Layman question
JQ
I know that in mammals, the sex-determinate chromosome is in the
sperm, and in birds (for example) it's in the ova. Since sex
presumably evolved long before bird-mammal speciation (at least I hope
it did! And don't bug me about the shaky terminology, I don't know
much about this), either the bird line or the mammal line went through
some serious changes. How is it possible for the sex-determining
chromosomes to turn up in the opposite gender like that? I can't see
any way in which the species could survive something like that (eg, I
can't see how reproduction in any intermediate stage could result in
anything except total mess).
Big K
sex is determined in reptiles and fishes You might figure out an answer.
Just a thought.
Big K
"JQ" <jac...@writeme.com> wrote in message
news:1177473164.9...@r30g2000prh.googlegroups.com...
Steven J.
crocodilians, for example, the temperature of the egg during incubation
determines whether the embryo will develop as male or female. Other
reptiles, and birds, use a WZ chromosome system: the male has two copies
of the W chromosome, while females have one copy of the W chromosome and
one copy of the Z chromosome. These are not homologs of the XY
chromosomes used in most mammals, in which two copies of the X
chromosome causes the embryo to develop as female, while one copy of
each produces a male. Note that one inherits one of these chromosomes
from each parent, whether one uses the WZ or XY system. Some insects
have a system in which males have no father and have only one copy of
each chromosome (these insects don't have sex chromosomes, of course),
while two copies of each chromosome produces a female.
Some mammals do things a bit differently; the Y chromosome can be lost
(and some of its functions taken up by genes on other chromosomes) or
its effects overridden, so that you have moles with XX males (as well as
XX females -- sex is no longer determined by these chromosomes), while
some mice have XY females (as well as XX females and XY males).
<http://genome.wellcome.ac.uk/doc_WTD020741.html>
Note that most of the genes for most of the parts that distinguish males
from females are not on the sex chromosomes (and some genes, like the
genes for color vision on the X chromosomes, on the sex chromsomes have
little or nothing to do with sex). Rather, some genes on sex
chromosomes are regulatory genes that trigger the activity of other
genes. So what one needs in order for new methods of sex determination
to evolve is mutations that change the way regulatory genes work, or
which create new regulatory genes or regions. I'm just a layman, and
not sure how any of this would work, but consider those mice with the XY
females: you can have a period when multiple regulatory systems are in
place at the same time, evolving to work together or so that one
replaces the other.
>
-- Steven J.
Perplexed in Peoria
"Big K" <big...@notmail.com> wrote in message news:VKAXh.79$uJ6...@newssvr17.news.prodigy.net...
> "JQ" <jac...@writeme.com> wrote in message
> news:1177473164.9...@r30g2000prh.googlegroups.com...
> >I have a question.
> >
> > I know that in mammals, the sex-determinate chromosome is in the
> > sperm, and in birds (for example) it's in the ova. Since sex
> > presumably evolved long before bird-mammal speciation (at least I hope
> > it did! And don't bug me about the shaky terminology, I don't know
> > much about this), either the bird line or the mammal line went through
> > some serious changes. How is it possible for the sex-determining
> > chromosomes to turn up in the opposite gender like that? I can't see
> > any way in which the species could survive something like that (eg, I
> > can't see how reproduction in any intermediate stage could result in
> > anything except total mess).
[rearranged to remove the top-post]
> I am just a layman also, but why don't you go back a step. Check and see how
> sex is determined in reptiles and fishes You might figure out an answer.
> Just a thought.
> Big K
Also a layman, but I will add a bit to what Big K wrote. Some fish are
sequential hermaphrodites - they are female when young, but may become
male when they get older. And some reptiles have their sex determined
by the temperature at which the eggs incubate. So, in this case, sex
chromosomes aren't the way it gets done.
Also, an odd group of mammals - the monotremes - uses something different
from the XY chromosome system used in all other mammals.
But it is a good question, and I would like to see a good tutorial treatment
of the whole subject. Maybe someone knows a good web site, or would like
to write a POTM or a FAQ page for the archive.
This, by the way, is another example of a common theme in this newsgroup.
Someone says they can't imagine a viable intermediate between life-strategy A
and life-strategy B. The evolutionary answer is that the intermediate used
a completely different strategy which was nothing like either A or B, but
which could evolve over time in either direction.
Big K
"Perplexed in Peoria" <jimme...@sbcglobal.net> wrote in message
news:SULXh.16350$Kd3....@newssvr27.news.prodigy.net...
Thanks for changing my top-post. I had just answered a few e-mails, those I
top-post. I do know better...
Big K
Lorentz
> chromosomes to turn up in the opposite gender like that? I can't see
> any way in which the species could survive something like that (eg, I
> can't see how reproduction in any intermediate stage could result in
> anything except total mess).
ancestor of birds and mammals, don't determine their sex through a
single chromosome. In alligators, which is close to the bird line
(really, it isn't a lizard) determine their sex as follows. The
alligator lays its eggs in a nest, and the eggs that hatch in the warm
parts of the nest produce males and the eggs that hatch in the cold
parts of the nest produce females. There is no chromosomal mark to
distinguish the genders.
The sex chromosome feature evolved independently in bird and
mammal ancestors. Its an example of parallel, or maybe convergent,
evolution.
I heard that! Really, the alligator-bird connection was known
decades before the current bird-dinosaur controversy. This isn't
something we just made up, honest. Both alligators and birds share a
lot more in common than turtles and lizards. The resemblance of an
alligator to a lizard is superficial.
While we are at it, there is another difference between birds and
mammals that appears like a reversal. That is our four-chambered
hearts. The bird heart looks like a mirror image of the mammal heart,
although both have four chambers.
Again, it is probably because the common ancestor of birds and
mammals had a three-chambered heart. The fourth chamber evolved
independently in mammals and birds. As an example, the alligator has a
three-chambered heart. That probably reflects the ancestoral condition
of the birds. Again, an example of parallel or convergent evolution.
John Harshman
Pretty simple, really. There are really no sex chromosomes, per se.
There are sex-determining genes or alleles, and they live on particular
chromosomes. In some species there is a single such gene. In other
species there are several. And additional genes can evolve as auxiliary
sex-determiners. Now imagine that there are genes on separate
chromosomes that both influence sex determination, and imagine that the
balance between them is subject to evolutionary variation. Voila:
gradual transformation of sex determination. And this in fact happens
over and over in evolution. Genes gradualy change function, are
gradually replaced by other genes, etc. Sex-determination genes are no
different in this respect.
For a real-world example of changing sex-determination see this: Ogata,
M., H. Ohtani, T. Igarashi, Y. Hasegawa, Y. Ichikawa, and I. Miura.
2003. Change of the heterogametic sex from male to female in the frog.
Genetics 164:613-620.
John Harshman
Actually, alligators have four-chambered hearts.