How Did an All-Female Species Survive Sans Sexual Reproduction for Millennia?

By all means, the Amazon molly should have gone extinct by now. Except it has not.

An AMazon molly (right) caught in action while seducing a male sailfin molly to steal sperm. Caption and credit: Manfred Schartl

An Amazon molly (right) caught in action while seducing a male sailfin molly to steal sperm. Caption and credit: Manfred Schartl

On paper, the Amazon molly should have been extinct by now. But it continues to thrive in diverse habitats. A new study has now found the secret to its reproductive success in its genome: a combination of genetic diversity and heightened immune responses has helped the Amazon molly defy evolutionary rules.

Sexual reproduction is important to sustain (most) life (as we know it). It enables the genetic diversity that helps ward off diseases and helps species adapt to new environments. Without such diversity, harmful mutations could accumulate or organisms wouldn’t be able to evolve new defences against new threats. Either way, the species would go extinct soon.

This said, there are advantages to asexual reproduction as well. A female that produces only daughters has a two-fold reproductive output than a female who produces equal numbers of daughters and sons. This is because, in the former case, all the progeny contribute to population growth whereas in the latter, only half of the population does so.

But the advantages of sexual reproduction easily shadow its disadvantages – and that’s why it has become the preferred way of procreating on Earth.

Indeed, asexual vertebrates are very rare. The Amazon molly was the first asexual vertebrate to be described, back in 1932. This freshwater fish reproduces by producing exact copies, or clones, of the mother. And so male Amazon mollies are done away with. In fact, the ‘Amazon’ in its name alludes to the legendary warriors of Greek mythology and not to the river signposting its habitat. The Amazon molly actually inhabits the backwaters and streams of northeastern Mexico and southern Texas.

The fish is a natural hybrid of two distantly related sexual species – the Atlantic molly and the sailfin molly – that currently share its habitat. Previous studies have found that the Atlantic molly was the maternal ancestor and the sailfin molly, the paternal. The Amazon molly practises sexual parasitism. Though it needs sperm from a related sexual species to trigger the egg’s development, the resulting egg is an exact clone of the mother with no contribution from the sperm. In rare cases, however, genetic material from the male does seep in.

Nonetheless the daughters are exact clones of the mothers, with no any genetic recombination in play.

Normally, this would have posed a problem. With no new genetic variations in the offspring, deleterious mutations should have piled up. The molly should have also been more susceptible to certain parasitic infections. By all means, the species should have gone extinct by now.

Except it has not. By studying ancestral history data and its complete genome, researchers from 20 institutions around the world have now explained why the species has outlived its predicted extinction time.

The researchers measured parental contributions in the Amazon molly’s genome by comparing it with contributions from the Atlantic and sailfin mollies. They expected to see some form of genetic decay. “We were surprised when our data revealed no obvious differences when compared to the sexual parents that created Amazon molly and share their same aquatic habitats,” Wesley Warren, the lead author of the study, told The Wire.

Curiously, the researchers also found that genes that are no longer useful in an all-female fish – such as those aiding male development – are present in the Amazon molly, and without being corrupted. This could be because not enough time has passed for these traits to be irrelevant.

One reason why the Amazon molly has been successful could be because of the paternal DNA that ‘leaked’ into the fish and which has contributed to some genetic variation. Paternal leakage is neither equivalent to nor as effective as genetic recombination due to sexual reproduction. The ‘leakage’ is only part of the story.

The other part is that the Amazon molly is a functional hybrid: one that expresses genes roughly equally from each of its parents. This means that genetic diversity in its lineage has been “frozen” at some points on the genome. But there can be variation in the rest of the genome within a lineage or between lineages.

Though species that produce clones, like the Amazon molly, should have less variation in genes associated with their immune systems, the researchers found otherwise. It seems the Amazon molly has been able to duck genome decay thanks to this diversity.

Ingo Schlupp, who studies the ecology and evolution of live-bearing fishes at the University of Oklahoma, said this diversity explained why he and his team members were unable to detect a disadvantage for Amazon mollies relative to their parasites. “That a species can be that old and show virtually no signs of a negative impact of asexuality on the level of the genome is pretty cool,” Schlupp, who wasn’t associated with the study, told The Wire.

(Disclosure: During his postdoc, Schlupp was mentored by the study’s principal investigator.)

Aracely Newton from Harvard University, who was not involved in the study either, also finds the diverse immune system genes unexpected and interesting – something she feels should be investigated further by evaluating genetic variations among different clones across the whole genome and then check how many of these variants are actually functional. This would provide a stronger basis for concluding that higher diversity in these genes makes the individuals more immune.

Given that the Amazon molly is a hybrid species, there have been attempts to “create” it in laboratory settings. All attempts thus far have failed. The study’s authors think that recreating the Amazon molly has eluded them because “the genomic combinations that allow successful survival and reproduction are very specific.”

But Newton has her doubts. “The observation that there are one or few origins does not necessarily mean that it is the biggest barrier,” she said. “Perhaps there are many hybridisations, but much fewer can form stable lineages.”

The Amazon molly’s is an important model used to figure out the origins and evolution of sexual reproduction as well as understand how traits have adapted with time. ”This study is a breakthrough in the field and finally provides us with the genome of one of the best studied unisexual vertebrates,” Schlupp said. He feels that this genome is going to be an important resource for a large community of researchers interested in the evolution of recombination or hybridisation.

It is also easy to breed these mollies in captivity, making it useful for scientific studies. Since the species reproduces by producing clones, they are used for studying genetically identical individuals. Warren hopes their findings will help increase scientific interest in this species since it might hold clues to how species change over time when chance, random events occur.

There are other implications as well: “Perhaps similarities to the clonal evolution of cancer cells can be gained from a deeper study of this species,” Warren said. He and his team now plan to study the process by which some small portions of the paternal DNA leak into the Amazon molly’s genome, and how mutations change over time when no genetic recombination is involved.

The sex life of the Amazon molly holds a lot of secrets. Unlocking them might help us better understand how we came to be.

The study was published in the journal Nature Ecology and Evolution on February 12, 2018.

Rashmi Bhat is a wildlife researcher with an overarching interest in applied conservation biology. She is based in Bengaluru, India.

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