Biology Reference
In-Depth Information
sequences appear to buck the trend; thus, the evolutionary rate of the noncoding
region of the immunoglobulin-
gene is greater in hominoids than in Old World
monkeys (Kawamura
et al
., 1991).
2.3.3 Chromosome evolution in primates
Old World primates
In dim outline, evolution is evident enough but that particular and essential
bit of the theory of evolution which is concerned with the origin and nature of
species remains utterly mysterious.
W. Bateson
William Bateson, Naturalist
(1928)
The evolution and probable phylogeny of primate chromosomes has been exten-
sively reviewed by both Rumpler and Dutrillaux (1990) and Clemente
et al
.
(1990). The interested reader is referred to these reviews for detailed accounts.
Some chromosomes appear to have been relatively protected from change during
primate evolution, for example human chromosomes 19 and X. By contrast, other
chromosomes have been prone to significant reorganization, for example human
chromosomes 1, 3, and 7 (
Figure 2.5
).
The most frequent types of chromosomal change detected in primate evolution
are inversions (especially pericentric, see Haaf and Bray-Ward, 1996), changes in
the amount and localization of heterochromatin, fusions and fissions, and
changes in the location of centromeres due to activation/inactivation. Reciprocal
translocations, deletions and insertions are much less frequent. Human chromo-
some 18 differs from the homologous chromosomes in the great apes by a peri-
centric inversion and it is thought that one inversion breakpoint may have been
located at or within the centromere (McConkey, 1997). Pericentric inversion
breakpoints have also been identified on the chimpanzee equivalents of human
chromosomes 4 (4p14, 4q21), 9 (9q22), and 12 (12p12 and 12q15) and these appear
to coincide with the locations of either fragile sites or tumor-associated break-
points (Nickerson and Nelson, 1998). Pericentric inversions may have played an
important role in establishing reproductive isolation and speciation during the
evolution of the higher primates.
There are at least a dozen blocks of X-Y sequence homology outwith the
pseudoautosomal region of humans but these blocks occur in a very different order
and orientation on these chromosomes (Vogt
et al
., 1997). This may be accounted
for in terms of the occurrence of a number of different inversions, transpositions
and other rearrangements during primate evolution (Bickmore and Cooke, 1987;
Lambson
et al
., 1992; Mumm
et al
., 1997; Page
et al
., 1984; Yen
et al
., 1988). One
example of a human-specific inversion is that involving the short arm of the Y
chromosome (Schwartz
et al
., 1998). Since humans from different racial groups
(Caucasian, African, and Asian) all possess this Yp inversion, the rearrangement
must have occurred prior to the divergence of the human racial groups.
Studies of chromosome banding patterns and hybridization homologies
between ape and human chromosomes have provided evidence for human chro-
mosome 2 having arisen from the fusion of two ancestral simian chromosomes.
IJdo
et al
. (1991) showed that this probably occurred by telomere-telomere fusion
