University of Wuerzburg researchers just deciphered the DNA of the Amazon molly fish. They found that the all-female fish, one of the few vertebrate species to rely on asexual reproduction, has no genetic flaw, according to the study published in the Nature Ecology & Evolution journal.

There was no accumulation of harmful mutations or a lack of genetic diversity, contrary to the expectations of the scientists, Reuters reported. The species has a strong genetic makeup which makes it equally fit compared to fish that use sexual reproduction and pass maternal and paternal genes along to the offspring.

Fish still needs to mate

The Amazon molly does not need a male sperm cell to fertilize its egg. The species produces clones of itself by initiating embryogenesis by luring a male of a closely related species to mate; however, none of the male genomes is passed to the offspring, Wesley Warren, a geneticist at the McDonnell Genome Institute at Washington University in St. Louis, said.

Gynogenesis of the asexual mode of reproduction means the sperm cell activates the egg cells of the Amazon molly. But the sperm cell degenerates without fusing with the nucleus of the egg. The species usually mates with Poecilia latipinna, Poecilia Mexicana, or Poecilia latipunctata, or the Sailfin molly, Atlantic molly, and Broadspotted molly.

Laurence Loewe, an evolutionary biologist at the University of Edinburgh, said the mollies defy ecological theory and manage to co-exist. Hanna Kokko, an evolutionary ecologist at the Helsinki University in Finland, created a mathematical model based on the idea that if male members of the two sexual species are able to discriminate between females of its own species and females of the asexual species, the complex has a better chance of persisting.

The males would still mate with their female species more often than provide sperm for the asexual female species. The model also suggested that if the males are relatively efficient, they can continue servicing both sexual and asexual females as the population numbers go up, and then the three species should get along. But the system also collapses, with the molly species going locally extinct once every four years on the average, Kokko said, The Scientist reported.

The origin of the species is the mating of the Atlantic molly and the Sailfin molly about 100,000 to 200,000 years ago. Warren said that the stars are apparently aligned for the species because the hybridization of the genomes of two different species into a new one would need near perfect compatibility between the elements of the parent genome to bypass sexual reproduction which is practiced by most vertebrate species.

Asexual reproduction

When the Amazon molly was discovered in 1932 as the first asexual vertebrate, the fish created a sensation and excitement in the scientific community because the belief then was that vertebrates would not be able to exist using asexual reproduction to increase but are now thriving along the Mexico-Texas border.

Warren pointed out that the expectation was that asexual organisms are at a genetic disadvantage because if no DNA is introduced during reproduction, harmful gene mutations can accumulate over successive generations. It can lead to an eventual extinction of the species. Asexual reproduction also limits the genetic diversity within a species that the animal will eventually become unable to adapt to changes in the environment.

However, contrary to those beliefs, the Amazon molly is doing quite well. Since the first mating of the Atlantic molly with the Sailfin molly, the pair had spawned about 500,000 generations up to the present day, Warren estimated. He noted that the expectation of many harmful mutations which would accumulate did not materialize.

Since then, researchers have identified about 50 vertebrates that use asexual reproduction, including fish, amphibians, and reptiles. Newswise added that only about one in 1,000 of all living vertebrate species is into asexual reproduction.

Genomic secrets remain hidden

But while the Amazon molly has thrived for thousands of years and defied expectations, the species has remained resistant to give away its genomic secrets. Manfred Schartl, the chair of Biochemistry at the University, said it could be that the species has the best of both worlds. It apparently has some advantages seen in species that reproduce sexually and other advantages normally seen in species that produce offspring asexually such as large population sizes.

He pointed out that based on established theories, the species should no longer exist. It should have become extinct during the course of evolution, Sky News noted.

Michael Lynch, the director of the Biodesign Center for Mechanisms of Evolution at the Arizona State University, said the research caps an intensive, collaborative study. It marked the first glimpse of the genomic features of an asexual vertebrate and set up a platform for future molecular, cellular, and developmental work in the Amazon molly species.

[researchpaper 리서치페이퍼=​Vittorio Hernandez 기자]