Evolution of the human genome - Human Gene Evolution - page 65

AMELX
POLA
DMD
F8C

AMELX
POLA
DMD
F8C

AMELX
PDHA1
POLA
DMD
CYBB
TIMP1
GATA1
ALAS2
DXF34
CEN
AR

22
21
11

Xp

13
21
22
23
24
25
26
27
28

LAMP2
CYBB
ALAS2
AR

LAMP2

COL4A5

AMELX
PDHA1
DXF34
CEN
ALAS2

Xq

COL4A5
LAMP2

AR
ALAS2
CYBB

GATA1
TIMP1
CYBB LAMP2

COL4A5 F8C

Human X

F8C

DMD
POLA

AR

Amg
Alas2
cen

Amg

cen
DXF34
Gata 1
Cybb
Timp 1
Lamp 2

1
5
6
7
1
2
3
1
2
3
4
5

COL4A5

A

Cf-8
Dmd
Pola
Ar

B
D
E

Lamp2 Col4a5

Alas2
Pdha 1
Amg

Cybb
Timp

F

Gata1
cen

Col4a5

Mouse X

Figure 2.2. Possible events that may have led to the current arrangement of homologous
blocks on the human and mouse X chromosomes (from Blair et al. , 1994). On the left, a
postulated ancestral X chromosome is shown. Three inversion events are indicated. Each
homologous block is shaded in the same manner throughout and the proximal and distal
loci that define the blocks are given. Dotted lines between loci indicate the positions of
evolutionary breakpoints. The comparative maps of the human and mouse X
chromosomes are shown alongside their cytogenetic maps.

Conservation of synteny may extent beyond the mammals. For example, 11/18
genes from the chicken Z chromosome have orthologues on human chromosome
9pter-q22, albeit in a different order (Nanda et al ., 1999). What are the reasons for
this degree of conservation? One reason may have been functional, for example in
order to allow coordinate regulation of the genes involved (see Chapter 8, section
8.5) or in order to avoid possible meiotic disturbance consequent to a major chro-
mosomal rearrangement. In the case of the HLA system, synteny may have indi-
rectly promoted the generation of diversity by optimizing the potential for gene
conversion. Alternatively, it is possible that insufficient time has passed for ances-
tral linkages to have been broken up completely. Synteny may be conserved in one
phylogenetic group but not in another. Thus, the surfeit genes are tightly clus-
tered in human ( SURF1 , SURF2 , SURF3 , SURF4 , SURF5 ; 9q34.1) and chicken
but not in invertebrates where the Surf genes are unlinked (Duhig et al ., 1998).
Assuming a human-rodent divergence time of 80 Myrs, Collins and Jukes
(1994) estimated the rate of silent substitution to be 2.9 × 10 -9

site -1 year -1 .