Biology Reference
In-Depth Information
A
Duplication
A
'A'
Alu-Alu
recombination
'B /E'
A
Figure 4.15.
Evolution of the
human glycophorin genes,
GYPA
,
GYPB
and
GYPE
(after Ondo
et al.
, 1993)
.
Duplication
A
B
E
and Martindale, 1998) and which are directly involved in the control of embryogen-
esis in animals as diverse as nematodes, insects and vertebrates. The expression of
these genes is spatially and temporally regulated during embryonic development and
plays an important role in establishing the system of positioning along the anterior-
posterior axis of the embryo. HOX proteins share a common homeobox domain and
influence gene transcription by binding to a 4 bp core sequence in the promoters of
their target genes.
The
HOX
genes appear to have been duplicated on several occasions during evo-
lution, perhaps even before the origin of angiosperms, fungi and the metazoa
(Bharathan
et al
., 1997). Early in vertebrate evolution, an ancestral
HOX
gene clus-
ter present in invertebrates and the cephalochordate
Amphioxus
was duplicated
twice probably by whole genome duplication (see Chapter 2, section 2.1; Bailey
et
al
., 1997; Garcia-Fernàndez and Holland, 1994; Holland and Garcia-Fernàndez,
1996; Schughart
et al
., 1989) to give rise to the four linkage groups (HOX A, HOX
B, HOX C, HOX D). From studies of
HOX
gene number, this duplication event is
likely to have occurred after the divergence of ray-finned and lobe-finned fishes
but before the radiation of the teleosts (Amores
et al
., 1998). A scheme for the evo-
lution of mammalian
HOX
genes is presented in
Figure 4.16
. It can be seen that
cluster duplication must have been followed by the loss of some specific
HOX
genes and indeed this is evident from comparison of
HOX
gene number between
clusters (
Figure 4.17
). The physical order of genes within the clusters has however
been conserved during vertebrate evolution (Schughart
et al
., 1989) perhaps as a
result of enhancer sharing between different
HOX
genes (Mann, 1997).
In human, most of the
HOX
genes are located in four clusters containing
between them 39
HOX
genes (Acampora
et al
., 1989). The HOX A cluster is
located at 7p14-p15 and contains 11 genes:
HOXA1
,
HOXA2
,
HOXA3
,
HOXA4
,
HOXA5
,
HOXA6
,
HOXA7
,
HOXA9
,
HOXA10
,
HOXA11
and
HOXA13
. The
HOX B cluster is located at 17q21-q22 and contains 10 genes:
HOXB1
,
HOXB2
,
HOXB3
,
HOXB4
,
HOXB5
,
HOXB6
,
HOXB7
,
HOXB8
,
HOXB9
and
HOXB13
.
The HOX C cluster is located at 12q13 and contains 9 genes
HOXC4
,
HOXC5
,
