Home » Eyes Rolling, Human evolution » Male sex chromosome losing genes by rapid evolution, study reveals

Male sex chromosome losing genes by rapid evolution, study reveals

Friday, 24 July 2009 01:05 ALEXANDER CHIEJINA With Agency Report

Scientists have long suspected that the sex chromosome that only males carry is deteriorating and could disappear entirely within a few million years. However, until now, no one has understood the evolutionary processes that control this chromosome’s demise. Now, a pair of Penn State scientists has discovered that this sex chromosome, the Y chromosome, has evolved at a much more rapid pace than its partner chromosome, the X chromosome, which both males and females carry.

Read more…

Here is some relief:

“Even though some of the genes appear to be important, the team thinks there is a chance that the Y chromosome eventually could disappear.
If this happens, it won’t be the end of males. Instead, a new pair of non-sex chromosomes likely will start on the path to becoming sex chromosomes.”

But what if the female species takes another evolutionary direction and becomes asexual, like the Mycocepurus smithii ant (otherwise known as “Queen B”)?  Will this be the end of evolution for humans?

No variation, no diversification, no evolution!  Oh darn, all women will look the same.  :(

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23 Responses to Male sex chromosome losing genes by rapid evolution, study reveals

  1. Deteriorating? Isn’t that kind-of the reverse of what we have in mind (picture thinking) when we were taught evolution?

    Oops I forgot, it’s directionless.

    Except in the unobservable past.

  2. It has long been known that sex chromosome systems are characterized by the deterioration of the Y (or whatever one identifies the heterogametic sex). The reason for that is, the mechanism that keeps the sex chromosomes separate is greatly reduced recombination between them. That isn’t a problem for the X chromosome because in the homogametic (i.e., XX) sex, recombination occurs freely. But it does mean that the Y will accumulate deteriorating mutations over time.

  3. So if recombination is limited deterioration is inevitable?

    But wait males also have an X, therefor that X should also show signs of deterioration.

  4. Josesph,

    You need to think of this as deterioration over time in a population, not an individual.

  5. Dave,

    Individuals make up the population.

    Males are part of the population.

    Males have gametes that undergo recombination.

    Males have an X chromosome which cannot recombine- according to you.

    Therefor if a man has a son, who then has a son, who then also has a son, we should be able to see some X chromosome deterioration somewhere along that patrilineage in at least some of the gametes.

  6. BTW each male carries a population of gametes.

    In that population we should see the X chromosome deteriorate over time.

    That is if your premise is correct.

  7. Joseph,

    Males contribute X chromosomes to their daughters, which can then undergo recombination and be passed on to their sons and daughters, and so on. In other words, every male receives his X chromosome from his mother, where it underwent recombination with its homologous X. Surely this doesn’t have to be explained to you further.

  8. Did you have a point?

    How did what you posted address:

    BTW each male carries a population of gametes.

    In that population we should see the X chromosome deteriorate over time.

    That is if your premise is correct.

    My point is that a dad can give a deteriorated X to his daughter.

    And you still haven’t addressed:

    So if recombination is limited deterioration is inevitable?

    IOW how does recombination prevent deterioration?

  9. joseph,

    My point is that a dad can give a deteriorated X to his daughter

    And my point is that X can then undergo recombination with its homologue, which can ameliorate the overall degenerative effect. The Y cannot do that at all.

    And you still haven’t addressed:

    So if recombination is limited deterioration is inevitable?

    IOW how does recombination prevent deterioration?

    If you cannot see how a chromosome that never recombines with its homologous chromosome can avoid eventual deterioration, then you need to brush up on your basic population genetics. The situation is similar to what happens with asexual populations. The genomes of asexual populations inevitably accumulate deleterious and degenerative mutations under a process known as Muller’s Ratchet. Without recombination, the Y will accumulate degenerative mutations in the same way, with no means of bringing in “healthier’ DNA from a homologous chromosome.

  10. Luckily, Joseph didn’t bring up Platypus sex chromosomes. I must admit that I indeed would be interested to see a calculation of the FSCI content of human and Platypus XY systems. Not to forget the WZ chromosomes of birds, the sex determination in Drosophila and the sex inheritance in the strictly monogenic fly Chrysomya rufifacies.

  11. joseph,

    A good explanation of Muller’s Ratchet and recombination can be fo8und in Joe Felsenstein’s paper on the advantages of recombination:

    Felsenstein J (1974). The evolutioinary advantage of recombination. Genetics 78: 737-756

  12. Umm but there isn’t any guarantee that the other X chromosome is OK.

    IOW what happens when two degenerative X chromosomes start swapping sequences?

    But anyway perhaps there is just an error-correction problem with the Y chromosome.

    I would think that a robust error-correction setup would alleviate any deterioration.

    BTW I understand that recombination provides variation and variation is key to survival.

