Male vs. Female Chromosome War?
May 8, 2005 10:32 AM   Subscribe

Are there really such things as "scars on chromosomes" and "X vs. Y" 'wars' between the male- and female-determining genes?

Next-to-last paragraph at this exitmundi page sez

"Still, like most of the scenarios outlined on Exit Mundi, the one with the chromosomes already happened once - almost. Somewhere in our prehistoric past, X seems to have launched a massive attack on Y. It must have been a very close call. But Y succeeded in repairing itself at last. We can tell this has happened because of certain scars that are still detectable on the male chromosome."

What the hell is THAT all about? Also, is this an accurate picture of the X and Y chromosomes? It's a bit deflating for a guy to see...
posted by rleamon to Science & Nature (15 answers total)
Don't anthropomorphize chromosomes. They don't like it.

Yeah, the human Y chromosome is a stunted little thing. There aren't very many genes left on the Y chromosome, it is true.

But attacks? Scars? That's crazy talk.
posted by grouse at 11:34 AM on May 8, 2005

Also, let me add that the Y chromosome is deteriorating over evolutionary time. Since, unlike the X chromosome, it doesn't ever get to recombine with a sister chromosome, deleterious mutations just stick around. There are some areas of the Y chromosome that have multiple copies so that recombination can still occur within the same chromosome, but it's a poor substitute.
posted by grouse at 11:38 AM on May 8, 2005

"Anthropomorphize chromosomes" -- how's THAT for a feedback loop?
posted by rleamon at 11:39 AM on May 8, 2005

...I don't think that the people who wrote that page have much understanding of how biology works. Parts of the Y chromosome are homologous with the X chromosome - that is, they contain the same genes [if not necessarily the same phenotypes], and can recombine during meiosis. The Y chromosome is much smaller because a large percentage of the genes on it are sex-related, while most of the X chromosome doesn't relate to sexual characteristics. The SRY [sex-determining region y], for example, is one of the most important genes on the Y chromosome. So yeah, the Y chromosome is much smaller than the X, but that's because most of the genes on the X chromosome have nothing to do with sex characteristics and are active in both genders, while those in the Y chromosome are mostly sex-related.

So where's the problem in their argument? Well, the X chromosome doesn't 'know' what the sex-related genes on the Y chromosome code for. The exitmundi page is a mess of anthropomorphization - the X chromosome is 'angry' at the Y chromosome for the way sperm are designed. This makes _absolutely no sense_, biologically. Sperm have to be equipped to deal with the environment of the vagina and uterus, which is generally fairly hostile to outside biological intruders, but there's no way that the X chromosome could 'know' about that, and no way it could _choose_ to retaliate by damaging the Y chromosome chemically. Heck, what's it supposed to do, synthesize new genes to make proteins that damage the Y chromosome only? Riiight. [Note also that sperm don't 'shut down womens' immune systems' - if they did, you'd see a lot of women getting very very ill after sex.]

Many of the rest of the arguments are full of shit, too. People who've had a deletory mutation in their SRY region will show female characteristics, just as people who've had a deletion in a gene related to sight will show the characteristics of blind people - it's the result of random chance, not chemical warfare against men [or against sighted people.] Plenty of people are infertile, both male and female [although the site does not note that women can also be infertile, of course.] The woman who gave birth only to girls [not such a terribly odd occurance to begin with] probably did so as the result of random chance, unless she had a truly staggering number of children. Her husband's sperm might have had something to do with it as well, but it certainly wasn't her X chromosome figure out how to 'end the male problem.' Species may have differences in the number of males and females born [even in humans the rate is not 50/50], as well as the survival rate of a given gender. Again, this has nothing to do with chromosomal warfare, and everything to do with reproductive strategies, physiology, etc.

Chromosome numbers change over time - chromosomes are combined, split, etc. These changes happen through mistakes in mitosis and meiosis, and through mutations. That generally doesn't mean that the genetic data is lost, but rather that the data is part of a new chromosome. Thus, if the Y chromosome, as such, disappeared, one would find that the sex-related genes tacked on to another chromosome, or perhaps in their own even smaller new chromosome. So certainly, the Y chromosome, along with the rest of the genome, has changed since the origin of humans. [Chimps, for example, have 24 chromosome pairs while humans have 23.] But these kind of changes aren't a 'quirk of nature that astounds scientists', they're a regular part of genetics.

