The origin and fate of the Y chromosome

Factual Questions
1

Bryan Sykes has a new book called Adam’s Curse which is an alternate selection for the Scientific American Book Club. I haven’t read teh book yet, but this is an organization I would assume uses at least some screening so I wouldn’t dismiss what I see here.

The description of the book tells of the deterioration of the Y chromosome because it alone amongst human chromosomes has no pair with which it can swap genes and repair possible mutations. In fact, it concludes with the following “‘The historical process of decay which is all too evident in the wretched condition of our Y chromosomes is far from over.’ In 125,000 years Sykes estimates, no male chromosomes will survive.’”

I have heard this before, though not this specific prediction.

I have two questions:

I know (at least some) reptiles do not have a comparable sex chromosom, but that incubation temerature determines sex. When in evolutionary history did a specific sex chromosome arise? Do all primates have it? All mammals? Most mammals?

Is there evidence of deterioration in the Y chromosome in other animals? 125,000 years sounds very short in terms of a species’ life. If humans are this close, one would think that other species might be even closer to losign their Y chromosome.

Any thought?

2

I don’t know anything about losing the Y chromosome, but I can say that some species all up and down the evolutionary tree have one, including fruit flies, at least some birds (aside: chickens have something called the W chromosme, because its presence creates females, and the lack thereof defaults to male), some reptiles. It doesn’t seem like something detrimental or easily lost.

In support of his arguement, it does seem like sex determination is generally a trait that changes very rapidly (in evolutionary time). You’ve heard about reptiles where incubation temperature determines sex, and probably about ants where haploid or diploid genomes (1 or 2 copies of each chromosome) determine sex, and maybe about ants where nutrients fed to the developing larvae determine sex. Variety does seem to be a general trend. For instance, if you look at the fly species closely related to fruit flies, there are a wild array of sex determination methods even among these closely related species. These include:

[ul]Fruit flies themselves. They have a Y chromosome, but it doesn’t necessarily determine maleness, the ratio between X and Y chromosomes does. So a single X is female, XY is male, XXY is female, XXYY is male.[/ul]

[ul]Another species is similar, but the ratio between Y chromosomes and the autosomes (non-sex chromosomes) determines the sex.[/ul]

[ul]In another, the sex of the offspring is determined by the genotype of female that lays them. A given female lays only males or only females.[/ul]

[ul]In one, every embryo inherits two X chromosomes and two Y chromosomes, then seems to “lose” some during development. What’s left after they are “lost” determines sex.
[/ul]

The counterarguement to this is that sex determination seems to have become “fixed” in mammals. All mammals (that I am aware of) use the same XY system that humans do, with very minor varitions (single X females are fertile in some species). If humans were to lose the Y chromosome, we would need to evolve a whole new sex determination system. Since our entire genomes are now set up around the one we have, this doesn’t seem so likely.

All of that said, I haven’t read the book either, so I don’t know how much sense this guy is making.

mischievous

3

All mammals do, indeed, have the XY system. Birds have a WZ system, wherein a WZ chromosome pair results in a female, and a ZZ pair results in a male. It is believed that the evolution of differentiated mammalian sex chromosomes occurred in about four discrete stages over a period of about 300 million years. See here for some (relatively) recent research in the evolution of XY chromosomes. Similar findings (though with an obviously different time period) have been found with respect to avian sex chromosomes - see here, for example.

4

Thanks for the replies.

Given that all mammals have XY sex determination and the separation of the X Y chromosomes dates to some 300 million years ago. It woudl seem that all mammals should tehn be suffering from the same Y degradation discussed in this book and all mammals should be heaging for an absence of the Y chromosome – unless something particular happened to humans to accelerate this process. If Sykes’ hypothesis is correct, we’re then in the last 125,000/300,000,000 < 0.04% of this period. That’s hard to believe.

5

Well, the fact that the Y chromosome can indeed crossover with the X chromosome would indicate that this guy doesn’t know what the hell he’s talking about. What does “deterioration” of the Y chromosome even mean? Sure, it’s small. But so what? It still contains important genes.