Chimpanzee Genome
555
that is synchronized in all species under
consideration, those species with a smaller
genetic distance are more closely related.
1.1
Phylogenetic Analyses Based on Protein
Comparisons
1.1.1
Immunological Comparison of
Proteins
T
h
ef
r
s
ta
t
t
em
p
tt
or
e
s
o
l
v
ep
r
im
a
t
e
phylogeny using molecular evidence was
based on immunological comparisons oF
their serum proteins. As the underlying
principle, an antiserum against a protein
From species X is obtained by immunizing,
F
o
re
x
am
p
l
e
,r
a
b
b
i
t
so
rc
h
i
c
k
e
n
s
.T
h
e
degree oF similarity between the protein oF
species X and the corresponding protein
in a second species Y is then determined
by the extent oF cross-reactivity oF anti-
X-serum and Y-protein. A quantitative
measure oF similarity is achieved since
the antiserum comprises a mixture oF
antibodies, each recognizing a distinct
surFace structure (epitope) oF the protein
used For immunization. The more closely
two species are related, the more similar
are their proteins and thus, the more
epitopes are shared among them. As a
consequence, the extent oF cross-reactivity
increases.
Initial immunological comparisons oF
primate proteins used antisera against
whole blood serum, a rather poorly defned
protein mixture. Their results supported
the rough classifcation oF anthropoid
primates into the major groups already
suggested by morphologists (±ig. 1). How-
ever, no Further resolution was possible
since the antigenic properties oF hominoid
blood sera are too similar to each other to
be diFFerentiated by these means.
In later years, increasing understand-
ing oF the molecular processes oF immune
response in combination with the ability
to generate antisera against individual pro-
teins Facilitated the development oF more
sensitive immunological assays. On the ba-
sis oF such improved methods, the AFrican
apes and not orangutans were assigned as
the closest relatives oF humans. But sub-
sequent determination oF the branching
order oF the individual AFrican ape species
and humans Failed For mainly the Follow-
ing reason. ±or proteins that diFFer only
slightly, the correlation between the ob-
served immunological diFFerence and the
genetic distance is not absolute. As an
example, protein A From species X diF-
Fers From the corresponding protein in
species Z in three positions. By chance,
none oF these diFFerences aFFect the anti-
genic properties oF the respective proteins;
thereFore, no diFFerence will be detected
in an immunological assay with anti-A
X
-
serum. Protein A From a third species, Y,
however, diFFers only in a single position
From A
X
which, again by chance, aFFects
an epitope recognized by the anti-A
X
-
serum. The resulting lower cross-reactivity
with anti-A
X
-serum erroneously suggests
a higher genetic distance between species
Y and X than between species Z and X
and thus, a grouping oF X and Z to the
exclusion oF Y. This Fuzziness in combina-
tion with a substantial experimental eFFort
For the individual assays renders immuno-
logical comparisons not suitable to inFer
the evolutionary relationships oF closely
related species.
1.1.2
Electrophoretic Comparison of
Proteins
Proteins contain amino acids with basic or
acidic side chains and thus are charged
molecules
under
most
pH-conditions.
This circumstance is exploited to sepa-
rate proteins on a matrix according to their
size and their net charge by application oF
previous page 1229 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 1231 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off