Biologists Debunk the "Rotting Y Chromosome" Theory

An anonymous reader writes "Biologists have previously predicted that that the male sex-determining Y chromosome, which once carried around 800 genes, like the X, has lost hundreds of them over the past 300 million years, will mutate itself out of existence, leading to the eventual extinction of men. However, researchers of a study published in the latest issue of Nature found evidence to suggest that the Y chromosome will not shed any more of the 19 ancestral genes that it is left with."

Comments

Both sexes are valuable

[Anonymous Coward]

While our Y chromosome may make us (men) more susceptible to genetic diseases, it also allows for more rapid adaptation and spread if a mutation is beneficial. I certainly wouldn't want it to go away.

Re:Both sexes are valuable

[Darinbob]

Except that they were talking about the Y chromosome. The problem with it is that it doesn't get combined with genetic material from the mother, it's passed on as-is. So over time it can degrade due to mutations, and it has done this in the past. However natural selection is strong enough to maintain it.

Re:Both sexes are valuable

[reverseengineer]

Well, there's no specific reason to favor the XY system of sex determination over some alternative arrangement, like the ZW system in birds (females are ZW, males are ZZ). In that case, the Z chromosome is larger and has more genes than the W. On the other hand, there's really no evidence to suggest that the XY system is any worse than the alternatives, or at least worse enough to support some sort of changeover (or lead to the extinction of placental and marsupial mammals). It does make sense to let Y "rot" to a certain extent: letting Y "cross over" with X is hazardous. It leads to the possibility of producing gametes that contain X chromosomes with male-sex determining genes, and gametes that contain Y chromosomes that lack those male-determining genes. It is to the system's benefit that X and Y are completely non-homologous, even factoring in the problem of X-linked diseases. It's theoretically possible that the function of Y could be captured in a single gene. However, chromosomes are also physical structures that have to be able to be manipulated by the machinery of the cell. It's likely that there is a minimum size for that to be done without high risk of error, which means that Y is safe.... for now.

Re:Both sexes are valuable

[reverseengineer]

Crossing over is normally a beneficial practice for chromosomes, and is a key advantage for sexual reproduction. See Muller's ratchet for the case of deleterious effects piling up in organisms who do not use recombination to shuffle around their genes. Therefore, it generally makes sense for chromosomes to pair up, so that at each generation, offspring get a mix of ancestral genes rather than a perfect (aside from mutation, of course) clone of a chromosome from parent's set. Being able to swap homologous genes dramatically increases the diversity of offspring. The X and Y pair is an exception because it would be a problem for recombination to occur. The genes on Y are supposed to be unique to males- if some regions of Y containing them recombined with X, those male-development specific genes could be traded away in exchange for nothing, likely leading to sterile offspring. Therefore, it has been to the advantage of XY-determination organisms to have X and Y as different as possible so that there is basically no recombination. The drawback is that Y doesn't get to recombine with anything (X can still crossover with another X), so a son's Y is essentially his father's Y and grandfather's Y, though with whatever errors have accumulated. This has led to a pruning of genes on Y over time- it appears from this paper that this deletion will not necessarily go to nothing, and that a "minimal Y" may be stable for many millions of years. It does however underline that the X/Y pair is a special case, that the endpoint of the asymmetric relationship that they have is for one chromosome to dwindle to a single purpose. The Y chromosome has 19 genes: X has around 2000.

Re:Both sexes are valuable

[Anonymous Coward]

Once again with the subject of genetics on Slashdot, we have a shocking level of confident ignorance on display (aided and abetted by the equally clueless moderators).

Please, evolution is not synonymous with natural selection. If all you know about genetics is what you learned in Biology 101, perhaps supplemented by a Dawkins book, you're missing out on most of the picture.

The degeneration of the Y chromosome was made possible by the lack of recombination along most of its length (Muller's ratchet/Hill-Robertson effect), which allowed the combined effects of mutation (including deletions) and genetic drift (which is much stronger on the Y due to there being 1/4 the number of Y chromosomes in a population than a given autosome) to very slowly truncate it. There's really no need to invent post-hoc selective stories to explain this; it's all pretty basic stuff.

Of course, you are correct that this doesn't mean that males would (or could) go extinct if the Y somehow did disappear. No competent scientist would ever claim this; most likely the sex-determining genes would move to other chromosomes.

Summary of this story in Nature

The origins of genome complexity

Why would anybody think otherwise?

[Rich0]

So, genders have been around for hundreds of millions of years - why would anybody think that evolution would suddenly make them go away?

In humans it probably doesn't make so much sense to have lots of sex-linked characteristics, so it makes perfect sense that the contents of the Y chromosome would dwindle over time to just the minimal set of genes necessary to confer gender. After that there should be strong selective pressure to conserve things.

