Gene conversion is a DNA repair process found not only in Amazon mollies but in many organisms.
In sexually reproducing species like humans, each individual carries two copies of each gene—one from the mother and one from the father. If UV radiation damages DNA, cells use the intact gene to repair the damage.
This process allows immediate correction of DNA damage in humans and other sexual species. In species like the Amazon molly, gene conversion appears to be the primary strategy for genome maintenance.
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Reissmeier and his team compared DNA from mollies across different generations.
Their analysis showed that the mollies’ DNA evolves not through sexual reproduction but by gene conversion.
In other words, gene conversion compensates for the role sexual reproduction plays in preventing harmful mutations in the genome.
Studies also confirm that the Amazon molly, like other asexual animals, originated from a hybridization event between two ancient species.
Like many hybrids, this cross initially produced sterile offspring but eventually gave rise to a species capable of reproducing without males.
Today, every Amazon molly carries genes from two ancestral species.
This dual heritage likely facilitates the extensive gene conversion observed within mollies.
Image credit, DEA/C.DANI/De Agostini via Getty Images
Interestingly, gene conversion is more prevalent in a limited part of the genome.
Despite 100,000 years without sexual interaction, this fish species exhibits good genetic health.
Implications for Human Science
Understanding how genetic errors arise and are managed could be beneficial for humans.
Harmful mutations are not exclusive to asexual species.
“Cancer arises because of mutation,” explains Reissmeier. Although he refrained from sharing many study results, he noted that learning how nature copes with mutations naturally would provide long-term benefits.
Scientists have yet to fully understand how mechanisms outside of sexual reproduction can maintain genomic stability.
However, in the case of the Amazon molly, evolutionary theory seems insufficient as its genetic health is stronger than expected.
“We used to think that sexual reproduction was the only way to preserve the genome… but now we see there is another way,” Reissmeier concluded.