Cladogenesis and Chomosome Number changes- good reference?
Carter Sanders
mechanisms for a change in a species' Chromosome Number. I am not a biologist or
geneticist and would consider myself a moderately well-read laymen on this topic. I've
read some Dawkins and Gould and a few other books, but I haven't found a discussion of
this yet.
Related questions:
1. Isn't a change in chromosome number a normally fatal defect (eg Turner's, Downe's)?
2. Why would such a change be beneficial (or even neutral)
3. Are snippets like "Chimps and Humans have %98 of the same genetics" fallacious because
of the differing chromosome number?
4. Is there a better phrase for "change in chromosome number"? One of David Froelich's
posts uses the term "karyotype divergence". Is that the right phrase, or is it too broad?
--
-------------------------------------------
| What ARE you doing with that rivet gun? | Carter Sanders
| You'll shake free all the snot and | zanz...@mindspring.com
| bailing wire! |
-------------------------------------------
R C Jenkins
>
> Does anyone have a recommendation for books or articles that describe the genetic
> mechanisms for a change in a species' Chromosome Number. I am not a biologist or
> geneticist and would consider myself a moderately well-read laymen on this topic. I've
> read some Dawkins and Gould and a few other books, but I haven't found a discussion of
> this yet.
>
> Related questions:
> 1. Isn't a change in chromosome number a normally fatal defect (eg Turner's, Downe's)?
Down's syndrome and Turner's syndrome are only fatal in the same way that
life is eventually fatal. Admittedly they can be associated with
life-threatening complications and reduced life-expectancy. They are
certainly an evolutionary dead-end in view of the effects on fertility.
Richard
John Ladasky
Carter Sanders <zanz...@mindspring.com> wrote:
>Does anyone have a recommendation for books or articles that describe
>the genetic mechanisms for a change in a species' Chromosome Number.
>I am not a biologist or geneticist and would consider myself a moderately
>well-read laymen on this topic. I've read some Dawkins and Gould and a few
>other books, but I haven't found a discussion of this yet.
Since you are a well-read layman, I would suggest a college-
level textbook rather than primary literature. Check out _An_Introduction_
_to_Genetic_Analysis_, by Griffiths, Miller, Suzuki, Lewontin, and Gelbart,
W.H. Freeman and Co., New York. You will be interested in Chapter 8,
"Changes in Chromosome Structure", and Chapter 9, "Changes in Chromosome
Number".
>Related questions:
>1. Isn't a change in chromosome number a normally fatal defect
>(eg Turner's, Downe's)?
These aren't the kinds of changes that are believed to occur
in the generation of normal chromosome diversity. The kinds of things
that are likely to occur under normal circumstances do not change the
*quantity* of DNA present. Two of these do not change the number of
chromosomes, though they can change the organization of the chromosomes.
These are called translocations (two chromosomes exchange pieces with
each other) and inversions (a piece in the middle of a chromosome is
"turned around" with respect to the ends). The other two types of
changes will change the chromosome number -- these are known as Robert-
sonian fissions (a chromosome breaks into two) and fusions (two
chromosomes join end to end).
What you're thinking about, the kinds of changes found in
Turner's or Down's Syndrome, is a change in the chromosome number arising
from gaining an entire extra copy of a chromosome (trisomy), or by
losing a chromosome (monosomy or, in the case of a sex chromosome,
nullisomy). In this case, the *amount* of DNA in a cell changes.
Thousands of genes are suddenly present at 1.5 times the normal amount
when a chromosome is gained, or at zero or 0.5 times the normal amount
when one is lost. These types of defects are so bad that they are usu-
ally lethal. The syndromes you mentioned, along with trisomy 14 and
trisomy 18, are actually the only known *non-lethal* defects in which
an entire chromosome is gained or lost.
>2. Why would such a change be beneficial (or even neutral)
As long as chromosomal breaks and splices occur in areas of
the genome which are non-coding or which serve no regulatory function
(and, as far as we can tell, that's about 90% of the genome), they
should be neutral. As for why they might be advantageous, well, that's
a long story involving a lot of hand-waving. I don't think that anyone
really knows why there is an advantage to chromosome diversity. Yet
in fruit flies (Drosophila) there are chromosome variants containing
inversions which persist, and are maintained by balancing selection.
People have sequenced the inversions and the breakpoints, but have
found nothing that resembles any known genes or regulatory elements.
