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Worm Species without Males Loses 25% of Genes!

  1. Jan 10, 2018 #1


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    Here is an article in the NY times about a species of round worm which evolved from having males and females to having self-fertilizing hermaphrodites.
    This released from selective pressure many genes involved in being male and male reproduction. They acquired random mutations until the decayed away over generations. About 1/4 of the genome (7,000 genes) was lost.

    Quite a genetic load to carry around and maintain a system of sexual recombination.
  2. jcsd
  3. Jan 12, 2018 #2

    jim mcnamara

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    Staff: Mentor

    Sexual reproduction has a lot of advantages, and this round worm just lost access to them. Dandelions (Taraxacum spp.) did the same thing, parthenogensis. And they seem to be doing exceptionally well, especially in lawns:-p They have also evolved ways of avoiding being mowed. The flower stem is super short compared to ones not inhabiting your lawn. The mower just whizzes on over the flower head. (Un-)Natural Selection?

  4. Jan 12, 2018 #3

    Buzz Bloom

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    Hi Jim:

    I think one of us has misunderstood what the cited paper reports.
    As I understand how ordinary hermaphrodite reproduction works, two individuals contribute genes to an offspring, which preserves the benefits of sexual reproduction. What is missing is just the a distinction between the X and Y chromosome. Each individual hermaphrodite can produce both eggs and sperm. It is sometimes possible for an individual to produce both an egg and sperm, and the sperm fertilizes the individual's own egg, but some hermaphrodite species have mechanisms to avoid this, for example: an individual cannot produce both eggs and sperm at the same time.

    I do not have access to the cited article, so I don't know the details of this particular worm's reproductive process.

    I found the following in the Times.
    But only a small minority of C. briggsae are males. The rest are hermaphroditic females that reproduce by self-fertilizing, or selfing. They have evolved the ability to produce sperm that merge with their own eggs.​
    This is apparently saying that individuals of this species do fertilize it's own eggs, but the species also has some males which to some limited extent preserves sexual reproduction. There is not enough information in the Times article to make an estimate about how much sexual reproduction takes place.

    This article has details about reproduction. The following describes the fraction of individuals which are males.
    Many scientists still raise the strain that Sydney Brenner selected in the early 1960s. Those worms will typically produce one new male for every thousand females. The frequency of males can by higher in wild populations, though. In some places, a third of the worms turn out to be male.​

    Last edited: Jan 12, 2018
  5. Jan 13, 2018 #4


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    There are self-fertilizing hemaphrodites. They breed with themselves, which is the same genetically as breeding with an identical twin. This is often called "selfing".

    In a purely inbreeding population, this will, in the long run, result in a reduction of their genetic heterozygozity (AKA genetic diversity), due to the random mechanisms of inbreeding. Each generation will have the number of genes that are heterozygous reduced by 50%, until the rate of mutation production equals the rate at which gene diversity is lost.
    Because there are many different cells undergoing this inbreeding process in parallel, many clones are created that derive from the founder, but which have all taken there own path of homozygozing their genes, so each individual can have different genetics until they are completely inbred.
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