How can chromosome numbers change?

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Erland Gadde

Posted: Sun Feb 18, 2007 7:14 pm

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Human beings have 46 chromosomes (23 pairs), while other apes,
including chimpanzees, have 48 (24 pairs). This means that at some
time since the split of the ancestral lineage connecting humans and
chimpanzees, a human ancestor lost a chromosomal pair, problably by
two pairs merging into one.

But how can that have occurred? If a child of parents with 48
chromosomes by some reason got 46 chromosomes, how could that
individual mate with other specimen with 48 chromosomes? Or should we
assume that that individual had a sibling who also had 46 chromosomes,
and that those siblings mated and that all humankind descends from
those two siblings? Or did the bond where two chromosome pairs were
merged to one evolve during the generations, so that it was quite
loose from the beginning, so that originally it could weaken and split
the cromosomes and than tying it up again, forward and back in the
cells of an individual, allowing those individuals to sometimes mate
with individuals with 48 chromosomes, when their gamete chromosomes
were temporarilly untied, but that the bond evolved and became
stronger during the generations, until it it became unbreakable?

I would be interested in learing more about this...

Regrads,

Erland Gadde

Tim Tyler

Posted: Tue Feb 20, 2007 12:16 am

Guest

Erland Gadde wrote:

Quote:

See:

How can genetic mutations change the n chromosome number of a species?

http://www.madsci.org/posts/archives/2001-05/989331026.Ev.r.html
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|im |yler http://timtyler.org/ tim@tt1lock.org Remove lock to reply.

Guest

Posted: Tue Feb 20, 2007 12:16 am

On Feb 18, 12:14 pm, "Erland Gadde" <erl...@bredband.net> wrote:

Quote:

There's an interesting talk by Ken Miller which touches on this as
well:
http://www.youtube.com/watch?v=Gs1zeWWIm5M

Guest

Posted: Tue Feb 20, 2007 12:16 am

Quote:

But how can that have occurred? If a child of parents with 48
chromosomes by some reason got 46 chromosomes, how could that
individual mate with other specimen with 48 chromosomes?
A chromosome fusion does not have to result in a complete

reproductive barrier. A chromosome fusion can be one of the less
disruptive types of mutation, depending on the details of how the
fusion occurred.
1) A chromosome fusion does not change the genome. The one gene formed
by fusion has the same genes as the two chromosomes it started with.
Transposition of chromosome segments also doesn't change the genome. I
think the small effect of these two mutations is the reason that
fusion and transposition are so common in the primate lineages. Gene
fusions and gene transpositions survive better in the long run.
2) During meiosis, the two unfused genes from one parent can pair with
the fused chromosome from the mutated parent. The resulting hybrid
will have the same complement of genes as an homogenous individual
with unfused chromosomes.
3) The change in phenotype caused by a fused chromosome could be very
small. Such a change in phenotype could be an advantage, or a
disadvantage, in terms of natural selection.
4) Separate traites that were previously uncorrelated would in
proceeding generations become strictly correlated. This would not be
catastrophic. Correlation could be a plus in the long run if the two
traites were synergistically beneficial.
5) If a partial hybridization barrier resulted, natural selection
could eventually remove it in the fashion. Mating with an incompatible
individual would be a risky preposition, because resources would be
squandered on the at-risk hybrids. The population with the unfused
chromosome would go extinct, or separate from the population with the
fused chromosomes.
6) There are other types of mutation that automatically produce huge
changes in the genome. Two examples are chromosome duplication and
gene knockouts. Duplicating a chromosome adds genes to the genome,
resulting in chemical imbalances that usually result in inviable
offspring. An example would be Down's syndrome. Knockout gene
mutations can also be catastrophic. The homeotic mutations in flies
studied by Morgan are gene knockouts.
7) Chromosome fusion mutations occur very rarely compared to
chromosome duplication mutations and gene knockout mutations. Also,
chromosome fusion mutation is less likely to be noticed when it does
occur. Catastrophies attract attention. Therefore, not much
information is available on the phenotypic effects of chromosome
fusion.

Guest

Posted: Tue Feb 20, 2007 12:16 am

On Feb 18, 12:14 pm, "Erland Gadde" <erl...@bredband.net> wrote:

Quote:

The chromosomes 2q and 2p (which appear in apes) is fused in the human
lineage into a single chromosome (chromosome 2). (You can see this in
the illustration here: http://www.gate.net/~rwms/hum_ape_chrom.html)
They fused at the telomeres. (So the human lineage actually has a
telomere in the middle of chromosome 2 and two centromeres.) There
are species alive today who have variable numbers of chromosomes, and
it doesn't seem to affect an organisms ability to interbreed with
another organism with a different number of chromosomes. My guess is
that when a fused-chromosome organism mates with an unfused-chromosome
organism, there might be a bias towards breaking the fused chromosome
into two or fusing the two chromosomes into one. If the bias remains
constant, it would eventually lead to a species with a chromosomes
that are universally fused or split. I believe that neanderthals had
the same fused chromosome structure as humans, which tells us the
chromosome fusion happened a while ago in the human lineage.

There's some more information here: http://www.gate.net/~rwms/hum_ape_chrom.html

Guest

Posted: Tue Feb 20, 2007 12:16 am

Quote:

But how can that have occurred? If a child of parents with 48
chromosomes by some reason got 46 chromosomes, how could that
individual mate with other specimen with 48 chromosomes?

I googled an article on chromosome fusion for you. The following
article describes a type of grasshopper, raised in the laboratory,
which has a tendency to produced chromosome fusion mutations. The
resulting offspring vary in viability. However, some cross breeding
did occur over several generations. Apparently, chromosome fusion
doesn't have to result in strict reproductive barriers. The article
citation is:

"The recurrence of chromosome fusion in inter-population hybrids of
the grasshopper Atractomorpha similis"
Journal Chromosoma
Publisher Springer Berlin / Heidelberg
ISSN 0009-5915 (Print) 1432-0886 (Online)
Subject Biomedical and Life Sciences and Medicine
Issue Volume 85, Number 3 / July, 1982
DOI 10.1007/BF00330356
Pages 323-347
SpringerLink Date Friday, December 03, 2004

Guest

Posted: Tue Feb 20, 2007 7:28 pm

I gave an incomplete reference. The complete citation:

G. B. Peters, "The recurrence of chromosome fusion in inter-population
hybrids of the grasshopper Atractomorpha similis," Journal Chromosoma
V85(#3), pp323-347 (July 1982).

I haven't read the entire article (which cost $32), but I did
read its' abstract which was pretty informative. As I understood it
(as a nonprofessional), there is a laboratory breed of grasshoppers
that has a much higher rate of chromosome fusion mutations than wild
populations. The mutation in this lineage hasn't occurred just once,
but is a recurring mutation. The chromosome fused mutations are
viable, and have a variable degree of ability to interbreed with
normal individuals. In fact, several generations of these individuals
have been cross bred with normal individuals. However, the
crossbreeding creates some type of chromosomal instability in that
further mutations with chromosomal abnormalities often occur. These
are also often viable.
The researchers conjecture that the original tendency toward
chromosomal abnormalities resulted from the initial selection of
grasshoppers. The grasshoppers in the wild had formed a cline, so that
there was a slight incompatability between individuals from distant
parts of the geographical region. The grasshoppers chosen to start the
breed were captured from far apart regions. The incompatibility
started this tendency to create chromosome fused mutations.
I wonder if this type of scenario could explain evolutionary
changes in chromosome number in other types of animals? Like us great
apes?

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