An all-female fish species is challenging how scientists think about asexual reproduction

The surprising resiliency of the Amazon molly.
The surprising resiliency of the Amazon molly.
Image: AP Photo/Steven Senne
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For all intents and purposes, the Amazon molly should be on a crash course to extinction. Shockingly, it’s not only survived, but thrived.

Over time, the all-female species of the Amazon molly, a freshwater fish native to the border region of Texas and Mexico, has figured out how to clone itself without any male DNA. They technically mate with males in a similar species, and the sperm from the male does pierce the female ovum—but then the Amazon molly’s eggs destroy any trace of male genes and the cloning process begins.

Scientists have long theorized that this form of sexual reproduction—called gynogenesis—would usher in extinction for the Amazon molly. Most species that employ (or had employed) asexual reproduction are marked by a lack of genetic variation. Scientists believe animals and other living things that reproduce in this way aren’t equipped to survive the ravishes of new pathogens and other dangers that arise as environments change. Take for example the Penicillium marneffei, an asexual fungus native to Southeast Asia that’s been unsuccessful in spreading into new environments.

But new research into the Amazon molly shows how its been able to sidestep such a fate. In a study published this week (Feb. 12) in the journal Nature Ecology & Evolution, researchers mapped the Amazon molly’s genome and compared it to the genomes of two related fish species. They found a high level of genetic variability in the Amazon molly’s immune-system genes, which they believe enables the fish to adapt to dangers in its surroundings.

“Unexpectedly, we found no widespread signs of genomic decay,” the researchers write. In other words, the fish’s genes evolved along with the its surroundings, rather than stagnated.

There was another strange finding: the Amazon molly appears to have kept some of the sexual organs it doesn’t even use. That’s uncommon, as many other fish have evolved to lose organs they stopped needing. For example, the cave-dwelling Mexican tetra lost its eyes. The same has happened with some amphibians. It may be that the Amazon molly’s evolutionary process hasn’t played out long enough yet, in which case it’s setting something of a record, according to the study.

The finding is forcing scientists to reconsider how they think about asexual reproduction. The Amazon molly—known technically as Poecilia formosa—is the sexual ancestor of two parent fish called Poecilia latipinna and Poecilia mexicana. It may be that the coming together of those two fish was something of a perfect storm of genes. Rather than seeing the resulting asexual species as inferior, researchers are considering the hybrid genome as a strength. They’re calling it “rare-formation hypothesis.”

“We propose that genetic diversity between clones offers at minimum a short-term benefit to the asexual species in coping with environmental challenges,” the study states. “Those clones that acquired new adaptive mutations will thrive, while others that are less fit…will disappear.”