Suppose for the sake of argument somebody is born with a Y' chromosome that doesn't confer maleness. Either they'll have non-functional reproductive organs, or functional female ones. In the former case they're an evolutionary dead-end. In the latter case and they reproduce with an XY man then 25% of their children will be normal XX females, 25% will be Y'Y offspring that won't make it to birth lacking an X chromosome, 25% will be normal XY males, and 25% will be XY' like the mother. So, in 75% of those cases the Y' chromosome is lost. And all that assumes that there aren't any deformities/etc that make reproduction less likely. I can't see how such a situation could ever become dominant. It would likely reach some low frequency equilibrium even if not harmful.

The fact that it hasn't already happened makes me think that it is not likely to do so.

Re:Why would anybody think otherwise?

[FrootLoops]

It's worth noting that conditions apart from standard XX female and XY male do occur in humans:

Turner syndrome: usually, single X chromosome, no second X or Y. Creates females who are almost always infertile with varying physical problems. Incidence is around 1 in 2000 to 1 in 5000 (phenotypic) females.
Triple X syndrome: XXX chromosomes. Makes females with essentially no physical differences from XX females (including reproductively). Incidence: 1 in 1000 females.
Klinefelter's Syndrome: XXY chromosomes. Produces sometimes-infertile males, sometimes with developmental problems. 1 in between 500 and 1000 males affected.
XYY Syndrome: XYY chromosomes. Almost no physical differences with XY males (slightly taller). 1 in 1000 males.
XX Male Syndrome: XX chromosomes. Produces always-infertile males who usually appear to be XY males. 4 or 5 in 100,000 people.
Swyer Syndrome: XY chromosomes. Produces females without developed gonads, though a developed uterus may be able to carry another person's embryo.

The above is only a partial list. There are quite a few related conditions that fall under the general heading of "Intersex" (sometimes you see the acronym LGBTI; that's the I). They vary widely from producing (some type of) hermaphrodites to causing a large number of non-standard sex characteristics. From the article,

According to Fausto-Sterling's definition of intersex, on the other hand, 1.7 percent of human births are intersex.

and

Between 0.1% and 0.2% of live births are ambiguous enough to become the subject of specialist medical attention, including surgery to disguise their sexual ambiguity.

To give a very approximate comparison (these numbers vary a lot by region, time period, and definition), around 1% of the population is bisexual, and around 5% is gay. It's perhaps even more difficult to get an accurate transgender incidence number; I've seen between 0.2% and 0.003%. Those who get sex reassignment surgery are in the minority. (There's a lot more to gender than the type of gonads you have, and female-to-male surgery isn't terribly effective.)

link to the source, please

[rritterson]

Not like I love the Nature Publishing Group (NPG) very much, but let's link to the source to help give the original authors credit. (Which, as far as I can tell, the medical daily article doesn't even do!)

Here is a link to the original paper

For those who aren't molecular biologists or geneticists, here is a link to the Nature news article on the scientific paper

Re:Seems like a non-issue either way

[John Hasler]

> ...evolution will see that...

Don't anthropomorphize evolution. It doesn't like that.

Re:correct me if I'm wrong

[samoanbiscuit]

No, the XX/XY system of sex determination is just one of the many types used in nature, mostly by mammals. Some animals use the XX(female)/X0(male) system, like ants and bees, while reptiles, birds and some other use the ZZ(male)/ZW(female) system. As you can see, in birds and reptiles (not crocodilians or turtles those have a temperature based sex determination mechanism) the ZZ chromosome configuration (the default) is male, while the ZW configuration causes female development. However, back to the disappearing Y chromosome, it is the fastest mutating chromosome in the human genome (and in all mammalian genomes) because it does not recombine with an analogue chromosome, the way two X chromosomes would. However, just because genes are "lost" does not mean it is shrinking, and research stated here shows it to be the case.

Re:Time scale

[artor3]

If "the right combination" is intelligence + virtually no trace of autism symptoms, then why not just take the intelligence part and leave off the crippling disability? Autism is not a prerequisite to high intelligence. There are plenty of truly brilliant people out there who aren't autistic at all.

I used to work in a school for autistic children. I was just the IT guy, but I was in the classroom at times and saw how hard the kids had to work to grasp things that most toddlers can do intuitively. I think it's unfortunate that when kids like that overcome their illness and do great things, people think "Wow, he sure benefited from those genes" instead of "Wow, that guy must have worked really hard to become so brilliant despite his disability."

Re:correct me if I'm wrong

[Michael Woodhams]

They Y chromosome not only evolves fast because of lack of recombination, but also because sperm are very many more cell division generations away from the original copy (fertilized ovum) than ova are. The Y chromosome spends 100% of its time in males, normal chromosomes 50%, X chromosomes 33.3%.

Ref: "Male-Driven Sequence Evolution", pg 225, "Molecular Evolution" by Wen-Hsiung Li (1997).