Biology Reference
In-Depth Information
4.2 Multigene families
4.2.1 Gene families
Dayhoff
et al
. (1978) defined a gene family as a group of related genes encoding pro-
teins differing at fewer than half their amino acid positions. Owing to complex evo-
lutionary histories, both the size of gene families and the relationship between their
constituent members are very variable (Henikoff
et al
., 1997). In principle, the mem-
bers of multigene families can evolve in three different ways—by concerted evolu-
tion, divergent evolution or by a birth-and-death process (Ota and Nei, 1994;
Figure
4.2
).
Concerted evolution
may operate through either unequal crossing over or gene
conversion (Chapter 9, section 9.5) both of which serve to homogenize multigene
family members. Examples of this process include the rRNA and histone genes
(Sections 4.2.2,
Histone genes
and 4.2.2,
Ribosomal RNA genes
). By contrast, some dis-
tinct groups of genes within multigene families appear to have been maintained over
long periods of evolutionary time. An example of this
divergent evolution
is provided
by the immunoglobulin V
H
genes (Section 4.2.4,
Immunoglobulin genes
), the diversifi-
cation of which may have conferred a selective advantage. In other multigene fami-
lies, gene duplication serves to create new genes whilst other genes are inactivated by
deleterious mutation or are eliminated by unequal crossing over ('
birth-and-death
').
Once again, the immunoglobulin V
H
genes provide an example of this process. It can
be seen therefore that some multigene families possess members that serve to illus-
trate the action of all three of the above processes. No one multigene family can be
regarded as being archetypal. Each illustrates certain principles and so a variety of
examples will be discussed.
Actin genes.
A total of six actin genes are found in mammals although a consid-
erably larger number of pseudogenes may be found (Engel
et al
., 1981; Erba
et al
.,
Time
Ancestral
species
Species
1
Species
2
Species
1
Species
2
Species
1
Species
2
(a) Concerted
evolution
(b) Divergent
evolution
(c) Evolution by birth
and death process
Functional genes
Pseudogenes
Figure 4.2.
Three different modes of evolution of multigene families (after Ota and Nei
1994).
