Contractions and expansions in gene size and number - Human Gene Evolution - page 355

some 1q21-q22 (Gibbs et al ., 1993). The central segments of the encoded polypep-
tides are composed of tandemly repeated units of either eight or nine amino acids.
Thus the consensus octamer PKVPEPCH is found 6 times in SPRR1A and
SPRR1B , the nonamer PKCPEPCPP three times in SPRR2 and the octamer
TKVPEPGC 14 times in SPRR3. This is consistent with a process of internal
duplication that began after the divergence of the SPRR genes into three distinct
subfamilies. It is evident that, during the evolution of the SPRR gene family,
there has been a bias toward either intragenic or intergenic duplications. Thus in
SPRR2, intergenic recombination has occurred more frequently than intragenic
recombination since there are seven SPRR2 genes each with three repeats. By
contrast, in SPRR3, no intergenic recombination has occurred but intragenic
recombination has occurred frequently to generate 14 repeat copies. Interestingly,
the percentage of amino acid conservation (relative to the consensus for each type
of gene) is significantly higher for the SPRR2 repeats than for either SPRR1 or
SPRR3 suggesting that intergenic homogenization may be more effective than
intragenic homogenization.
Concerted evolution is not however an obligatory property of internally repeti-
tive proteins. Take for example the case of the

-spectrins. Spectrin is a red
blood cell cytoskeletal component which consists of a tetramer of two antiparallel
ab spectrin dimers. The

- and

-spectrin ( SPTB ; 14q22-
q23) genes evolved by duplication of a common ancestral gene which existed
before the divergence of the vertebrate and arthropod lineages ~600 Myrs ago.
The structure of both protein subunits is consistent with successive intragenic
duplications. The

-spectrin ( SPTA1 ; 1q21) and

-subunits consist of tandemly repeated segments of 106
amino acids of which 20 occur in

- and

-spectrin, 17 in -spectrin. Although the -

spectrin segments appear to have evolved in homogeneous fashion, the -spectrin

segments exhibit considerable heterogeneity (Muse et al ., 1997; Thomas et al .,
1997). One explanation for this difference is that some segments with specific
functions may have evolved differently from others. Indeed, on the basis of the
similar locations of the heterogeneous

- and -spectrin segments, Muse et al .

(1997) suggested that the -spectrins have co-evolved, and those segments

that are intimately involved in subunit dimerization have been evolutionarily
constrained by the structures of their binding partners. Muse et al . (1997) found
no evidence for interrepeat exchanges and therefore concluded that neither gene
conversion nor recombination had operated. At some stage, probably before the
divergence of arthropods and vertebrates, concerted evolution may have operated
but this initial phase probably ceased as the individual segments began to diverge
at the DNA level (Thomas et al ., 1997).

- and

8.6.4 The emergence of primordial genes by oligomer duplication

It is possible that primordial coding sequences emerged by a process of sequential
duplication of base oligomers to yield 'genes' encoding polypeptides with signifi-
cant periodicity (Ohno, 1984; Trifonov and Bettecken, 1997). Ohno (1987) cites
the example of a putative primordial rhodopsin gene, which gave rise to this
ancient family of seven transmembrane domain-containing proteins including
among others, bacterial rhodopsin and vertebrate retinal opsin, β 2 -adrenergic

receptor and muscarinic acetylcholine receptor. This primordial gene originally