thermore, the members of a species or of a reproductive population are never completely identical. With the help of the newly developed
methods of isoenzyme separation by gel electrophoresis, it was shown at the end of the 1960s that
the genetic polymorphism in natural populations
was far larger than previously assumed, so large
that the individuals in a bisexual, reproductive
population could never be genetically identical.
Even genetically identical individuals arising by
asexual reproduction, such as twins or members
of a clone, vary phenotypically due to environmental factors [23].
The characteristic variety of living organisms is
apparent at all levels of complexity, from individuals through organs, tissues, cells and cell organelles down to individual macromolecules. For
example, the human body is made up of 200 different cell types. Also, single cells of the same
type in an organ or tissue show large differences
[12]. The heterogeneity of the mitochondria in a
cell or a cell type can be so large that they may be
classified into several populations with different
characters [10]. Basically, the variety of species,
individuals, cells and cell components is related
to molecular differences: to differences in the
structure of the molecular building blocks and/or
to differences in their arrangement. This molecular variation is the central theme of the present
book.
1.3 The Subjects of Comparative
Biochemistry
Biochemistry textbooks usually do not present
the composition and metabolism of particular
animals or cells; more often they describe a basic
scheme of biochemical organization, a biochemical "archetype" as Pette called it [27]. Because
the origins of biochemistry lie in human medicine, this basic scheme is mostly only valid for
man and mammals, even when research results
from yeast and bacteria are included.
It should never be assumed that the statements
in biochemical textbooks are completely valid for
lower vertebrates or for invertebrates. It is more
often the case that the composition and metabolism of different animal groups or species must be
specifically examined and presented; thus there
exist, besides human and mammalian biochemistry, further presentations of "special biochemistry", e.g. the biochemistry of insects, crustaceans and molluscs. The "comparative bioche1.3 The Subjects of Comparative Biochemistry
3
mistry of animals" uses the same data but through
the comparative approach comes to new conclusions. It can contribute to general biochemistry
both by revealing similarities, and by pointing to
experimental models that are suitable for investigating the general laws. The thesis known as the
''August Krogh principle", named after the great
Danish physiologist, proposes that for every
problem there is an ideal animal model; this has
found many applications in biochemistry [22].
The main task of comparative biochemistry is to
describe the molecular variety of organisms and
to explain both their biological importance and
their development during evolution.
The present book restricts its view of comparative biochemistry to animals. The classification
of living organisms in the animal kingdom usually
presents no problem, but in specific cases may be
difficult. Thus, the classification of flagellate species to the plant "phytoflagellates" or to the animal "zooflagellates" on the basis of the presence
or absence of photosynthetic capacity has long
been considered questionable. The classification
by several authors of the "slime moulds" Physarum and Dictyostelium in the animal kingdom is
also undoubtedly controversial. Paramecium,
belonging to the Ciliophora, is today a favourite
object of first-year undergraduate, zoology practical classes. However, because of the fundamental characteristics of their nucleic acid and protein
sequences, the Ciliophora assume such a special
position amongst the eukaryotes that they cannot
logically be classified as either animals or plants.
There are compelling reasons to assume that their
divergence from the eukaryotic evolutionary line
occurred before the separation of the lower fungi,
higher plants and animals (see pp. 13 and 170).
Only a very small number of the known animal
species has ever been used for biochemical
investigations; biological, methodological and
economic factors play a role in the choice of
investigated species [19]. Species-rich groups that
have been studied physiologically have been preferred; in addition to the mammals, examples
include the amphibians, fish, insects, crustaceans,
molluscs and echinoderms. Because biochemical
investigations require large amounts of genetically uniform material, which in most cases cannot
be obtained from the field, species which can be
reared in the laboratory have been used. Particular attention has been paid to species of practical
importance, such as domestic animals, farm animals, pests and pathogens. Comparative biochemistry concerns itself with differences in composition and metabolism not only between members
methods of isoenzyme separation by gel electrophoresis, it was shown at the end of the 1960s that
the genetic polymorphism in natural populations
was far larger than previously assumed, so large
that the individuals in a bisexual, reproductive
population could never be genetically identical.
Even genetically identical individuals arising by
asexual reproduction, such as twins or members
of a clone, vary phenotypically due to environmental factors [23].
The characteristic variety of living organisms is
apparent at all levels of complexity, from individuals through organs, tissues, cells and cell organelles down to individual macromolecules. For
example, the human body is made up of 200 different cell types. Also, single cells of the same
type in an organ or tissue show large differences
[12]. The heterogeneity of the mitochondria in a
cell or a cell type can be so large that they may be
classified into several populations with different
characters [10]. Basically, the variety of species,
individuals, cells and cell components is related
to molecular differences: to differences in the
structure of the molecular building blocks and/or
to differences in their arrangement. This molecular variation is the central theme of the present
book.
1.3 The Subjects of Comparative
Biochemistry
Biochemistry textbooks usually do not present
the composition and metabolism of particular
animals or cells; more often they describe a basic
scheme of biochemical organization, a biochemical "archetype" as Pette called it [27]. Because
the origins of biochemistry lie in human medicine, this basic scheme is mostly only valid for
man and mammals, even when research results
from yeast and bacteria are included.
It should never be assumed that the statements
in biochemical textbooks are completely valid for
lower vertebrates or for invertebrates. It is more
often the case that the composition and metabolism of different animal groups or species must be
specifically examined and presented; thus there
exist, besides human and mammalian biochemistry, further presentations of "special biochemistry", e.g. the biochemistry of insects, crustaceans and molluscs. The "comparative bioche1.3 The Subjects of Comparative Biochemistry
3
mistry of animals" uses the same data but through
the comparative approach comes to new conclusions. It can contribute to general biochemistry
both by revealing similarities, and by pointing to
experimental models that are suitable for investigating the general laws. The thesis known as the
''August Krogh principle", named after the great
Danish physiologist, proposes that for every
problem there is an ideal animal model; this has
found many applications in biochemistry [22].
The main task of comparative biochemistry is to
describe the molecular variety of organisms and
to explain both their biological importance and
their development during evolution.
The present book restricts its view of comparative biochemistry to animals. The classification
of living organisms in the animal kingdom usually
presents no problem, but in specific cases may be
difficult. Thus, the classification of flagellate species to the plant "phytoflagellates" or to the animal "zooflagellates" on the basis of the presence
or absence of photosynthetic capacity has long
been considered questionable. The classification
by several authors of the "slime moulds" Physarum and Dictyostelium in the animal kingdom is
also undoubtedly controversial. Paramecium,
belonging to the Ciliophora, is today a favourite
object of first-year undergraduate, zoology practical classes. However, because of the fundamental characteristics of their nucleic acid and protein
sequences, the Ciliophora assume such a special
position amongst the eukaryotes that they cannot
logically be classified as either animals or plants.
There are compelling reasons to assume that their
divergence from the eukaryotic evolutionary line
occurred before the separation of the lower fungi,
higher plants and animals (see pp. 13 and 170).
Only a very small number of the known animal
species has ever been used for biochemical
investigations; biological, methodological and
economic factors play a role in the choice of
investigated species [19]. Species-rich groups that
have been studied physiologically have been preferred; in addition to the mammals, examples
include the amphibians, fish, insects, crustaceans,
molluscs and echinoderms. Because biochemical
investigations require large amounts of genetically uniform material, which in most cases cannot
be obtained from the field, species which can be
reared in the laboratory have been used. Particular attention has been paid to species of practical
importance, such as domestic animals, farm animals, pests and pathogens. Comparative biochemistry concerns itself with differences in composition and metabolism not only between members