4.2.7 Gene Transfer Between Species
and "Molecular Lamarckism"
If the discovery of the dynamic state of the molecules of inheritance and the mobility of DNA
sequences in the genome radically changed our
ideas about the molecular mechanisms of evolution, then the transfer of genes across species
boundaries has opened a whole new dimension in
evolutionary theory. Gene transfer between prokaryotes is well known. However, there are also
examples of the transfer of genetic information
between prokaryotes and eukaryotes. For example, a large plasmid is responsible for the induction of crown-gall tumours in plants by Agrobacterium tumefaciens, and it is, in part, incorporated
into the genome of the plant and expressed. This
DNA includes genes for the synthesis of specific
substances ( opines) which are otherwise never
produced in the cells of higher plants. The prokaryote Progenitor cryptocides secretes in culture a
protein that is apparently encoded by the gene for
human chorion gonadotropin. These two cases
are both pathological; only systematic DNA
sequence comparisons will show to what extent
gene transfer between species is important as a
normal mechanism of evolution of the eukaryotes. Artifical gene transfer has already been
achieved in a whole series of eukaryotic cells. As
the enzymes and gene vectors used in these
experiments are all of natural origin, what the
genetic engineer has carried out should also be
possible in evolution [122].
The following cases have been offered as evidence of gene transfer during evolution.
1. The symbiotic photobacterium Photobacter
leiognathi, regularly found in the teleost Leiognathus splendens, is one of a few prokaryotes known to possess a Cu, Zn superoxide dismutase. This is the typical cytosolic enzyme of
the eukaryotes; only the Mn and Fe superoxide dismutases are found in other prokaryotes
(p.706).
2. Escherichia coli possesses two GAPDH genes,
only of which is a typical prokaryotic gene; the
other shows great similarity to the GAPDH
genes of the eukaryotes. E. coli apparently
took over this second gene from a eukaryote
[97].
3. The repetitive DNA family 2108 is very similar
in the sea urchins Strongylocentrous purpuratus, Tripneustes gratilla and others, although
these species separated at least 200 million
4.2.8 Adaptive and Innovative Protein Evolution
127
years ago and display large differences in
single-copy DNA.
4. The histone gene h19 of the North Atlantic sea
urchin Psammechinus miliaris differs by only
1.3 % from that of the Pacific species Strongylocentrous miliaris, although these species
separated at least 65 million years ago.
Thoughts of gene transfer are supported by the
very similar sequence found in the species
Strongylocentrous droebachensis, which colonized the North Atlantic only about 5 million
years ago.
5. The fact that haemoglobin is widely found in
animals but is restricted to a few groups of
plants suggests gene transfer, perhaps with a
virus as the vector.
At the beginning of the 1980s, there was controversy, under the heading of "Lamarckism", about
whether the immunotolerance conferred upon
new-born mice by inoculation of spleen or bonemarrow cells was inherited by their progeny.
Repetition of the experiments led, in all cases, to
negative results [37]. In fact, confirmation of the
results would in no way have been support for
Lamarckism against Darwinism. In the view of
Lamarck, the assumed process would certainly
not have been a case of an acquired phenotypic
character having a shaping effect on the genotype. It would have been much more a question
merely of gene transfer between somatic cells and
the germline, a process for which there are no
precedents but which, from present-day knowledge, can certainly not be excluded [122]. The
enhancement of the mutation rate in stationaryphase bacterial cultures by a potential substrate,
and thus the appearance of enzymes required for
its assimilation, has also been interpreted as the
inheritance of an acquired character. This also
conforms with neo-Darwinite ideas in that the
single-stranded DNA arising during transcription
is more easily mutated, and therefore substrate
induction can quite possibly increase the mutation rate [85].
