All-female fish species shows sex is overrated

An all-female freshwater fish species called the Amazon molly is living proof that sexual reproduction may be vastly overrated, scientists say after analyzing its DNA.

Amazon molly has healthy, diverse genome even though it has reproduced by cloning for 100,000 years

This is an Amazon molly (right), caught in action while seducing a male Sailfin molly to steal sperm. (Manfred Schartl)

An all-female freshwater fish species called the Amazon molly that inhabits rivers and creeks along the Texas-Mexico border is living proof that sexual reproduction may be vastly overrated.

Scientists said on Monday they have deciphered the genome of the Amazon molly, one of the few vertebrate species to rely upon asexual reproduction, and discovered that it had none of the genetic flaws, such as an accumulation of harmful mutations or a lack of genetic diversity, they had expected.

They found that the Amazon molly, named after the fierce female warriors of ancient Greek mythology, boasts a hardy 
genetic makeup that makes it equally fit, or even more so, than fish using sexual reproduction in which both maternal and paternal genes are passed along to offspring.

"The Amazon molly is doing quite well," said biologist Manfred Schartl of the University of Wuerzburg in Germany. 
"Unexpectedly, we did not find the signs of genomic decay as predicted."

Sperm stealer

The fish reproduces using a strategy in which a female's egg cell develops into a baby without being fertilized by a male's sperm cell. But that does not mean the fish does not need some hanky panky.
"The Amazon molly female produces clones of itself by duping a male of a closely related species to mate with her. The asexual mode of reproduction termed gynogenesis requires the female to mate with a male but none of the male's genome is passed to the offspring," said geneticist Wesley Warren of the McDonnell Genome Institute at Washington University in St. Louis.
The Amazon molly's egg cells are activated to develop into an embryo by a sperm cell that degenerates without fusing with the egg's nucleus.
The fish is up to about 3 inches (8 cm) long and eats insects, plants, algae and other food. The study showed it originated when two other species, the Atlantic molly and the Sailfin molly, mated about 100,000 to 200,000 years ago.
Animals that reproduce asexually are rare compared to the overwhelming majority that exist as males and females and 
reproduce sexually.

"It was long thought that vertebrates would not be able to exist in such a way. It was a sensation when the Amazon molly was the first asexual vertebrate discovered in 1932," Schartl said.
About 50 vertebrates are known to use asexual reproduction including fish, amphibians and reptiles.
The research was published in the journal Nature Ecology & Evolution