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."

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

[similar_name]

I never quite understood the idea of men going extinct. I'm not a biologist but it seems the last place survival of the fittest is going to stop working is on the first step. I mean once an x chromosome in a male stops functioning it's not going to get passed on. The ones that continue to function will continue to be passed on.

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:Time scale

[macraig]

That divergence might occur upstairs between the ears. Some groupings of autistic traits seem to be early precursors of that divergence. Call it a disability if you must, but there's gold in them genes for some folks who get the right combination.

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 [wikipedia.org]. 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: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).

Re:correct me if I'm wrong

[Michael Woodhams]

It just occurred to me that these two effects can be disentangled by looking at birds. The Z chromosome spends 2/3 of its time in males, so should evolve faster than normal (autosomal) chromosome, but it can recombine. The W spends 100% of its time in females, but has no recombination. The 'many times more sperm than ova therefore faster evolution (more errors) in males' may not hold for all animals, but it should hold for birds.

While I'm at it, I keep pointing out that a cophylogeny of mitochondria and W chromosomes could potentially measure the rate of 'paternal leakage' of mitochondria in a bird species, but so far as I know nobody has tried this.

Re:Both sexes are valuable

[Darinbob]

I think sometimes there's a misconception of natural selection. All traits and features from big to tiny are not necessarily selected for. Sometimes there are mistakes. Sometimes there are things that just happen without evolution being the answer. I see this especially in the social sciences, and I've heard things like "what is the evolutionary reason for having grandmothers" which may not even have a reason other than mothers not dying or losing maternal instincts. But people assume there must be an "evolutionary reason" too often. Another faulty thinking I see sometimes is the assumption that evolution leads to more advanced life forms and that nothing ever goes backwards (as in the saying "more evolved than that").

So in this case, my answer is that natural selection may not have favored the shortening of the Y, it just happens and it's not perfect and entropy is winning. Sometimes mutations are mistakes and are not weeded out, they're neither harmful nor beneficial and they don't degrade chances of reproducing. For instance there's likely no evolutionary advantage to hemophilia and it's more likely it's just a defect that pops up now and then; maybe it'll diminish over the eons.

Putting in a religion metaphor, I've seen people who want to portray god as a micro-manager so that anything that happens must be caused directly by god. But this is a rather naive theological stance that ignores things like free will. So on the evolutionary side I see the same thing, people wanting to treat evolution as a micro-manager.

Just my evolutionary pet-peeve...

Re:Both sexes are valuable

[jpapon]

Stop repeating things without fully understanding them. Making broad statements such as "free-will doesn't exist" is completely irresponsible, since it has in no way been proven experimentally. If one defines free will as "the ability of the conscious mind to make long term plans and see them to fruition", then neuroscience has, as of yet, nothing concrete to say on the subject.