All-female salamander species steals sperm but doesn’t actually use it
Unisex amphibians beat the evolutionary odds. Stephen Fleischfresser reports.
Sex is expensive: it eats time, energy and resources. But the genetic diversity it produces is key to long-term evolutionary health, with asexual species doomed to extinction because they miss out on this variety.
At least, that’s the theory. New research published in the journal Evolution shows that, at least for one salamander species, the celibate life isn’t as bad as many might think.
The discovery in 1932 of a fish called the Amazon molly (Poecilia formosa) debunked the theory that asexual vertebrates could not exist. The all-female molly, from Mexico, reproduces without the genetic input of a sexual partner, a process known as ‘parthenogenesis’.
However, asexual vertebrates are very rare, and face two major hurdles.
The first is known as the Red Queen hypothesis, and argues that species must continue to evolve in order to survive the pressures of environment, diseases and parasites. As the character of the Red Queen from Lewis Carroll’s Through the Looking Glass said, “it takes all the running you can do, to keep in the same place”. Those species who stand still, in evolutionary terms, will be wiped out.
The second problem is something known as ‘Muller’s ratchet’, named after the Nobel prizewinning geneticist Hermann Joseph Muller. As asexual reproduction creates offspring that are essentially clones of the parent, any damaging mutations that arise in the genepool will be conserved. This means that over time these deleterious mutations accrue, leading to the eventual extinction of the species.
But all this pessimism seems slightly misplaced. Recent research showed that the Amazon molly is in better evolutionary health than predicted, and now a trio of scientists from Ohio State University in the US have found another surprisingly robust all-female species.
Using new sequencing and analytical tools, project leader Rob Denton, Ariadna Morales and senior author H. Lisle Gibbs, have uncovered the reproductive history of the stunningly long-lived, unisexual Ambystoma salamander.
Ambystoma reproduces using a form of parthenogenesis called ‘kleptogenesis’. The female salamanders steal sperm from closely related sexually reproducing species. However, the stolen goods are only used to stimulate egg production, meaning its genetic material is excluded from the genomes of offspring.
However, every now and then some of their genetics are incorporated, adding much needed genetic diversity to the genepool. The team hypothesised that this must have happened regularly for Ambystoma because it is such an old species.
“Most asexual lineages blink out after 100,000 years,” says Denton. “We think these have been around for 5 million years.”
The researchers discovered that while there is evidence of genetic influx over the last half a million years, for most of their evolutionary history the salamanders rarely borrowed genes.
“This research shows that millions of years went by where they weren't taking DNA from other species, and then there were short bursts where they did it more frequently," says Denton.
Yet somehow, they have escaped what seemed like certain doom.
"Surprisingly, it doesn't look like they're suffering any ill genetic effects. It's a mysterious scenario that an animal can avoid sexual reproduction for millions of years and not suffer the consequences of that,” he adds.
“These salamanders are just sort of plodding through evolutionary time doing strange and surprising things.”