4.2.8 Adaptive and Innovative Protein
Evolution
Even though many amino acid substitutions
occurring during evolution may be selectively
neutral, adaptive evolution of proteins does certainly occur. This book is full of examples of the
adaptation of proteins to the specific requirements of individual animals. The evolution of
single proteins relies upon the optimization of dif·
and "Molecular Lamarckism"
If the discovery of the dynamic state of the molecules of inheritance and the mobility of DNA
sequences in the genome radically changed our
ideas about the molecular mechanisms of evolution, then the transfer of genes across species
boundaries has opened a whole new dimension in
evolutionary theory. Gene transfer between prokaryotes is well known. However, there are also
examples of the transfer of genetic information
between prokaryotes and eukaryotes. For example, a large plasmid is responsible for the induction of crown-gall tumours in plants by Agrobacterium tumefaciens, and it is, in part, incorporated
into the genome of the plant and expressed. This
DNA includes genes for the synthesis of specific
substances ( opines) which are otherwise never
produced in the cells of higher plants. The prokaryote Progenitor cryptocides secretes in culture a
protein that is apparently encoded by the gene for
human chorion gonadotropin. These two cases
are both pathological; only systematic DNA
sequence comparisons will show to what extent
gene transfer between species is important as a
normal mechanism of evolution of the eukaryotes. Artifical gene transfer has already been
achieved in a whole series of eukaryotic cells. As
the enzymes and gene vectors used in these
experiments are all of natural origin, what the
genetic engineer has carried out should also be
possible in evolution [122].
The following cases have been offered as evidence of gene transfer during evolution.
1. The symbiotic photobacterium Photobacter
leiognathi, regularly found in the teleost Leiognathus splendens, is one of a few prokaryotes known to possess a Cu, Zn superoxide dismutase. This is the typical cytosolic enzyme of
the eukaryotes; only the Mn and Fe superoxide dismutases are found in other prokaryotes
(p.706).
2. Escherichia coli possesses two GAPDH genes,
only of which is a typical prokaryotic gene; the
other shows great similarity to the GAPDH
genes of the eukaryotes. E. coli apparently
took over this second gene from a eukaryote
[97].
3. The repetitive DNA family 2108 is very similar
in the sea urchins Strongylocentrous purpuratus, Tripneustes gratilla and others, although
these species separated at least 200 million
4.2.8 Adaptive and Innovative Protein Evolution
127
years ago and display large differences in
single-copy DNA.
4. The histone gene h19 of the North Atlantic sea
urchin Psammechinus miliaris differs by only
1.3 % from that of the Pacific species Strongylocentrous miliaris, although these species
separated at least 65 million years ago.
Thoughts of gene transfer are supported by the
very similar sequence found in the species
Strongylocentrous droebachensis, which colonized the North Atlantic only about 5 million
years ago.
5. The fact that haemoglobin is widely found in
animals but is restricted to a few groups of
plants suggests gene transfer, perhaps with a
virus as the vector.
At the beginning of the 1980s, there was controversy, under the heading of "Lamarckism", about
whether the immunotolerance conferred upon
new-born mice by inoculation of spleen or bonemarrow cells was inherited by their progeny.
Repetition of the experiments led, in all cases, to
negative results [37]. In fact, confirmation of the
results would in no way have been support for
Lamarckism against Darwinism. In the view of
Lamarck, the assumed process would certainly
not have been a case of an acquired phenotypic
character having a shaping effect on the genotype. It would have been much more a question
merely of gene transfer between somatic cells and
the germline, a process for which there are no
precedents but which, from present-day knowledge, can certainly not be excluded [122]. The
enhancement of the mutation rate in stationaryphase bacterial cultures by a potential substrate,
and thus the appearance of enzymes required for
its assimilation, has also been interpreted as the
inheritance of an acquired character. This also
conforms with neo-Darwinite ideas in that the
single-stranded DNA arising during transcription
is more easily mutated, and therefore substrate
induction can quite possibly increase the mutation rate [85].
4.2.8 Adaptive and Innovative Protein
Evolution
Even though many amino acid substitutions
occurring during evolution may be selectively
neutral, adaptive evolution of proteins does certainly occur. This book is full of examples of the
adaptation of proteins to the specific requirements of individual animals. The evolution of
single proteins relies upon the optimization of dif·