Biology Reference
In-Depth Information
Coevolution occurs between genes encoding different subunits of het-
erodimeric proteins, for example the
myc
proto-oncogene (
MYC
; 8q24) and its
dimerization partner
max
(
MAX
; 14q23) (Atchley and Fitch 1995), the genes
encoding the type I and II keratins (Section 4.2.1,
Keratin genes
) and the genes
encoding the
-integrin chains (Hughes 1992; Section 4.2.1,
Integrin genes
).
Coevolution can also occur between agonists and antagonists, for example inter-
leukins 1
- and
(
IL1B
; 2q13-q21), and the interleukin 1 receptor
antagonist (
IL1RN
; 2q14.2) which binds to the IL1 receptor and blocks IL1
(
IL1A
; 2q13) and 1
and
IL1
binding without inducing a signal of its own; all three genes share a com-
mon ancestry (Eisenberg
et al
., 1991). The cytokines and their receptors have both
arisen by a process of gene duplication and divergence and although the relative
timing of these duplicative events is still unclear, evidence is emerging for ligand-
receptor coevolution (He and Wu, 1993; Kosugi
et al
., 1995; Shields
et al
., 1995).
One prediction of Fryxell's (1996) hypothesis is that duplication/divergence
events in functionally related gene families may be temporally correlated. This
prediction appears to be bourne out at least for the
-integrin chain genes
(Hughes, 1992) and the fibroblast growth factor/fibroblast growth factor receptor
genes (Coulier
et al
., 1997).
Whilst numerous examples of 'ligand promiscuity' have been recognized e.g.
the type I interferon genes clustered on 9p21 (see Section 4.2.3,
Interferon genes
),
'receptor promiscuity' appears to be much rarer (Ahuja
et al
., 1992). Examples of
receptor promiscuity include the interleukin 8 receptors (
IL8RA
,
IL8RB
; 2q35;
Ahuja
et al
., 1992) and the interferon receptors
- and
,
β,
and
, 1 (
IFNAR1
) and 2
(
IFNAR2
) and interferon receptor
2 (
IFNGR2
) whose genes are closely linked to
each other on 21q22.1.
The coevolution of ligand-receptor pairs by parallel pathways of gene duplica-
tion and functional divergence has probably been facilitated in two quite distinct
ways. Firstly, the close linkage of multiple genes encoding either ligands or their
receptors will have served to promote gene duplication. Secondly, whole genome
duplications early on in vertebrate evolution (Chapter 2, section 2.1) may have, by
simultaneously increasing both ligand and ligand receptor diversity, provided the
raw material for selection to recruit novel receptor-ligand interactions thereby
potentiating the dramatic increase in the biochemical and physiological complex-
ity characteristic of the vertebrates.
References
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