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Part IV: The Fitness of Living
The Nature of Dauermodifications
THE TERM dauermodifications (original German:
dauermodificationen) seems to have been first used by
V. Jollos in 1913. (9)
It has still not found its way into evolutionary literature in
general, because the climate of opinion is not sufficiently favourable
toward the concept for which it stands. Yet the phenomena which
it was coined to define have been observed and demonstrated experimentally
for many years. Dauermodifications are the kind of modifications
which are observed in living things in response to environmental
pressures and which, when they occur in one generation, appear
to be inherited by the next.
the term to describe what he observed to be long-lasting changes
induced in paramecia by heat treatment and by various chemicals,
which he was persuaded were being transmitted through the cytoplasm
rather than the nucleus. Moreover, he noted that such induced
changes, or "modifications," continued to be propagated
over successive generations even after the inducing agent had
In a textbook on
organic evolution in 1952, A. W. Lindsey reported experiments
by F. B. Sumner and others, reinforcing the evidence for cytoplasmic
inheritance of this kind. (10) Sumner raised white mice at 20-30 degrees C. and
found that at the higher temperatures they developed longer bodies,
tails, ears, and hind feet. In these experiments Sumner took
normal mice and exposed them to an environmental pressure in
the form of a higher temperature than they were accustomed to;
he discovered that within a few generations the mice had modified
their bodies to improve their chances of survival by increasing
the amount of body surface area
9. Jollos, V., "Experimentelle Untersuchungen
an Infusorien," Biol. Zblt., vol.33, 1913, p.222-36.
1 of 6
10. Lindsey, Arthur Ward, Principles of Organic Evolution,
Mosby, St. Louis, 1952, p.342.
from which heat could
be radiated. This included elongation of the body, enlarging
the ears and tail (both of which are excellent heat exchangers),
and also enlarging the hind feet for reasons which are not altogether
clear at the moment. These modified animals were then returned
to a normal environment and mated. Their offspring were raised
at temperatures normal to the species. It was found that these
offspring retained the modified form for some generations even
though they were no longer being subjected to above-normal temperatures.
Thus Sumner demonstrated
experimentally that the modified form had become inherited. He
also demonstrated that with a return to a normal environment,
the inherited modification only gradually reverted to original
type. This seemed to be clear evidence that the elongated shape
which was an acquired character in response to heat had indeed
become inherited, but only in a semipermanent way. Sumner was
convinced that this was a form of cytoplasmic inheritance, since
it had been demonstrated so clearly by others that the nuclear
genes are not subject to environmental influences.
That the cytoplasm
was capable of influencing the form and function of daughter
cells had already been argued by a number of developmental physiologists
and embryologists on the following grounds. Since all cells in
an organism share the same nucleus and yet differentiate specifically
into different kinds of tissue ‹ bone, tendon, nerve, muscle,
skin, and so forth ‹ the power of differentiation was presumed
to be under the control of the cytoplasm rather than the nucleus.
Moreover, since millions of cells retain their ability
to produce any one of these specifically different structures
in the body, there must be some inheritable factors unique to
the controlling cytoplasm which governs the proliferation of
cell lines in certain directions. Boris Ephrussi put it succinctly:
Unless development involves
a rather unlikely process of orderly and directed gene mutation,
the differential must have its seat in the cytoplasm.
If the cytoplasm causes differentiation,
it must be endowed with the power of perpetuation of cell type.
- Bone cells
continue to reproduce bone and not skin ‹- not because their
nuclei are different from cells producing skin, but because their
proliferation as bone cells is under some cytoplasmic control
which so directs them. Since these cells replicate as bone and
not as, say, muscle, the control must be passed on from cytoplasm
to cytoplasm by some process of inheritance. We seem therefore
to be driven to the conclusion that there
- 11. Ephrussi, Boris, Nucleo-cytoplasmic
Relations in Micro-Organisms, Oxford University Press, 1953,
is a cytoplasmic form
of inheritance as well as a nuclear gene form of inheritance,
and it seems likely to be of a somewhat similar particulate nature.
difficulties continued for many years to leave the matter in
doubt, especially by contrast with the easily demonstrable and
therefore undoubted hereditary factors in the nucleus. Lindsey
complained that the experimental evidence of cytoplasmic inheritance
existing by 1952 was ignored by most geneticists because of their
anti-Lamarckian bias. It was simply denied that any environmental
pressure could influence the nuclear genes, which were held to
be the sole determiners of inheritable characters.
In 1953 Boris Ephrussi
published a report of his work with paramecia in such a lucid
manner as to draw fresh attention to the evidence of cytoplasmic
inheritance. He wrote: (12)
These studies confirm the view that
cytoplasm, like the genes, is endowed with genetic continuity.
The genes are therefore no longer to be regarded as the sole
cell-constituent with this property.
is a delight to read. He is full of enthusiasm for his subject,
and this enthusiasm is communicated to the reader in a flow of
language which seems easily to be able to handle the most complex
details. At that period Ephrussi did not seem certain that the
mechanism always involved active particles of some kind
in the cytoplasm which would be comparable to the genes in the
nucleus. But certainly the mechanism of this type of inheritance
resided in the cytoplasm and not in the nucleus. Toward the end
of his book he wrote: (13)
Considering that embryonic development
results in a restriction (and some widening, too) in different
cell lineages of the manifold potentialities originally carried
by the egg, we may picture the process of differentiation as
consisting, for example, in the segregation or sorting out of
an initially mixed population of cytoplasmic particles. Or we
may suppose that the egg, to begin with, contains a mixed population
of inactive particles and that development consists in the activation
by nuclear genes of different sorts of lineages.
In 1959 C. L.