In short, men are in no danger of disappearing, the X chromosome isn't waging a vicious microscopic against the beleaguered Y chromosome, and is full of shit.
posted by ubersturm at 11:40 AM on May 8, 2005

It sounds like the description you describe may be based on a chapter in Matt Ridley's book Genome To wit:

"A piece of simple statistics: because females have two X chromosomes while males have an X and a Y, three-quarters of all sex chromosomes are Xs; one quarter are Ys. Or, to put it another way, an X chromosome spends two-thirds of its time in females, and only one-third in males. Therefore, the X chromosome is three times as likely to evolve the ability to take pot shots at the Y as the Y is to evolve the ability to take pot shots at the X. Any gene on the Y chromosome is vulnerable to attack by a newly evolved driving X gene. The result has been that the Y chromosome has shed as many genes as possible and shut down the rest, to ‘run away and hide’ (in the technical jargon used by William Amos of Cambridge University). (“Chromosomes X and Y: Conflict”) "

That doesn't mean it's not correct, to some extent, but not in the anthropomorphic sense of the X chromosome hiding in the trenches and lobbing Artillery-style bombs at the Y.
posted by greatgefilte at 11:45 AM on May 8, 2005

Is it purely mutations is the question -- is there something in the womb that literally attacks sperm cells in such a way as to harm them even further?
posted by rleamon at 11:45 AM on May 8, 2005

rleamon, we're not talking about sperm cells, just the genetic material that resides in all diploid cells. Y has less opportunity to repair itself, so "attacks" (meaning deleterious events) from anywhere (not just the X) have more of an opportunity to diminish its size and influence.
posted by rxrfrx at 11:56 AM on May 8, 2005

Well, grouse is mostly right, thought there's some recombination between the homologous regions of the X and Y chromosomes. One suspects that there will be some sort of chromosomal reorganization in the future, certainly. In Drosophila, for example, sex is determined by ratio of X and Y chromosomes. In birds and butterflies, males are the ones with the matching chromosomes [ZZ] while females have a mismatched set [ZW.] I'm sure there are other systems extant that I don't know about. The fact that somehow, the distinction between male and female has managed to survive for hundreds of millions of years [and presumably longer than that] suggests that the male sex genes will end up surviving in some fashion, even if the Y chromosome itself does not.

Rleamon, the cervical mucus provides an environment that is more acidic than is ideal for sperm. The vagina's a pretty hostile place for them in general. We're not talking "antibodies against sperm" here; the vaginal canal is simply too stressful for most sperm. It should be noted that the environment isn't mutagenic - what it does is kill sperm outright. If a sperm makes it to the egg, it won't have incurred genetic damage from the vaginal environment.
posted by ubersturm at 12:00 PM on May 8, 2005

Also, about the linked article, I should say that it's a ridiculously uninformed analysis. To say that "females are winning" in a way that is relevant to humans is just dumb. The method of sex determination and dynamics of reproduction are so massively diverged between even similar species... it's clear that these things are very malleable to fit a particular evolutionary niche. This doesn't mean that sex determination in humans is going to somehow change in the next few (thousand) years.
posted by rxrfrx at 12:01 PM on May 8, 2005

is there something in the womb that literally attacks sperm cells in such a way as to harm them even further?

On preview, what rxrfrx said. And besides, it would be self-defeating to damage sperm cells already in the female reproductive tract, since that would prevent any of the parents' chromosomes from being passed on to progeny.

Some women produce anti-sperm antibodies, but as far as I know, it's very rare and certainly not helpful (in teleological terms) to the X chromosome.
posted by greatgefilte at 12:01 PM on May 8, 2005

Just for the beauty of it, here's a nice colourful picture of human chromosomes. And no, they aren't coloured that way in real life. :)
posted by greatgefilte at 12:04 PM on May 8, 2005

Greatgefilte - sperm are pretty much safe once they're into the uterus. At most points in the female reproductive cycle, the vaginal region is downright hostile to sperm - acidic, and the cervical mucus forms a thick gel that makes it difficult for sperm to make it through. Around ovulation, conditions become more sperm-friendly, though the environment still isn't ideal. So when women are at their most fertile, the vaginal environment is at its least hostile, but most of the time it's pretty hard for sperm to survive. At least, that's my understanding of it. I realize that my previous comments made it sound like the vagina is always very hostile to sperm, which isn't the case.
posted by ubersturm at 12:17 PM on May 8, 2005

And my understanding is that the vaginal secretions during sex raise the pH thereby making the environment less hostile. Meaning it's acidic normally but not when conception is a possibility. Correct me if I'm wrong.
posted by peacay at 12:19 PM on May 8, 2005

Not smack on topic, but my views on considering males and females part of the same species, rather than in uneasy symbiosis, changed drastically on reading Dr. Tatiana's Sex Advice to all Creation. That, and this thing about ligers.
posted by Aknaton at 2:28 PM on May 8, 2005

Here's an excellent article from the New York Review of Books about the idea of shrinking Y chromosomes.
posted by Termite at 11:46 PM on May 8, 2005

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