From: Preston Garrison (garrisonp@uthscsa.edu)
Date: Wed Apr 30 2003 - 02:03:37 EDT
> >Naturally advantageous changes in chromosomes among mammals is
>difficult to imagine, especially in terms of reproduction.<
>
>As fas as I know, the variations in chromosome number in mammals are
>thought to be random variants that happened to succeed via genetic
>drift, rather than advantageous. Thus, groups that frequently have
>small, isolated populations might be more prone to such changes. It
>is also possible that some taxa are better able to deal with such
>changes than are others. For example, I think I remember that a
>particular gene had been identified in wheat that enables it to sort
>out the three sets of chromosomes properly during meiosis. If this
>gene is not working or absent, the equivalent chromosomes from the
>three ancestral lineages tend to get mixed up, and meiosis fails.
>
Changes in chromosome number in mammals have generally been from
fusion or fission of chromosomes, I believe. One problem that you
could anticipate would be absence of a centromere (the structure on
the chromosome that attaches to the mitotic spindle) in one new
chromosome from a fission. In that regard, I stumbled on the review
below recently.
Preston G.
Am J Hum Genet 2002 Oct;71(4):695-714
Neocentromeres: role in human disease, evolution, and centromere study.
Amor DJ, Choo KH.
Murdoch Childrens Research Institute, Royal Children's Hospital,
Parkville, Victoria, Australia.
The centromere is essential for the proper segregation and
inheritance of genetic information. Neocentromeres are ectopic
centromeres that originate occasionally from noncentromeric regions
of chromosomes. Despite the complete absence of normal centromeric
alpha-satellite DNA, human neocentromeres are able to form a primary
constriction and assemble a functional kinetochore. Since the
discovery and characterization of the first case of a human
neocentromere in our laboratory a decade ago, 60 examples of
constitutional human neocentromeres distributed widely across the
genome have been described. Typically, these are located on marker
chromosomes that have been detected in children with developmental
delay or congenital abnormalities. Neocentromeres have also been
detected in at least two types of human cancer and have been
experimentally induced in Drosophila. Current evidence from human and
fly studies indicates that neocentromere activity is acquired
epigenetically rather than by any alteration to the DNA sequence.
Since human neocentromere formation is generally detrimental to the
individual, its biological value must lie beyond the individual
level, such as in karyotype evolution and speciation.
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