In the owl monkey (Aotus), many animals found in the wild have a
mismatched karyotype containing one large metacentric chromosome with
no matching partner. Instead, there are two acrocentric chromosomes
which, if joined, would match the unmatched big one. This suggests that
the fission/fusion event is being maintained in the population, just
like the inversions in Drospohila.
>3. Are snippets like "Chimps and Humans have %98 of the same genetics"
>fallacious because of the differing chromosome number?
Probably not. They're looking at segments of the DNA, not the
whole genome. The gene for DNA polymerase III might be in one place in
the human genome, and another in the chimp genome. But when you ignore
the chromsome you got it from, line the two sequences up and compare
them, they agree almost completely.
>4. Is there a better phrase for "change in chromosome number"? One of
>David Froelich's posts uses the term "karyotype divergence". Is that
>the right phrase, or is it too broad?
To my ears, "karyotype divergence" would include inversions
and translocations, and not just changes in chromosome number.
--
Rainforest laid low.
"Wake up and smell the ozone,"
Says man with chainsaw. - John Ladasky
Enkidu
> >2. Why would such a change be beneficial (or even neutral)
>
> As long as chromosomal breaks and splices occur in areas of
> the genome which are non-coding or which serve no regulatory function
> (and, as far as we can tell, that's about 90% of the genome), they
> should be neutral. As for why they might be advantageous, well, that's
> a long story involving a lot of hand-waving. I don't think that anyone
> really knows why there is an advantage to chromosome diversity. Yet
> in fruit flies (Drosophila) there are chromosome variants containing
> inversions which persist, and are maintained by balancing selection.
> People have sequenced the inversions and the breakpoints, but have
> found nothing that resembles any known genes or regulatory elements.
> In the owl monkey (Aotus), many animals found in the wild have a
> mismatched karyotype containing one large metacentric chromosome with
> no matching partner. Instead, there are two acrocentric chromosomes
> which, if joined, would match the unmatched big one. This suggests that
> the fission/fusion event is being maintained in the population, just
> like the inversions in Drospohila.
>
An increase in chromosome number is beneficial because of the better
'genetic mixing possibilities' of organisms with higher chromosome
numbers.
A too high chromosome number isn't beneficial because of the higher risk
of something going wrong with the division of the chromosomes between
the two child-cells.
I'm not a biologist and I don't know if this is correct.
I have another question:
Species A evolves into B.
A has n chromosomes.
B has n+1 chromosomes.
How did this transition happen?
At some point there must have been individuals with n and individuals
with n+1 chromosomes. What about their 'mating compatibility'? Isn't the
first individual with n+1 chromosomes incompatible with the rest? He
will die without having reproduced, so there will be no transition from
n to n+1 chromosomes.
--
Spammers should reply to the fake adress
Others are invited to use vee...@wxs.nl
John Ladasky
Enkidu <Enk...@fake.adress.com> wrote:
>John Ladasky wrote:
>
>> >2. Why would such a change be beneficial (or even neutral)
>>
>> As long as chromosomal breaks and splices occur in areas of
>> the genome which are non-coding or which serve no regulatory function
>> (and, as far as we can tell, that's about 90% of the genome), they
>> should be neutral. As for why they might be advantageous, well, that's
>> a long story involving a lot of hand-waving. I don't think that anyone
>> really knows why there is an advantage to chromosome diversity. Yet
>> in fruit flies (Drosophila) there are chromosome variants containing
>> inversions which persist, and are maintained by balancing selection.
>> People have sequenced the inversions and the breakpoints, but have
>> found nothing that resembles any known genes or regulatory elements.
>> In the owl monkey (Aotus), many animals found in the wild have a
>> mismatched karyotype containing one large metacentric chromosome with
>> no matching partner. Instead, there are two acrocentric chromosomes
>> which, if joined, would match the unmatched big one. This suggests that
>> the fission/fusion event is being maintained in the population, just
>> like the inversions in Drospohila.
>>
>An increase in chromosome number is beneficial because of the better
>'genetic mixing possibilities' of organisms with higher chromosome
>numbers.
Yes, for things that *need* to be mixed up in combinations in order
to provide their advantage, more chromosomes are better. But for things
that need to stay together in order to function their best, it's better to
have them on the same chromosome, and preferably close to each other so
they don't get split up by a recombination event.
>A too high chromosome number isn't beneficial because of the higher risk
>of something going wrong with the division of the chromosomes between
>the two child-cells.
Butterflies have as many as 200 chromosomes. So the practical
limit for the number of chromosomes that can segregate correctly at mitosis
would seem to be pretty high. The fruit fly (Drosophila) has only four
chromosomes.
