Good golly, miss molly!
Asexual fish species confounds expectations and genetic predictions. Stephen Fleischfresser reports.
Right on the heels of surprising research into the species of self-cloning, macromutant crustaceans known as the marbled crayfish comes yet more insight into the bizarre world of asexually reproducing vertebrates.
Asexual vertebrates are rare. Indeed, up until the 1930s it was thought that they didn’t, and couldn’t, exist. Putting paid to that theory was the 1932 discovery, by ichthyologist husband and wife team Carl and Laura Hubbs, of Poecilia formosa, or the Amazon Molly.
This small freshwater fish from north-eastern Mexico is so named as an allusion to the Amazons of Greek myth – a tribe of women warriors. While the prowess of the molly in battle is unknown, they are, just like their classical namesakes, all female.
This is because the Amazon molly, the hybrid offspring of two other molly species, reproduces by parthenogenesis: it can produce young without the genetic input of a sexual partner. It does this, however, in a most peculiar way – by something called “gynogenesis”. The fish steals the sperm of sexually reproducing molly species and uses it to trigger embryonic development of its own eggs. However, the sperm’s DNA is not incorporated into the embryo, meaning that all offspring are true clones of the mother.
That’s clever, but even after the subsequent discovery of somewhere between 50 and 100 other fully or partially parthenogenic vertebrate species, there is still doubt as to long-term viability of the strategy.
There are two major reasons for this.
The first is “Muller’s ratchet”, a term coined to honour Hermann Joseph Muller, the Nobel prizewinning geneticist who devised the idea. This suggests that in asexual clonal lineages any deleterious mutation will be passed on to all daughter clones. Each new damaging mutation is thus conserved in the genepool, and the accumulation of such will lead eventually to decay of the genome and, inevitably, the extinction of the species.
Alluding to a character from Lewis Carroll’s Through the Looking-Glass, the second reason, the Red Queen hypothesis, suggests that species must continually adapt to overcome the negative impacts of parasites, pathogens and environmental pressures: a sort of arms race. Asexual species have reduced genetic diversity – they are, after all, clones – which means their ability to evolutionarily adapt is severely restricted. The Red Queen hypothesis thus predicts asexual species will eventually be overwhelmed by antagonistic pressures and wiped out.
An international team led by biologist Manfred Schartl of the University of Würzburg, Germany, reports that the species is faring far better than the predictions would suggest.
The team sequenced the genome of the fish and compared it with its two parent species, Poecilia mexicana and Poecilia latipinna, only to discover a surprise. As biologists Pedram Samani from the Georgia Institute of Technology, US, and Max Reuter from University College London, UK, write in an accompanying editorial: “The main finding of the paper is that the species is in remarkably good genomic health.”
Indeed, so good is the genomic health of the molly that Schartl and colleagues estimate it has survived for 100,000 years, “several-fold beyond its Muller’s ratchet-based predicted extinction”.
The authors argue that some unique qualities have helped to keep disaster at bay. First is the fact that on rare occasions the DNA of the stolen sperm does in fact contribute to the makeup of P. formosa individuals, thus helping to alleviate genetic damage accrued over time.
Perhaps more importantly, the very first hybrid Amazon molly must have had very high levels of genetic diversity within its genome, a condition known as high “heterozygosity”. It is a characteristic that the species still displays. Indeed, the diversity of the molly is 10 times that of its parent species. This unique state of affairs, the researchers contend, is a “sufficient starting point” for the fish to be able to adapt enough to escape Red Queen dynamics.
These characteristics, on top of the fact that asexual species have an advantage in that 100% of individuals can produce offspring, show how parthenogenic vertebrates can be successful.
Their rarity, the authors conclude, is not because they are inferior to sexual species, but rather “because the genomic combinations that allow successful survival and reproduction are very specific”.
Thus, the “difficulty of creating a functioning asexual organism from a sexual ancestor,” write Samani and Reuter, is why asexual vertebrates are few and far between.