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Part I

Part II

Part III

Part IV



     

Part IV:  The Fitness of Living Things: Dauermodifications

Chapter 2

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.
     Jollos originated 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 been removed.
     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.
10. Lindsey, Arthur Ward, Principles of Organic Evolution, Mosby, St. Louis, 1952, p.342.

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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: (11)

     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, p.4.

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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.
     Certain experimental 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.

     Ephrussi's book 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.

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    Now, 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 for a
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.

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    Alfred 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 be doubted.
     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 fashion.

     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.
 

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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.
     We shall now examine some of the growing evidence that such a mechanism does exist. 

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Copyright © 1988 Evelyn White. All rights reserved

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