Prosser was able to report: (14)
Several types of non-genic inheritance
and of indirect effects of environmental selection on the genotype
are recognized. Cytoplasmic inheritance is being discovered in
more and more groups of organisms, and cytoplasm is more readily
influenced by the environment than is the nucleus.
- 12. Ibid., p.6.
- 13. Ibid., p.100.
- 14. Prosser, C. I., "The Origin After
a Century: Prospects for the Future" in American Scientist,
vol.47, 1959, p.545.
the ovum in many species, including man, is much larger than
the spermatozoon. In man the ratio is about 500 to 1. Since the
nucleus is of equal size in both, the difference in mass results
from the far greater amount of cytoplasm which the ovum contains.
It is considered
that this fact is related to the greater importance (in some
matings) of the female contribution rather than that of the male,
and it results in such instances in a greater resemblance of
the offspring to the female parent. This in itself reinforces
the likelihood that some real contribution to inherited factors
is made specifically by the cytoplasm. In his book on the architecture
of the cell, Verne Grant proposes that if a cytoplasmically controlled
character does not persist for more than a generation or two,
it could be explained as a maternal effect in which the nuclear
genes of the mother, by imposing some condition on the cytoplasm
of the egg, predetermines a phenotypic trait of the offspring.
The trait in question is not therefore carried by some particles
in the cytoplasm acting autonomously.
On the other hand,
if the cytoplasmically controlled characteristic persists for
several generations but still eventually disappears, it should
be regarded as a dauermodification. The decisive test is persistence
number of generations even when the stimulus which was the determining
factor is removed. As Verne Grant observed: (15)
We are forced to conclude that
particles with a gene-like property of self-reproduction exist
in the cytoplasm. Inheritance through the cytoplasm has been
verified for a number of plants, animals, protista, and fungi.
- A little later
Grant refers to a plant experiment undertaken by P. Michaelis
(16) in which enucleated
cells retaining only the original cytoplasm were supplied with
nuclei from other cells. These structurally modified cells were
then cultured, and it was demonstrated that "the cytoplasmic
constituents responsible for the characters in question maintain
their identity and produce their specific action even though
under the influence of a foreign nucleus for 24 generations."
(17) This would
seem to indicate that the hereditary factors in the cytoplasm
do, in some cases, have genuine autonomy. Grant was writing in
1964. Since that time the principles of cytoplasmic inheritance
have been elaborated somewhat, as may be seen from Alfred Kuhn's
treatment of the subject.
- 15. Grant, Verne, The Architecture of
the Germplasm, Wiley, New York, 1964, p.15.
- 16. Ibid., p.19.
- 17. Micehaelis, P., "Cytoplasmic Inheritance
in Epilogium and its Theoretical Significance" in Advances
in Genetics, vol.6 1954, p.287-401.
Kuhn published his lectures on developmental physiology in 1971.
He observed: (18)
The form and size of [certain
protozoans] can be modified strongly and in various ways by environmental
factors. Certain modifications of form are retained as dauermodifications
for a long time after the conditions change, and it often takes
a large number of generations before a new form corresponding
to the new conditions is acquired.
And we shall
see in the next chapter, the response of the organism to the
environment may be extraordinarily rapid, even in man. Toward
the end of his volume, Kuhn wrote: (19)
In dauermodifications the consequences
of transient environmental influences can last for many cell
generations in single-celled organisms and for several individual
generations in multi-cellular organisms. The norm of reaction
of the cells is in all these cases controlled by alterations
of the cytoplasm. . . . In dauermodifications, cytoplasmic
components with altered properties must replicate.
Thus, in order to understand the nature of
determination one is led to the possibility that certain cytoplasmic
structures are capable of self-replication and that their relative
numbers and properties can he altered by appropriate conditions.
That part of the hereditary mechanism lies in the cytoplasm cannot
A cytoplasmic property which shows
extra-nuclear inheritance has been called the plasmagene or
plasmon by von Wettstein. The name plasmagene has been
given to the bearer of properties inherited in an extra-nuclear
At this point
Kuhn lists a number of references to work in this area by E.
Caspari (1948-55), F. Ochlkers (1952), P. Michaelis (1954), and
R. Hagemann (1964).
Thus we come in
a kind of circular course from a general acceptance of Lamarck's
common-sense doctrine of the inheritance of acquired characters
in the early eighteenth century to a position of uncertainty
by the mid-nineteenth century, followed by outright rejection
in the first half of the twentieth century. And now we are back
again to the original position, but on an entirely new basis.
As soon as the doctrine began to receive more favourable attention
by a few members of the scientific community whose opinion was
not to be lightly set aside, then a host of lesser authorities
suddenly began to observe any number of potential examples of
cytoplasmic inheritance, and a whole new field of experimental
inquiry was opened up.
Today there is
a wide measure of agreement that organisms have the power to
improve their fitness by
18. Kuhn, Alfred, Lectures on Developmental
Physiology translated by Roger Milkman, Springer-Verlag.
New York, 2nd edition, 1971, p.83.
19. Ibid., p.489.
adjusting their form
and function and passing on these adjustments to their offspring.
Nuclear genes do not seem to be involved, and for the most part
the older established doctrines of nuclear genetics remain valid.
Nuclear genes are indeed surprisingly impervious to environmental
pressures, but plasmagenes are not. A way is thus opened for
any organism to contribute to the greater fitness of its descendants,
and the whole of nature is in a position to reinforce the fitness
of things without becoming in bondage to an altered form which
in a later reversion of the environment would spell its doom.
Copyright © 1988 Evelyn White. All rights
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We shall now examine
some of the growing evidence that such a mechanism does exist.