Evolution of the human genome - Human Gene Evolution - page 60

Table 2.2 continued

1 11 12 15 19

FUT4 FUT1

SIAT4C FUT2

MANA1 MANB

CKMT1 CKM

GANAB GANC

CAPN2 CAPN1

CAPN3 CAPN4

MUC1 MUC2,

MUC5B
MUC5AC

AT3 C1NH

AGT
PFKM

PFKX

FDPSL1 CHR39B

ACTA1 ACTC

POU2F1 POU2F2

TNNI1 TNNT1

SNRPE SNPRN SNRPA

SNRP70

TGFB2 AMH

TGFB1

CTSE CTSD CTSH

REN PGA3-5

duplication (Burger et al ., 1994; Hill and Hastie 1987; Kurihara et al ., 1997; Ohta
1994; Wallis 1993). In such studies, positive selection is implicated in the process
of evolutionary change by the observation of a higher frequency of non-synony-
mous over synonymous substitutions (Hughes and Nei, 1988). Changes in the
expression patterns of the duplicated genes are sometimes also apparent as in the
neuronal and muscle expressed genes of the nicotinic acetylcholine receptor fam-
ily (Le Novre and Chaneux, 1995). In principle, gene conversion (see Chapter 9,
section 9.5) may either promote diversification of proteins encoded by duplicated
genes (Ohta and Basten, 1992) or promote homogenization (Sidow and Thomas,
1994) in which case the molecular evolutionary record is automatically erased. In
practice, however, at least for the HLA and immunoglobulin genes, gene conver-
sion is notable more by its absence: instead new genes tend to be created by a
'birth-and-death' process of duplication and deletion (Nei et al ., 1997).
A more likely scenario, however, is that the duplicated gene rapidly acquires
inactivating mutations and becomes a pseudogene. Indeed, assuming that a gene
duplication is not selectively disadvantageous, the duplicated genes can survive
in the genome for quite long periods (Clark, 1994; Loomis and Gilpin, 1986;
Nowak et al ., 1997). Arguably the best available model system to assess whether
duplicated genes will be retained or inactivated over evolutionary time is yeast
( Saccharomyces cerevisiae ), the organism with the best characterized genome