How the All-Female Amazon Molly Fish Defies Evolutionary Norms
- Researchers at the University of Missouri have identified the genetic mechanism that allows the Amazon molly, an all-female fish species, to thrive despite reproducing asexually.
- The key to the species' survival is a process called gene conversion, where one copy of a gene overwrites another.
- Animals that reproduce by cloning themselves typically face significant genetic disadvantages.
Researchers at the University of Missouri have identified the genetic mechanism that allows the Amazon molly, an all-female fish species, to thrive despite reproducing asexually. This discovery challenges the long-standing scientific belief that asexual reproduction is an evolutionary dead end due to the accumulation of harmful mutations and a lack of genetic diversity.
The key to the species’ survival is a process called gene conversion, where one copy of a gene overwrites another. By utilizing long-read sequencing, researchers Wes Warren and Edward Ricemeyer documented this process at the genetic level for the first time in the Amazon molly.
The Genetic Paradox of the Amazon Molly
Animals that reproduce by cloning themselves typically face significant genetic disadvantages. Over time, harmful mutations are expected to accumulate, and limited genetic diversity often reduces their ability to adapt to changing environments, which usually leads to extinction.
The Amazon molly (Poecilia formosa) defies these expectations. The species first emerged over 100,000 years ago from a rare hybrid pairing between a male Poecilia latipinna and a female Poecilia mexicana. Since that event, the hybrid has continued to clone itself.
The Amazon molly was the first vertebrate confirmed to be capable of asexual reproduction in 1932. Despite this, prediction models suggested the species should not have survived beyond 10,000 years. Instead, it has remained genetically healthy for more than ten times that duration.
Uncovering the Mechanism of Gene Conversion
In 2018, Wes Warren, a Curators’ Distinguished Professor in the School of Medicine and the College of Agriculture, Food and Natural Resources, mapped the full genome of the Amazon molly. He expected to find evidence of genetic decay caused by millennia of cloning, but found the DNA appeared healthy and similar to species that reproduce sexually.
Warren hypothesized that gene conversion allowed the fish to preserve and repair DNA inherited from both original parent species. This theory was recently proven using long-read sequencing, which allowed the team to compare DNA sequences from both parent species and measure their evolution.
The researchers discovered that the two genomes within the same cells were mutating at different rates. This finding was unexpected, as mutation rates are typically driven by external factors such as environment or population size, which should affect both genome sets equally.
Edward Ricemeyer, computational biologist
To have two genomes be present inside the same cells of the same fish doing two very different things in terms of mutation rates was shocking.
The study found that gene conversion occurred at an optimal rate. If the process occurred too frequently, it would limit genetic diversity; if it occurred too rarely, harmful mutations would accumulate. This balance allowed beneficial genes to spread while harmful ones were weeded out, mimicking the genetic health typically provided by sexual reproduction.
Broader Implications for Science and Medicine
This research alters the understanding of the evolutionary potential of asexual reproduction. Scientists may now investigate whether other asexual animals, such as New Mexico whiptail lizards and Komodo dragons, utilize similar gene conversion processes to maintain their genetic health.
Beyond evolutionary biology, advances in genome evolution research have practical applications in several fields:
- Improving animal and plant breeding techniques.
- Deepening the understanding of the causes of genetic diseases.
- Analyzing how genes mutate and repair themselves, which is critical for the development of cancer treatments.
The findings, which were published in the journal Nature, suggest that the Amazon molly possesses the genetic health associated with sexual reproduction without the need for male DNA.
Edward Ricemeyer, computational biologist
Better understanding the different ways that reproduction happens helps us better understand ourselves. How we got here, and where we may be headed.