of Heredity 28: 31-33 (1937)
THE PLACE OF IAROVIZATION IN PLANT BREEDING
A. J. Bruman
Division of Plant Exploration and Introduction U. S. Dept. of Agriculture
Editor's Note: Iarovization (pronounced "Yarovization" and also referred to as Vernalization) was first discussed in this Journal a little over four years ago. Since that time a considerable amount of literature dealing with the genetic and physiological bases of the phenomenon, and with agricultural applciations of the method in practice, have appeared in many languages. The term is now used sometimes rather loosely to include many biological processes involving the manipulation of such external elements as light, heat, and moisture.
A new conception of dominance in plants is offered by T. D. Lissenko, the Russian exponent of the Iarovization idea, and his associates working at the Odessa Institute of Genetics and Plant Breeding. Not only is dominance looked at from a, presumably, new angle but a purported way towards its control is indicated in a series of recent publications emanating from the above-mentioned institution.
The gap between abstract mathematical studies of factor assortment and factor expression is supposedly filled by the new findings relating to the biological processes accompanying the course of each developmental stage in the life of a plant.
Without rejecting the prevailing concept of the genotype as the basis of all hereditary manifestations, a more or less definite and, assertedly, very essential guiding principle for its expression is pointed out. This principle the alleged connecting link between the gene and its phenotypic expression is what Lissenko and his associates call the "biological requirements for adaptability" of every genetic factor. Each member of an allelomorphic pair of factors is said to have its own biological requirements for adaptability and expression and, except for these requirements, is equally capable of becoming manifested in the zygote.
The Battle of Allelomorphs
Dominance in any heterozygote then depends entirely on the conformance of the biological requirements of one or the other side of its "dual personality" with the external conditions surrounding its development. Only that side develops which meets the appropriate conditions for its expression. In other words, dominance depends on the particular environmental set-up at the time when the two "rival" allelomorphs vie for supremacy. Depending on the particular factor, this period may fall during any of the several stages in the life of a plant.
The iarovizationists do not suggest that any such type of externally controlled dominance necessarily exists in animals where conditions surrounding the development of the various organs in the embryo are rather beyond the influence of those outside elements which play so vital a role in the early life of a plant. Also a rather fine distinction is drawn between external conditions and environment. There can be a complete change in environment, say the iarovizationists, without any change in the specific conditions essential to development. On the other hand, the slightest changes in environment may be of great significance when essential conditions necessary to the passing of a particular phase are involved. And, finally, it should be pointed out that the characters with which Lissenko and his associates are working cold resistance, drought resistance, length of vegetative period, tolerance to excessive moisture, earliness and others of a similar nature are really quite inseparably connected with environment (or external conditions). At least, this connection is certainly closer than would he the case with any strictly morphological characters.
But what, if any, is the practical significance of this new idea of phenotypic expression? Since dominance is only a matter of the realization, so to speak, of an inherent adaptability to external conditions, it should become unnecessary, it is claimed, in breeding plants for certain characters to go through any preliminary crosses and to subject countless progeny to the test for dominance. Lissenko and his associates proceed to determine dominance from the parents directly. This is done by what they call a "biological analysis" which is in reality Lissenko's iarovization test.
Again, from what one knows of iarovization, it is difficult to conceive of any great number of factors besides those already mentioned that could be so tested. One could, of course, envision the possibility of elaborate chemical and electrical tests which may unmask a number of additional factors but thus far the elements of heat, light and moisture, involving principally characters of a physiological nature, appear to loom most prominently in all of the demonstrations and experiments one encounters in the literature on the subject. Objections could, therefore, justifiably be made to propounding the hypothesis in too general terms.
Granting, however, the undisputable importance of the factors mentioned, let us see how Lissenko applies his theory of dominance to practical hybridization. The first of the numerous demonstrations cited is one in which two varieties of wheat, Triticum hordeiform 2508 and T. melanopus 069 are involved. Both of these, we are told, are late maturing varieties unsuitable to the Odessa region. By applying the iarovization test to samples of each, it is found that "lateness" is brought about differently in the two varieties. In T. hordeiforme 2508 it is due to a slow, extended "thermal" stage under Odessa conditions. In T. melanopus 069, on the other hand "lateness" is found to be due to a slowly proceeding "light" stage, its "thermal" stage being sufficiently brief. In other words, in T. hordeiforme a rapidly proceeding "light" stage is dominant under Odessa conditions, while in T. melanopus a rapidly proceeding "thermal" stage is dominant under the same conditions.
After dominance and the conditions bringing it about are thus determined, it is a simple matter, says Lissenko, to cross two "properly selected" parents, thus "consciously, definitely passing the desired qualities to the offspring." In the heterozygous F1, it is reasoned, the two "shortcomings" (factors for lateness?) of the parents are "mutually liquidated" and an early maturing variety is thus obtained from two late ones. It is further argued that since the best possible combination of "rapidity" of all developmental stages under a given set of external conditions is always obtained in the F1, the necessity for any further crosses is obviated.
Iarovization vs. "Classical Genetics"
A good deal of support for this new theory of dominance is drawn by its proponents from the prolific work of I. V. Michurin, the late Soviet fruit breeder, who, himself, was the originator of not a few novel ideas in horticulture and genetics. On the other hand, there is evidence of considerable opposition on the part of many of the more conservative Soviet geneticists to Lissenko's approach to dominance and to his close linking of iarovization with genetics. A paper published in the latest number of Iarovizatsiia a journal devoted entirely to iarovizaion by I. I. Present, an associate of Lissenko, discloses the rather intense controversy aroused by Lissenko's new teachings among Soviet plant scientists. The article is a reply to an unpublished manuscript which is on file at the library of the Institute of Plant Industry in Moscow. This manuscript is by A. R. Zhebrak, a former student of T. H. Morgan. From the numerous citations quoted in Present's article, one gathers that Zhebrak's criticism is quite outspoken and that it attacks the entire trend of "iarovizationist genetics" in no uncertain terms.
Geneticists in general, however, have long ceased quibbling over the relative roles played by inheritance and environment. The great amount of accumulated genetic data emphasizes the futility of such argument. And what are the iarovizationists' "external conditions" or "essential conditions for the passing of a developmental stage" if not environment? A mere differentiation between what might be called the micro-environment and the general environment certainly could not be made the basis for any sweeping generalizations with regard to the nature of dominance.
That portion of Lissenko's work, therefore, which deals with the influence of external conditions on factor expression can only be added to all the other knowledge now available on the subject. But the attempt at linking genetic phenomena to the biology of development and, particularly, the attempt at direct factor determination through some sort of biological or other test, while still somewhat vague and not always fruitful, must be recognized as a possible step in the right direction. The illustration cited above, which may he interpreted differently by some geneticists, yet indicates that Lissenko and his associates are groping in what may prove to be a field of some promise.