Biology Reference
In-Depth Information
genes may be derived from genome duplications (Chapter 2, section 2.2) that
occurred early on in the evolution of the vertebrate genome, for example
FGF4
and
FGF6
on chromosomes 11 and 12, respectively, and
FGF1
and
FGF2
on
chromosomes 4 and 5, respectively.
The generation of FGF diversity may have played a role in the various innova-
tions of the vertebrate skeletal system and the novel FGF family members are
likely to have co-evolved with the FGFRs. FGFR genes are also found in inverte-
brate genomes (Coulier
et al
., 1997) and four are found in the human genome:
FGFR1
(8p11),
FGFR2
(10q26),
FGFR3
(4p16) and
FGFR4
(5q35-qter).
However, by contrast with the FGF family expansion, the FGFR gene family
appears to have undergone only a single phase of expansion (Coulier
et al
., 1997).
GABA receptor genes.
The
-aminobutyric acid (GABA) receptor is a pentameric
ion channel complex which mediates fast inhibitory synaptic transmission in the
central nervous system. The receptor exists as many different isoforms assembled
from different combinations of subunit subtypes,
ρ
1-2
. The
human GABA receptor genes are clustered at different chromosomal locations:
GABRA2
,
GABRA4
,
GABRB1
,
GABRG1
(4p12-p13),
GABRA1
,
GABRA6
,
GABRB2
,
GABRG2
(5q34-q35),
GABRA5
,
GABRB3
,
GABRG3
(15q11-q13),
GABRA3
,
GABRB4
,
GABRE
(Xq28),
GABRR1
,
GABRR2
(6q14-q21), and
GABRD
(1p) (McLean
et al
., 1995; Russek and Farb, 1994). The organization of this
gene family is consistent with an evolutionary model of intracluster gene duplication
1-6
,
1-4
,
γ
1-4
,
,
ε,
and
Present day mammalian
FGFs
FGFRs
6
7
8
9
10
11
R1
R2
1
2
3
4
5
R3
R4
FGFRa/
FGFRb
FGF4/FGF6
4
6
10
11
FGF10/FGF11
1
2
FGF1/FGF2
/23 /65
1
2
7
8
9
10/11
R3
R4
Ancestral
FGF gene
Ancestral
FGFR gene
Triploblasts
?
?
Figure 4.9.
A hypothetical scheme for the evolution of the fibroblast growth factors and
their receptors. The putative phases of gene duplications (on the left) are tentatively
related to a phylogenetic tree of the metazoa (on the right). After Coulier
et al
. (1997).
Since this phylogenetic study was performed, several further
FGF
genes have been
characterized in the human genome (see text).
