Biology Reference
In-Depth Information
differing tissue distribution (tissue-specific isoforms) have undergone duplica-
tions independently in vertebrates and arthropods after divergence of the verte-
brate and arthropod lineages. Iwabe
et al
. (1996) concluded that there was a good
correspondence between molecular evolution at the level of the gene, and tissue
and organismal evolution.
Several gene duplications have occurred during primate evolution. For exam-
ple, the tandem duplication responsible for the creation of the
2-
(
HBG2
) globin genes occurred prior to the divergence of Old World from New
World monkeys (Fitch
et al
., 1991). The 5.5 kb duplicated segment is bounded by
two related LINE elements suggesting that the duplication occurred via homolo-
gous unequal recombination (Fitch
et al
., 1991). (The role of repetitive DNA
sequences in mediating the recombinational events responsible for gene duplica-
tions is discussed in more detail in Chapter 9, section 9.4.1). Perhaps it was the
functional redundancy initially introduced by the duplication which created an
opportunity for the
1- (
HBG1
) and
-globin genes to evolve a fetal function to replace their orig-
inal embryonic function. Post-duplicational functional redundancy is sometimes
still apparent in some systems. For instance, the MyoD family of transcription
factors involved in myogenesis in skeletal muscle still exhibits functional redun-
dancy (between the proteins encoded by the
MYOD1
(11p15.1),
MYF5
(chromo-
some 12), and
MYF6
(chromosome 12) genes) and this is probably indicative of
overlapping functions (Atchley
et al
., 1994).
The primate glycophorin genes (
GYPA
,
GYPB
,
GYPE
; 4q28-q31) are thought
to have emerged by duplication, mediated perhaps by recombination between
Alu
sequences (Rearden
et al
., 1993).
GYPA
probably represents the ancestral gene
and is present in all primates studied. The
GYPB
gene is present in human, chim-
panzee and gorilla but not orangutan and gibbon, whereas the
GYPE
gene is pre-
sent in human, chimpanzee but intriguingly only in 7/16 gorillas tested (Rearden
et al
., 1993). The complement C4 (
C4A
,
C4B
; 6p21.3) genes and the cytochrome
CYP21
(6p21.3) gene also emerged in the primate lineage as a result of a single
duplication occurring prior to the divergence of the apes from the Old World
monkeys (Horiuchi
et al
., 1993).
There are several other examples of gene duplications occurring in only one
mammalian order. For example, the bovine-specific coglutinin (
Cgn1
) gene is
homologous to the human pulmonary surfactant protein D (
SFTPD
; 10q22-q23)
gene and is thought to have evolved by gene duplication in the Bovidae after their
divergence from the other mammals (Liou
et al
., 1994). Sometimes the number
and distribution of
Alu
sequences may help the reconstruction of the phylogeny
of a gene duplication event as in the case of the duplication of the apolipoprotein
CI (
APOC1
; 19q13.2) gene, timed at about 39 Myrs ago (Raissonier, 1991).
Some partially homologous genes may have undergone a process of duplication
and divergence but one or other copy may have either acquired or lost DNA
sequence at some stage thereby limiting the extent of observable homology
between them. One example of this is the human
-
N
-acetylgalactosaminidase
(
NAGA
; 22qter) and
-galactosidase A (
GLA
; Xq) genes (Wang and Desnick,
1991). Six of the seven
GLA
exons were identically positioned in the
NAGA
gene
but there was no similarity between the predicted amino acid sequences of
GLA
exon 7 and
NAGA
exons 8 and 9.