>I'm not a biologist and I don't know if this is correct.
>
>I have another question:
>
>Species A evolves into B.
>A has n chromosomes.
>B has n+1 chromosomes.
>
>How did this transition happen?
Let's assume that this occurred by a Robertsonian fission, as
I described in an earlier post. A chromosome in a germ cell of an
animal undergoes a double-strand break. Those pictures of chromosomes
we see show these nice X-shaped molecules, leading us to believe that
there is only one place on each chromosome that can function as the
centromere. Apparently not -- when a chromosome breaks, somehow there
are regions on both pieces which can function as centromeres. And
most of the time this segregates correctly -- when a cell divides,
the mitotic spindle gets one copy of the long chromosome and one copy
of each of its short sister chromosomes into each new cell.
>At some point there must have been individuals with n and individuals
>with n+1 chromosomes. What about their 'mating compatibility'? Isn't the
>first individual with n+1 chromosomes incompatible with the rest? He
>will die without having reproduced, so there will be no transition from
>n to n+1 chromosomes.
In at least two species, the extra chromosome has been shown to
be no problem in cell division or in mating. There's a species of South
American marsh rat, Holochilus brasiliensis, that has over a dozen known
karyotypes. They can all interbreed with each other, and all are fer-
tile. See Nachman and Myers, Proc. Nat. Acad. Sci. USA 86:6666 [1989].
The owl monkey (Aotus) shows similar chromosome polymorphism, and in the
wild there are many animals with an odd number of chromosomes (remember
that in diploid organisms, including all animals, the chromosomes come
in pairs except for eggs and sperm). Check out the works of Nancy Ma,
including Lab. Animal Sci. 26:1022 [1976].
In short, a chromosome break does not a species make. And since
it is unlikely to break both sister chromosomes at once, it takes at least
two generations before an animal that only has copies of the broken
chromosome and none of the original chromosome. To get there in just
two generations, too, would involve a brother-sister mating, which is
usually a genetic disaster. So it is likely that the intact chromosome
coexists with its broken counterpart for several generations. The reason
that two species that we observe toady have different numbers of chromo-
somes is probably not because the chromosome break isolated one population
from breeding with the other. Rather, the ancestral population probably
had both broken and unbroken chromosomes, and each descendant population
eventually became homogeneous through genetic drift.
mel turner
wrote..
>John Ladasky wrote:
[snip]
>> In the owl monkey (Aotus), many animals found in the wild have a
>> mismatched karyotype containing one large metacentric chromosome with
>> no matching partner. Instead, there are two acrocentric chromosomes
>> which, if joined, would match the unmatched big one. This suggests
that
>> the fission/fusion event is being maintained in the population, just
>> like the inversions in Drospohila.
>>
>An increase in chromosome number is beneficial because of the better
>'genetic mixing possibilities' of organisms with higher chromosome
>numbers.
>of something going wrong with the division of the chromosomes between
>the two child-cells.
Sounds iffy.
>I have another question:
>
>Species A evolves into B.
>A has n chromosomes.
>B has n+1 chromosomes.
>
>How did this transition happen?
>At some point there must have been individuals with n and individuals
>with n+1 chromosomes. What about their 'mating compatibility'? Isn't the
>first individual with n+1 chromosomes incompatible with the rest?
The short answer is no. There are plenty of wild species with such
Robertsonian chromosomal polymophisms both within and among populations.
Some show a whole range of different numbers. There are a little more
info. and a few refs. of examples in:
http://x8.dejanews.com/getdoc.xp?AN=368526087
http://x8.dejanews.com/getdoc.xp?AN=358643648
http://x8.dejanews.com/getdoc.xp?AN=291715615
>He
>will die without having reproduced, so there will be no transition from
>n to n+1 chromosomes.
No, individuals with the two chromosomal forms could well be
interfertile, and there may be pairing-up of the one chromosome from one
parental set with the corresponding two chromosomes from the other during
meiosis in individuals that are "heterozygous" for the change. See the
Aotus monkey example mentioned above.
cheers
Kim Hodgson
> (snip)
> In at least two species, the extra chromosome has been shown to
> be no problem in cell division or in mating. There's a species of South
> American marsh rat, Holochilus brasiliensis, that has over a dozen known
> karyotypes. They can all interbreed with each other, and all are fer-
> tile. See Nachman and Myers, Proc. Nat. Acad. Sci. USA 86:6666 [1989].
