Siren-like: The Amazon molly uses its powers of seduction to lure the males of related fish species into largely fruitless matings. For the most part its offspring are asexual clones, say researchers.
Credit: Dunja K. Lamatsch
SYDNEY: New research has shown that an all-female species of fish has survived without sexual reproduction for up to 100,000 years.
"Basically… the Amazon molly should have gone extinct already," said evolutionary biologist Laurence Loewe, from the University of Edinburgh in Scotland, as asexual vertebrate species are thought to rapidly suffer from damaging mutations to their DNA.
Though researchers have previously suspected the fish's surprising longevity as a species, this is the first formal evidence to confirm it, said Loewe. Along with biologist Dunja Lamatsch, of the University of Würzburg in Germany, Loewe is coauthor of a study reporting the find in the journal BMC Evolutionary Biology.
Independent women
Contrary to its name, the Amazon molly (Poecilia formosa) is found not in Brazil, but in the river systems of Mexico and the U.S. state of Texas. Small and silver-coloured, the uniqueness of the species derives from its unusual mode of asexual reproduction.
The mollies produce eggs complete with all the necessary genetic material for the development of their offspring. This means that the 50 per cent genetic contribution provided by sperm in most other animal species, is not required.
Its presence is still needed, however, with the sperm acting solely as a "mechanical trigger" to kick-start the embryo's development, said Loewe. For this reason the female fish get friendly with males from other species, including the cave molly (Poecilia mexicana) and the sailfin molly (Poecilia latipinna). The mollies use behavioural and chemical cues to "seduce" these males and "enforce copulation," he said.
But reproducing asexually, without the recombination of two sets of parental genes, should ultimately be harmful for a species, said Loewe. "There are many asexual species that are known to be quite short lived." This is because without the input of paternal DNA, harmful genetic changes tend to build up over many generations.
With this in mind, Loewe and Lamatsch used mathematical computer models to simulate the evolution of the Amazon molly. What they discovered was that the species may be up to 100,000 years old, and by their reasoning should have died out from DNA damage around halfway through this lifetime.
Sneaky survival skills
What this means, Loewe said, is that the fish must have up their sleeves several "genetic survival tricks" to keep mutations at a minimum. The researchers believe one likely trick involves sneaking bits of DNA from the other species they dupe into having sex with them.
"The DNA of a related sister species has been shown to be quickly degraded most of the time, and is thus not incorporated into the genome of the Amazon molly," Loewe said. "However… occasionally small bits of this DNA [might] make it into the developing egg."
Though these DNA fragments are usually lost, he said, if they survive frequently enough, and "if they carry enough healthy genes to compensate for the decaying genes in the Amazon molly… then such a flow of genetic material could help the fish survive."
However, more work needs to be done before this can be conclusively demonstrated, said the researchers.
The study is significant because it is the first time experts have quantified the rate at which this fish accumulates harmful mutations, said biologist Caitlin Gabor, from Texas State University in San Marcos, USA. Because this speed of "genomic decay" can now be calculated, this represents a turning point in our ability to study and understand asexual species, said Loewe.
It has also provided us with a new estimation of the age of the Amazon molly, Gabor added, placing the fish as a contender for the "oldest living unisexual vertebrate species."
###