    As Dr Spetner made clear this is evidence for non-random evolutionary processes.

    And if asexual populations inevitably accumulate deleterious and degenerative mutations why are there still asexual reproducing organisms left?

  13. Dave Wisker,

    What is there to prevent a couple Adams or at least one Adam from persisting and thus passing on to all his male heirs a good Y gene? So that in the population there will always be a perfect copy of the Y gene? Granted some of the heirs will have a defective Y gene but it seems impossible that all will be eliminated through mutation.

    Unrelated but related question. What other genes are on the Y chromosome that are specifically male oriented and cause variation in the phenotype/morphology of males?

  14. 14

    joseph,

    One of the advantages recombination gives is the ability to get rid of multip-le deleterious alles in one gamete by producing chromosomes with several bad mutations at once. Those get removed from the population via natural selection.

    If you give your asexual population question above a little more thought, you will see that that purely asexual populations will undergo a gradual loss of fitness. However, the vast majority of organisms that reproduce asexually do have means for recombination to occur at some level: bacteria, for example can do so via conjugation and lateral gene transfer. Dandelions reproduce asexuallly most of the time (via apomixis), but there is a small percentage of individuals that can reproduce sexually and thus allow low-level recombination to occur which allows fresh combinations of alleles to enter the population.

  15. 15

    Hi jerry,

    What is there to prevent a couple Adams or at least one Adam from persisting and thus passing on to all his male heirs a good Y gene? So that in the population there will always be a perfect copy of the Y gene? Granted some of the heirs will have a defective Y gene but it seems impossible that all will be eliminated through mutation.

    That’s a great question. First of all, much of the Y chromosome’s length has become inactivated by heterochromatin. Secondly, any severely deleterious mutations that are expressed will be eliminated via natural selection. What you end up with over time is a chromosome that expresses only those genes that are responsible for sex differentiation. It may be present polymorphically in the population, with some individuals having better copies than others. It depends on the level of selection. In some groups, copies of the genes for differentiation can end up on another chromosome (via transposon activity, for example), and the old chromosome can be lost, in which case the cycle can begin again, with selection for suppressed recombination on the new chromosome beginning again. In the cases I am familiar with in the insects, this doesn’t happen in a linear fashion along one lineage; instead, the process starts and is kept in a divergent lineage via speciation. The result is a genus or family with species showing differing stages of the process in progress.

    Unrelated but related question. What other genes are on the Y chromosome that are specifically male oriented and cause variation in the phenotype/morphology of males?

    I don’t know specifically. I thiunk I saw an article in Nature once that discussed that. I’ll see if I can find it for you.

  16. 16

    Hi Jerry,

    Here’s that reference:

    Skaletsky, H et al (2003). The male-specific region of the human Y chromosome is a mosaic of discrete sequence classes. Nature 423: 825-837

  17. The whole genome is devolving, not just the Y chromosome:

    http://www3.interscience.wiley.....6/PDFSTART

  18. 18

    sparc,

    Luckily, Joseph didn’t bring up Platypus sex chromosomes. I must admit that I indeed would be interested to see a calculation of the FSCI content of human and Platypus XY systems. Not to forget the WZ chromosomes of birds, the sex determination in Drosophila and the sex inheritance in the strictly monogenic fly Chrysomya rufifacies.

    There are also some astonishingly weird sex chromosome systems in beetles.

  19. If the y chromosome is deteriorating, was it at some sort of peak state in the past? How did it get there and why is it deteriorating now? Do evolution and deterioration happen simultaneously?

    Dave, you have mentioned numerous systems for sex determination. Did they evolve separately?

  20. 20

    Hi merlin,

    If the y chromosome is deteriorating, was it at some sort of peak state in the past? How did it get there and why is it deteriorating now? Do evolution and deterioration happen simultaneously?

    Note: for this I will refer only to the system used by the majority of mammals, that is, an X/Y system with the male-determining genes on the Y.

    The Y chromosome presumably started out as homologous the X. That is, it was an X. Sex determination systems that use chromosomes are characterized by a lack of recombination between the X&Y. Lack of recombination keeps the male-determining genes on the Y (they cannot, during meiosis, get exchanged during crossing-over and find themselves X) . This ensures that the male-determining genes are segregated during meiosis in a regular and stable way.
    Why this situation leads to gradual deterioration is due to a phenomenon in population genetics called “Muller’s Ratchet”. Let’s consider a case where recombination occurs freely between two homologous chromosomes, say on the X. Let’s also assume a deleterious mutation occurs on one X and a deleterious mutation occurs to a different gene on the other (assume they both occur in the same individual, not necessarily at th same time). If recombination occurs between the two chromosomes during meiosis, one of the gametes can have an X chromosome that doesn’t have either of the two deleterious mutations. Not only that, an X chromosome will also be generated with both deleterious mutations on it. This might sound like a very bad thing, because offspring could be produced with both bad mutations at once. Natural selection could eliminate such individuals. But consider what would happen if no recombination occurred. Half of the offspring would carry the first deleterious mutation, and the other half would carry the second. Recombination, therefore, enables populations to get rid of multiple deleterious mutations more efficiently, by concentrating them in fewer individuals. Without recombination, deleterious mutations stay on the chromosome they appeared on. Over time, those chromosomes accumulate deleterious mutations that they can’t get rid of. The ‘ratchet’ analogy comes into play here: one bad mutation establishes itself on a chromosome and stays there because recombination cannot move it, essentially ‘ratcheting’ up the bad mutation count by one. As additional bad mutations occur, the ratchet tightens.

