Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

No sex for all-female fish species

12.02.2018

They reproduce through gynogenesis. Their offspring are clones of the mother. According to established theories, the Amazon molly should have become extinct a long time ago. A new study shows how the fish avoids this fate.

Species that produce asexually are rare among vertebrates, making the Amazon molly (Poecilia formosa) the big exception. The small fish species, who is native to the border region of Texas and Mexico, does not produce any male offspring. The females reproduce asexually through gynogenesis, making their daughters identical clones of themselves.


The Amazon molly does not produce any male offspring. The females reproduce asexually through gynogenesis. Nevertheless they need sperm to trigger the cloning process.

Foto: Manfred Schartl

This type of reproduction also means that they need sperm to trigger the cloning process. So the Amazon molly mates with closely related Molly fish to obtain this sperm. The sperm cells even penetrate the egg cell; however, none of the male’s DNA is incorporated into the Molly’s eggs. Rather, the egg completely destroys the male genes.

"According to established theories, this species should no longer exist. It should have long become extinct during the course of evolution," Manfred Schartl explains. The biochemist holds the Chair of Physiological Chemistry at the Biocenter of the University of Würzburg.

Schartl with an international team of researchers explored how the Amazon molly has managed to survive in spite of this. For this purpose, the researchers sequenced the genome of the fish species and compared it with that of related species. The results of their research are published in the current issue of the journal Nature Ecology & Evolution.

Contradictory to established theories

There are two main reasons that argue against asexually reproducing species surviving in the long run: "Harmful changes occur in any genome at some point. In creatures whose offspring are pure clones, these defects would accumulate over generations until there are no more healthy individuals," Schartl explains. Species that reproduce sexually can easily eliminate such defects when the number of chromosomes is reduced by half during formation of egg and sperm cells to be recombined subsequently during fertilization from half of the maternal and paternal chromosomes, respectively.

There is another argument against the long survival of a species whose offspring are all clones of their mothers: "These species are usually not capable of adapting to environmental changes as quickly as their sexually producing counterparts," Schartl says. So within a few generations, they should be on the losing side of evolution which calls for the "survival of the fittest".

Unique genetic variability

To answer the question why this theory does not apply to the Amazon molly, the scientists studied their genome as well as that of two related fish species that reproduce sexually. The main insight: "We found little evidence of genetic degeneration in the Amazon molly, but rather a unique genetic variability and clear signs of an ongoing evolutionary process," Manfred Schartl says and he continues to explain that especially the genes relevant for the immune system exhibit a high level of genetic variability in the genome of P. formosa.

From this the authors of the study conclude that this variability combined with a broad immune response essentially contributes to the fact that the Amazon molly does not share the fate of many other species that reproduce asexually, namely to fall victim to pathogens.

Further results of the study
• Comparing the genome of the related fish species P. formosa, P. latipinna and P. mexicana shows that the differences are minor. All three carry 25,220 protein-encoded genes.
• Surprisingly, the genome of P. formosa also contains genes which a female fish does not need, for example genes for spermatogenesis, the development of males or the meiosis of egg and sperm cells.
• The absence of major genetic damages cannot be explained by the fact that P. formosa developed only a few generations ago. A look inside the genome shows that the species probably evolved some 100,000 years ago. With a new generation born every three to four months, this amounts to about 500,000 generations since P. formosa first existed, which is much longer than what standard theories predict as the time until extinction. By the way, this is also many generations more than Homo sapiens can look back on.
• P. formosa probably participates in evolutionary processes as well, however, within the boundaries of a selection process of naturally occurring mutations and the competing clones. In this respect, asexual reproduction even proves beneficial for the Amazon molly: Without the expense involved in maintaining two sexes, the fish population can grow more rapidly and achieve a significant size.
• All known vertebrates that reproduce asexually are hybrids – two specimens of P. latipinna and P. mexicana were the "parents" of Amazon molly. The scientists therefore assume that a hybrid genome is the driving force behind such species staying fit. This, however, requires the hybrid genes to be compatible with each other - which is rarely the case.
• Researchers propose a new theory called "rare-formation hypothesis" to explain the chances of survival of asexually reproducing species to replace the old theories. According to this theory, asexual species are rare not because they are inferior to other species but because the conditions for a hybrid genome, which is crucial to survive and reproduce successfully, are so specific.

Clonal polymorphism and high heterozygosity in the celibate genome of the Amazon molly. Nature Ecology & Evolution http://dx.doi.org/10.1038/s41559-018-0473-y

Contact
Prof. Dr. Manfred Schartl, T: +49 (0)931 31-84149, phch1@biozentrum.uni-wuerzburg.de

Corinna Russow/Gunnar Bartsch | idw - Informationsdienst Wissenschaft
Further information:
http://www.uni-wuerzburg.de

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: In best circles: First integrated circuit from self-assembled polymer

For the first time, a team of researchers at the Max-Planck Institute (MPI) for Polymer Research in Mainz, Germany, has succeeded in making an integrated circuit (IC) from just a monolayer of a semiconducting polymer via a bottom-up, self-assembly approach.

In the self-assembly process, the semiconducting polymer arranges itself into an ordered monolayer in a transistor. The transistors are binary switches used...

Im Focus: Demonstration of a single molecule piezoelectric effect

Breakthrough provides a new concept of the design of molecular motors, sensors and electricity generators at nanoscale

Researchers from the Institute of Organic Chemistry and Biochemistry of the CAS (IOCB Prague), Institute of Physics of the CAS (IP CAS) and Palacký University...

Im Focus: Hybrid optics bring color imaging using ultrathin metalenses into focus

For photographers and scientists, lenses are lifesavers. They reflect and refract light, making possible the imaging systems that drive discovery through the microscope and preserve history through cameras.

But today's glass-based lenses are bulky and resist miniaturization. Next-generation technologies, such as ultrathin cameras or tiny microscopes, require...

Im Focus: Stem cell divisions in the adult brain seen for the first time

Scientists from the University of Zurich have succeeded for the first time in tracking individual stem cells and their neuronal progeny over months within the intact adult brain. This study sheds light on how new neurons are produced throughout life.

The generation of new nerve cells was once thought to taper off at the end of embryonic development. However, recent research has shown that the adult brain...

Im Focus: Interference as a new method for cooling quantum devices

Theoretical physicists propose to use negative interference to control heat flow in quantum devices. Study published in Physical Review Letters

Quantum computer parts are sensitive and need to be cooled to very low temperatures. Their tiny size makes them particularly susceptible to a temperature...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

2nd International Conference on High Temperature Shape Memory Alloys (HTSMAs)

15.02.2018 | Event News

Aachen DC Grid Summit 2018

13.02.2018 | Event News

How Global Climate Policy Can Learn from the Energy Transition

12.02.2018 | Event News

 
Latest News

Researchers invent tiny, light-powered wires to modulate brain's electrical signals

21.02.2018 | Life Sciences

The “Holy Grail” of peptide chemistry: Making peptide active agents available orally

21.02.2018 | Life Sciences

Atomic structure of ultrasound material not what anyone expected

21.02.2018 | Materials Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>