Biology Reference
In-Depth Information
G
Duplication (35 Myrs)
G
G
G
A
C
1
(10 Myrs)
G
A
Speciation (5 - 6 Myrs)
G
A
G
A
C
2
G
A
C
3
G
A
C
4
(1Myrs)
G
A
G
A
Gorilla
Human
Figure 9.9.
History of γ -globin gene evolution and conversion events in the great apes
(from Scott
et al
., 1984). A duplication of a γ -globin gene encoding glycine at position 136
occurred in the early catarrhine primates about 35 Myrs ago. Since a glycine is encoded at
this position in both γ -globin genes of
Pongo
and Old World monkeys whereas a
replacement with alanine in the 3
gene is found in
Homo
and
Gorilla
, this change must
have occurred after the divergence of
Pongo
(about 8 Myrs ago) but before
Homo
and
Gorilla
branched off (about 6-7 Myrs ago). This replacement may have occurred before
the first conversion (C1), or it may have happened after it but still before the separation
of
Homo
and
Gorilla
. If the Gly
Ala replacement occurred before C1, the 3
boundary of
C1 can be placed in exon 3 at codon position 135 or nucleotide position 1543. If the
replacement occurred after C1, the 3
boundary can be placed at the start of the 3
untranslated region. C1 is common to both humans and gorillas and was estimated to
have occurred about 10 Myrs ago, using a replacement rate change of 1% for every 10
Myrs.
No further conversions have been identified in the gorilla lineage, but three have been
identified in the human lineage. C2 and C3 are estimated to have occurred about 2-3
Myrs ago either in a common ancestor of human chromosome types A and B or in an
early version of chromosome B itself. Conversion C2 is evident in the BAγ -gene from
positions 901 to 1128 and extending into the 'hot spot,' and C3 is also evident in the BAγ -
gene, but from positions 42 to 777. C4 extending over some 1500 bp is estimated to have
occurred no earlier than 1 Myrs ago on human chromosome type A. Its effects are evident
in the AAγ -gene from positions 42 to 1128 with its 3
boundary being located in the hot
spot region. Taken from Scott
et al
. (1994) with kind permission.
