Biology Reference
In-Depth Information
hyd
1
750 Myrs
2
600 Myrs
TPH
TH
3
PAH
Chromosome 11
Chromosome 12
Figure 9.3.
Genetic events in the evolution of the tryptophan hydroxylase (
TPH
),
tyrosine hydroxylase (
TH
) and phenylalanine hydroxylase (
PAH
) genes from an ancestral
hydroxylase gene (hyd). 1. Duplication giving rise to
TPH
gene. 2. Duplication giving
rise to
TH
and
PAH
genes. 3. Translocation resulting in
TPH
and
TH
remaining on the
same chromosome whilst
PAH
is separated from them.
possesses a single cluster on 1q. These data are consistent with the interpretation
that the tRNA
Asn
gene cluster split before the divergence of Old World monkeys
and hominoids about 30 Myrs ago. No simple inversion can account for this situ-
ation and, since this chromosome is well conserved in primate evolution,
Buckland
et al
. (1992) speculated that one tRNA
Asn
gene cluster could have been
relocated from one chromosome arm to the other by some form of replicative
transposition.
Another example of a gene translocation occurring during the evolution of the
primates is provided by the adenine nucleotide translocator 3 (
ANT3
) and steroid
sulfatase (
STS
) genes. The
ANT3
and
STS
genes are pseudoautosomal (Xp22.32)
in humans and other higher primates but both localize to an autosome in lemurs
suggesting that there was an autosome-to-X/Y translocation after the simians
diverged from the prosimians (Toder
et al
., 1995).
9.2.1 Pericentromeric-directed transposition
Several examples of pericentromeric-directed transposition have been described
in primate genomes and since these have been well characterized, they are worthy
of discussion in some detail here. Comparative FISH analysis of chromosomes
from various primates has shown that a 27 kb region of Xq28 containing the cre-
atine transporter (
SLC6A8
) gene and five exons of the 'CDM' gene (DXS1357E)
has been duplicated in its entirety and translocated to 16p11.1, probably within
the last 10 Myrs (Eichler
et al
., 1996;
Figure 9.4
). An inverted cluster of
Alu
repeats and a number of immunoglobulin-like CAGGG repeat motifs lie in the
vicinity of the Xq28 breakpoint. Either type of repeat could have mediated the
transposition event but the CAGGG repeats may be the more likely candidate for
involvement since they have been found to flank a number of other recently
