Biology Reference
In-Depth Information
such that those haplotypes that carry certain matched alleles would have a selec-
tive advantage in the population (Gruen and Weissman, 1997). If odorant dis-
crimination is learned, this could have implications for the population genetics
of species in which odorant-mediated kin recognition is important.
Intriguingly, Wedekind
et al
. (1995) have claimed there to be evidence in
humans for a preference for particular MHC-linked variations in sweat odors.
The common marmoset (
Callithrix jacchus
), a New World primate, possesses
limited MHC class II variability as a result of the inactivation of the MHC-DP
region and limited polymorphism at the MHC-DR and -DQ loci (Antunes
et al
.,
1998). This limited MHC class II repertoire could play a role in the apparently
increased susceptibility of this species to viral, bacterial, protozoan and helminth
infections.
Mucin genes.
Many epithelial tissues such as trachea, mammary gland, pancreas,
stomach, cervix and intestine produce high molecular weight glycoproteins
known as mucins which are the major proteins of mucus. The mucins display
only limited homologies with one another (Desseyn
et al
., 1997a) but do share the
property of containing extensive tandemly repetitive regions. These regions can
vary in length from 8 amino acid residues in MUC5AC to 169 residues in MUC6
and can be highly polymorphic. Several of the human mucin genes are located
within a 400 kb cluster on chromosome 11p15 (
MUC2
,
MUC5AC
,
MUC5B
,
MUC6
), consistent with a series of successive gene duplications (Gum 1992),
whereas others are solitary (
MUC1
, 1q21;
MUC3
, 7q22;
MUC4
, 3q29;
MUC7
,
4q13-q21;
MUC8
, 12q24) (Pigny
et al
., 1996). Although the human mucins dis-
play only a limited degree of homology with one another, some mucin genes pos-
sess exons of similar length and distribution consistent with their having evolved
from a common ancestor (Buisine
et al
., 1998; Desseyn
et al
., 1997a, 1998).
As noted above, the mucin genes often contain internal tandemly repetitive
domains. Thus the
MUC2
gene contains two regions with a high degree of inter-
nal homology but no homology with each other. The first region comprises mul-
tiple 48 bp repeats interrupted by 21-24 bp segments whilst the second region is
composed of 69 bp repeats arranged in a tandem array of up to 115 copies
(Toribara
et al
., 1991). The
MUC5B
gene contains an extremely large (10.7 kb)
exon which encodes a 3570 amino acid protein that contains 19 subdomains
which can be grouped into four larger composite units of 528 amino acids ('super-
repeats') (Desseyn
et al
., 1997b). Similarly, the
MUC4
gene contains an uninter-
rupted 18 kb exon encoding about 380 units of length 48 bp (Nollet
et al
., 1998).
Presumably these enormous exons have gradually become extended by serial
internal duplication events that have not altered the splicing pattern, merely the
length of the exon to be spliced. A model for the evolution of these complex and
highly variable genes is presented in
Figure 4.22
.
Genes encoding RNA-binding proteins.
RNA-binding proteins are involved in
a wide range of biological functions including mRNA splicing, processing and
translation. These proteins constitute a family insofar as they contain RNA-bind-
ing domains (
Figure 4.23
) which share a common evolutionary origin that
predates the divergence of prokaryotes and eukaryotes (Fukami-Kobayashi
et al
.,
