Smallness of the human Y chromosome

Andrew Gyles syzygium at alphalink.com.au
Fri Nov 3 18:12:31 EST 2000

In article <8tpitg$d5b$1 at mercury.hgmp.mrc.ac.uk>,
  Mary K. Kuhner <mkkuhner at kingman.genetics.washington.edu> wrote:
> Andrew Gyles  <syzygium at alphalink.com.au> wrote:
> >Under natural selection a human population in which the males had
> >the 'anti-X gamete' Y chromosome would suffer the disadvantages of a
> >surplus of males and a scarcity of females. In the long run natural
> >selection would favour subsequent mutants of the 'anti-X gamete' Y
> >chromosome in which those mutant genes that discriminated against the
> >production of X-bearing gametes in spermatogenesis or sperm
> >were deleted or rendered inactive.
> This is the step in the argument that I doubt.  A Y chromosome
> with meiotic drive *always* has a local advantage over one without,
> even when the population as a whole is suffering badly from
> excessive males.  Group selection might possibly be able to
> push down the frequency of driven Y, but even if one accepts that
> group selection is a realistic possibility here, it tends to be
> weak and slow compared to individual selection.
> On the other hand, the X and the autosomes see a straightforward
> advantage in not allowing the Y they're with to push them into
> a male zygote when males are wildly overrepresented, so X and
> autosomal suppressors are straightfowardly advantageous.  It
> seems to me, therefore, that the usual way a driven Y stops being
> driven is that a non-Y suppressor becomes fixed, not that the
> drive locus is damaged or deleted.
> There are a fair number of observations of meiotic drive in the
> literature; you could look for references to suppressors/revertants
> and see what chromosome they're on.  I'd predict they're usually
> not on the Y.
> Mary Kuhner mkkuhner at genetics.washington.edu

Thank you for your comments on a Y chromosome with meiotic drive, and
the likelihood that this drive will be opposed by
suppressors/revertants located on the X and autosomal chromosomes, not
(as I suggested) by deletions from the Y. These mechanisms will operate
during spermatogenesis.

I also mentioned the stage of sperm maturation, and here I was thinking
of the possibility that if X-bearing sperm could be detected by the
male organism while the sperm is maturing the organism might be driven
by its Y chromosome to kill the X sperm. Of course the X chromosome in
the male organism might drive the organism to kill the Y-bearing sperm
if the sex of that sperm could be detected.

It would be a bit like submarine warfare, in which the first rule of
the submariner is to escape detection - hence the quiet-running
propellors, and the prejudice against mechanics who drop monkey
wrenches in engine-room.

Saccone and her colleagues mentioned in a 1995 paper that "In mammals,
too, mtDNA contributes to the synthesis of a structural product (ND1)
in the maternally transmitted component of the minor histocompatibility
antigen". Histocompatibility antigens are exposed on the surface of a
cell. The mtDNA contribution will of course appear in both X sperm and
Y sperm, but I assume that the rest of the maternally transmitted
component of this antigen is coded for by genes on the X chromosome.

If there is no paternally transmitted analogue of this antigen
component, coded for by genes on the Y chromosome, the Y gametes might
be undetectable during maturation. Perhaps the battleground between the
Y and the X was tilted in the Y's favour when the Y lost genes that had
coded for a paternally transmitted component of a histocompatibility
antigen. In that case the Y sperm could not be detected and killed, but
the X sperm could be detected and killed. (I do not imply an
immunological attack.)

This killing activity by the Y chromosome could perhaps be countered by
natural selection only by favouring deletions of the genes on the Y
that were responsible for it.

In any evolutionary competition between the X and the Y to get into
most of the gametes formed in a male organism the Y seems to have a
couple of advantages. Firstly, if we look at many generations of
transmission, the X spends half its time in a male organism and half in
a female one. The Y spends all of its time in a male organism, so
mutations that allow it to reduce the number of X-bearing sperm cells
can always be selected for.

Secondly, I understand from a recent paper that the average mutation
rate in male organisms is about four times greater than that in female
ones. Since a Y spends all its generations in a male organism and an X
spends only half its generations in a male organism, the Y will be
mutated more quickly.

Andrew Gyles

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