    Dave, you have mentioned numerous systems for sex determination. Did they evolve separately?

    I don’t know. Some may have.

  21. Hi Dave,

    If there were two X chromosomes, then it must have been before sexual reproduction. So is the theory that an XX “female” by default budded off (or whatever) an X(deteriorating X), functioning male? Even if the X lost a large portion of its DNA, wouldn’t the rest need to be fine tune by NS to function. I assume this is just someone’s speculation.

    Furthermore, this deterioration had to be happening since the LCA of all XX,XY species, didn’t it?

    Have you read Genetic Entropy by John Sanford? He discusses the difficulty of eliminating deleterious mutations.

  22. Rash Human Y Demising?
    On The Male Sex Chromosome

    A. On Human male sex chromosome, aaagain?

    From “Male Sex Chromosome Losing Genes By Rapid Evolution, Study Reveals”
    http://www.sciencedaily.com/re.....201127.htm

    “…we also know that most of the genes were not important for survival because they were lost, which led to the very different numbers of genes we observe between the once-identical X and Y. Although there is evidence that the Y chromosome is still degrading, some of the surviving genes on the Y chromosome may be essential, which can be inferred because these genes have been maintained for so long.”

    “…found evidence that some others are on track to disappear, as well.”

    B. From “Spontaneous speciation?”
    http://www.the-scientist.com/blog/display/55825/

    And from “On The Origin And Tasks Of Brain Cells”
    http://www.the-scientist.com/c......page#2822

    And from “Conservation of Y-linked genes during human evolution revealed by comparative sequencing in chimpanzee”
    http://www.nature.com/nature/j.....04101.html

    Chimp’s genome has been continuing survival by physiologically adapting to changing environments,
    whereas Human’s genome continued survival mainly by modifying-controling its environment.

    C. AcademEnglish verbiage should be at least scientifically careful

    The rate of “losing genes” by an organism is not a constant value of a natural law. It is induced and set mostly by the rate and nature of the change of culture of the organism, which is induced, in turn , by various circumstential factors…

    IMO we can feel assured that the human Y is not on an accelerating course to oblivion…

    Dov Henis
    (Comments from 22nd century)

  23. Hi merlin,
    If there were two X chromosomes, then it must have been before sexual reproduction. So is the theory that an XX “female” by default budded off (or whatever) an X(deteriorating X), functioning male? Even if the X lost a large portion of its DNA, wouldn’t the rest need to be fine tune by NS to function. I assume this is just someone’s speculation.
    We don’t know exactly how it happened.
    Furthermore, this deterioration had to be happening since the LCA of all XX,XY species, didn’t it?
    Yes, but in assessing the fitness effect of the deterioration, one has to balance it against the very high selective value of the male-determining genes, and the effects of compensatory and beneficial mutations as well.
    Have you read Genetic Entropy by John Sanford? He discusses the difficulty of eliminating deleterious mutations.
    I have read Sanford’s book, but I didn’t find his argument particularly convincing. For one thing, he seems to have cherry-picked his references: Sanford makes much of Higgins & Lynch’s 2001 paper on mutational meltdown, but completely fails to note that in 2003 Estes and Lynch went on to examine experimentally how populations with very high genetic loads can recover fitness via compensatory & beneficial mutations (Sanford’s book was published in 2005). He also tries to argue against Crow’s quasi-truncation selection model in an inconsistent manner. One the one hand he criticizes the model for not being biologically realistic, while on the other he runs his own simulation with a population size of only 100 (which introduces stochastic effects such as drift and ignores the well-known fact that natural selection in large populations is more efficient overall, and which may help ameliorate the human situation, since our populations today is quite large). More recent work has not been kind to his thesis, either. A 2006 paper by Reed and Aquadro points out how quasi-truncation selection (as described by Kimura and Crow in 1978) is efficient enough to require only 2.2 offspring per couple to counteract the deleterious mutation rate, using human mutational load data. Sanford also ignores other mechanisms that populations use to eliminate deleterious mutations, such as gametic selection, which has the potential for removing a significant number of them as well. So color me skeptical about his thesis.

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