Arabic AR Chinese (Simplified) ZH-CN English EN French FR German DE Japanese JA Portuguese PT Russian RU Spanish ES Ukrainian UK

Amazon fish challenges mutation idea

Latest news

    Amazon mollyImage copyright Reuters
    Image caption The Amazon molly is thought to be a hybrid of two different species

    Evolutionary theory suggests that species favouring asexual reproduction will rapidly become extinct, as their genomes accumulate deadly mutations over time.

    But a study on an Amazon fish has cast doubt on the rapidity of this decline.

    Despite thousands of years of asexual reproduction, the genomes of the Amazon molly fish are remarkably stable and the species has survived.

    Details of the work have been published in Nature Ecology and Evolution.

    There are two fundamental ways in which new generations of life come to being – sexual and asexual reproduction.

    Sexual reproduction relies on special reproductive male and female sex cells, the eggs and sperm, joining together during the process of fertilisation.

    Each sex cell contains half the number of chromosomes of normal parent cells, then following fertilisation, when the egg and sperm fuse, the normal cell chromosome number is reinstated.

    Asexual reproduction is different.

    A life born of celibacy

    Instead of creating a new generation by mixing equal measures of DNA from the mother and father, asexual reproduction dispenses with the male and instead creates new offspring containing an exact copy of the mother’s genome – natural maternal cloning, if you like.

    This is an incredibly efficient way of creating new life. By not wasting genetic material on the creation of males, all offspring arising from asexual reproduction can go on to produce more.

    But there is a downside. Because the progeny are genetic facsimiles of the mother they exhibit limited variability.

    And genetic variability can provide a big advantage. It’s what allows populations to respond and overcome changes in environment and other selective pressures – it underpins survival of the fittest.

    Sexual reproduction provides lots of scope to generate genetic variability; when pieces of individual chromosomes recombine as the eggs and sperm are formed and when the unique combinations of chromosomes are merged at fertilisation.

    Image copyright SPL
    Image caption Hermann Muller (in dark suit), after whom Muller’s Ratchet is named, is pictured here with his students

    Another advantage of sexual reproduction is that harmful mutations, which accumulate naturally over time, are diluted out and their effects nullified during this genetic mixing.

    Organisms relying on asexual transmission are presumed to forfeit these advantages.

    Prof Manfred Schartl, who is based at the University of Würzburg and is one of the lead authors of the study, said: “The theoretical predictions were that an asexual species would undergo genomic decay and accumulate many bad mutations and, being clonal, would not be able to rely on high genetic diversity to react to new parasites or other changes in the environment.

    “There were theoretical predictions that an asexual organism would demise after around 20,000 generations.”

    In evolutionary biology circles, this gradual and fatal accumulation of deadly mutations is known as Muller’s ratchet, in honour of the Nobel prize-winning scientist Hermann Muller who came up with the theory.

    But the latest study of the long-term stability of the Amazon molly fish genome has thrown up some surprising new insights into the potential cost of asexual reproduction.

    Beating the odds

    The Amazon molly fish is thought to be a hybrid that arose following breeding between two related species of fish – the Atlantic molly and the Sailfin molly.

    In order to multiply, the resulting Amazon molly relies entirely on asexual reproduction; one of only a handful of backboned animals that reproduce this way.

    In an unusual twist, the Amazon molly female can only reproduce when exposed to the sperm of a related species of molly, but the sperm DNA doesn’t usually find its way into the offspring.

    To define the impact of its celibate lifestyle, the team compared the genome sequences of Amazon molly fish collected from various locations in Mexico and Texas.

    Using the genome sequences the research team were able to build a phylogenetic, or family, tree.

    This tree showed that all the fish shared the same ancestor and this progenitor fish is predicted to have swum the American waters around 100,000 years ago.

    Tenacious survivor

    The Amazon molly had been around for half a million generations – far in excess of what theory would suggest.

    Not only that, but when the scientists looked for hallmarks of long-term genomic decay there were very few, as Prof Schartl explained:

    “What we found is that this fish had preserved its hybrid genome and what we know from plant or animal breeding is that when we try to make something better we breed a hybrid”.

    And he thinks it’s this ‘hybrid vigour’ that underpins the Amazon molly’s tenacious survival.

    “What nature has done is create from the beginning a good hybrid, which then thrived.”

    “Of course it got mutations but what we feel has not been taken into consideration is that evolution will wipe out the deleterious mutations and only those that become better with good mutations will thrive.”

    Commenting on the significance of the work, Dr Laurence Loewe, assistant professor at the Wisconsin Institute for Discovery at the University of Wisconsin-Madison told the BBC:

    “Usually species without regular recombination are not very long-lived evolutionarily. However, the Amazon molly seems to have found a way of surviving for a surprisingly long time without accumulating signatures of genomic decay”.

    View the original article:

    “To figure out how, we will likely have to combine many of the exciting advances in evolutionary genetics from the last 100 years.”

    In the same category are

    Trump may extend UK visit to play golf in Scotland Image copyright Getty Images US president Donald Trump may extend his visit to the UK in July in order to play golf in Scotland. The property magnat...
    London Ambulance Service taken out of special measures Image copyright LAS Image caption Inspectors said the trust was "inadequate" in 2015 London Ambulance Service (LAS) has been taken out of special ...
    Can English remain the ‘world’s favourite’ language? Image copyright Getty Images English is spoken by hundreds of millions of people worldwide, but do the development of translation technology and "hy...
    Colombia conflict: Swapping the battlefield for the football pitch Image caption La Paz Football Club aims to bring together victims and perpetrators of the armed conflict On a Sunday afternoon on a football pitch...
    When lesbian activists invaded the BBC On 23 May 1988 a group of lesbian activists invaded a BBC TV news studio as it went live on air. They were protesting against the introduction of new ...
    Customer choice under threat at Britain’s banks Image copyright Getty Images Until this year, things were looking up for the new generation of banks trying to make their way in Britain's notorious...

    Leave a comment

    Your email address will not be published. Required fields are marked *