566
Chimpanzee Genome
First, corresponding DNA sequences in
the two species differ only in about 1.2%
of the positions. This reduces the risk
that individual positions have changed
more than once in one species, or par-
allel in both species since the sequences
last shared a common ancestor. Such
multiple or parallel substitutions blur
the correlation between the true and ob-
served number of changes and comprise
a substantial problem in the evolutionary
analysis of DNA sequences. Second, the
close relationship between chimpanzees
and humans justi±es the hope that factors
determining rate and mode of DNA se-
quence change do not differ substantially
between the two species. The approach
to analyze DNA sequence evolution by a
DNA sequence comparison between chim-
panzees and humans is straightforward.
Different regions in the human genome
vary in divergence to the chimpanzee.
Any genomic feature that is found to
covary with the divergence can serve as
a candidate to influence DNA sequence
evolution. However, in order to correctly
interpret the data, one pitfall needs to
be regarded that has been already men-
tioned in Sect. 1.3. Genomic regions can
differ in their divergence between humans
and chimpanzees even though they evolve
at the same rate. This problem arises
since the time point at which the hu-
man and chimpanzee DNA sequences last
shared a common ancestor is not the same
throughout the genome. It is required to
take this into account in order to identify
regions that truly differ in their evolution-
ary rate.
When DNA sequences from different
regions of the chimpanzee genome are
compared to the corresponding sequences
in humans, it is seen that different chro-
mosomes vary in the amount of DNA
sequence differences between humans
and chimpanzees (Fig. 5). Interestingly,
the extent of divergence for the individual
chromosomes is correlated with the evolu-
t
ionaryt
imetheyspendinthema
legerm
line. The X chromosome, which spends
the least time in the male germ line, dis-
plays also the least divergence (1.0%), the
Y chromosome, which is con±ned to the
male germ line, displays the highest di-
vergence (1.9%), and the divergence of
the autosomes, which spend an interme-
d
ia
tet
imeinthema
lege
rml
ineranges
between that of the sex chromosomes
(1.2%). Currently, it is assumed that a mu-
tation rate about threefold higher in the
male germ line compared to the female
germ line is responsible for this observa-
tion. However, it is noteworthy that the
evolutionary rates also differ among au-
tosomes that spend the same amount of
time in the male and female germ line
respectively. This indicates the existence
of further factors that influence the ac-
cumulation of DNA sequence differences
in the human genome. Recently, a sec-
ond genomic feature has been found to
covary with DNA sequence divergence
between humans and chimpanzees. Re-
gions with a high recombination rate
also display a high divergence between
the two species, while regions with a
lower recombination rate have diverged
less. This can be taken as an indication
that the process of recombination is mu-
tagenic and thus, drives DNA sequence
evolution.
The amount of comparative DNA se-
quence data between chimpanzees is still
limited. However, more extensive stud-
ies are likely to give further insights into
how different factors interact to shape the
rate and pattern of DNA sequence evo-
lution in the genomes of humans and
chimpanzees.
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