> The owl monkey (Aotus) shows similar chromosome polymorphism, and in the
> wild there are many animals with an odd number of chromosomes (remember
> that in diploid organisms, including all animals, the chromosomes come
> in pairs except for eggs and sperm). Check out the works of Nancy Ma,
> including Lab. Animal Sci. 26:1022 [1976].
(snip)
>
- John Ladasky
Thank you, John, for making this so clear. A year or so ago I asked the
same question in this newsgroup, "How does a transition of chromosome
number happen without similar and simultaneous mutations in two
oppositely sexed animals?" No one then had so clear an explaination as
yours.
An example of pocket gophers was cited. Supposedly in a small area
there are subpopulations with different numbers of chromosomes
(allokaryotic?) that do not interbreed. Are you familiar with these?
Another example cited was cutthroat trout, which have different
chromosome numbers in different drainages.
Finally, all the non-human apes have 24 pairs of chromosomes, suggesting
that humans changed, not the others. Can the number of chromosomes
decrease as well as increase?
Kim Hodgson
John Ladasky
Kim Hodgson <xHod...@slb.com> wrote:
>John Ladasky wrote:
>
>> (snip)
>> In at least two species, the extra chromosome has been shown to
>> be no problem in cell division or in mating. There's a species of South
>> American marsh rat, Holochilus brasiliensis, that has over a dozen known
>> karyotypes. They can all interbreed with each other, and all are fer-
>> tile. See Nachman and Myers, Proc. Nat. Acad. Sci. USA 86:6666 [1989].
>> The owl monkey (Aotus) shows similar chromosome polymorphism, and in the
>> wild there are many animals with an odd number of chromosomes (remember
>> that in diploid organisms, including all animals, the chromosomes come
>> in pairs except for eggs and sperm). Check out the works of Nancy Ma,
>> including Lab. Animal Sci. 26:1022 [1976].
>(snip)
>>
> - John Ladasky
>
>Thank you, John, for making this so clear. A year or so ago I asked the
>same question in this newsgroup, "How does a transition of chromosome
>number happen without similar and simultaneous mutations in two
>oppositely sexed animals?" No one then had so clear an explaination as
>yours.
Gosh, thanks! The fact that I am just finishing up my disserta-
tion on the owl monkey may have something to do with my familiarity with
this subject... In a couple of years, I hope to be applying for assistant
professorships. Can I use you as a character reference for my superior
teaching abilities? :^)
>An example of pocket gophers was cited. Supposedly in a small area
>there are subpopulations with different numbers of chromosomes
>(allokaryotic?) that do not interbreed. Are you familiar with these?
I can't say that I am. However, this sounds like certain popu-
lation pair in the owl monkey. As I said, there are plenty of weird
karyotypes in the owl monkeys that imply that interbreeding can occur
between a lot of them. But in one small region in Peru, there are two
karyotypic variants living side-by-side with no evidence of interbreed-
ing. Thus *some* chromosomal changes may be reproductively isolating.
(Of course, we don't know what *other* changes have occurred in the
animals' genomes -- the chromosome changes are just easily observed.)
But if chromosome changes can be reproductively isolating, it
probably takes more than one: these two variants of monkey, though
neighbors, have three or four chromosome mismatches. Both are believed
to be more closely related to other neighbors of theirs than to each
other. If a mutation arose in one of these non-interbreeding popula-
tions, it could probably get to the other group through a third popula-
tion that was compatible with both.
There. That blows a hole in the whole concept of "species."
>Another example cited was cutthroat trout, which have different
>chromosome numbers in different drainages.
>
>Finally, all the non-human apes have 24 pairs of chromosomes, suggesting
>that humans changed, not the others. Can the number of chromosomes
>decrease as well as increase?
I would certainly think so, though the mechanism is harder to
imagine. It's easier to think of something getting broken than mended.
I haven't talked about DNA repair mechanisms. When a chromosome gets
broken, it *usually* gets fixed, reattached back in its original location.
But the repair machinery sometimes makes mistakes. As it was explained
to me, the repair machinery looks for a matching sequence nearby. This
process is initiated pretty quickly after a chromosome break, but if it
takes too long to get rolling, the ends may have drifted apart. In
really bad circumstances the repair mechanism does whatever it can, finds
a sequence that is reasonably homologous to the broken end, and grabs on
to that. In this way, two chromosomes can fuse. I'd have to pull out
some notes from a class I took a few years ago to explain it in more
detail than this.
>Kim Hodgson