Somatic Mutation

SOMATIC MUTATION, a change in the genotype of somatic cells during the individual development of an organism. It was previously believed that mutations usually occur directly before the formation or in mature germ cells and in any case exclusively in cells of the germ line; precisely for this reason such germ cell mutations are reproduced by all cells arising during development from the zygote, in the formation of which the initial mutational gamete participated (see Mutation). At present, numerous facts speak in favor of the idea that mutations can occur at any moment during the individual development of an organism. In plants, S. m. are essentially known for a long time and are observed extremely frequently. Such are the so-called bud variations, described in detail by C. Darwin (1868). Bud or vegetative mutations are observed especially frequently in fruit trees and ornamental plants and are the most common method for breeding new varieties. Thus, a whole series of new varieties of oranges, plums, etc., originate from individual branches discovered that differ sharply from the whole tree by certain traits, for example, rate of ripening, size, shape, and number of fruits. Vegetative cuttings from such modified branches give trees that completely repeat the peculiarities of the mother branch. Obviously, one should assume that such branches originated from a mutational initial cell at the point of growth. Since plants do not have a sharply separated germ line, in a number of cases of vegetative mutations it is possible to prove their mutational nature by sexual reproduction. Sexual reproduction can be realized in a vegetative mutation in the case when it has affected the so-called subepidermal layer, from which the germ cells of plants are formed. This peculiarity of plants leads to the fact that often vegetative mutations can have the character of a chimera: in the same plant, some tissues affected by the mutation will differ from other tissues remaining unchanged. Due to the absence of vegetative reproduction and the separation of the germ line, only in the very recent time have S. m. been found in animals. With certainty, S. m. can be detected primarily in genetically well-studied forms, such as the fruit fly Drosophila. It turned out that all types and regularities that we know regarding germ line mutations are also observed in somatic mutations. Thus, it was possible to observe gene S. m. Indeed, in Drosophila, on males it was possible to detect areas of the body or individual organs, by color or shape completely different from the whole body and externally analogous to the action of well-known genes of the sex chromosome -{body color, eye color, wing shape, bristle shape, and many others). Chromosomal aberrations also occur in S. m. Especially many cases of chromosomal elimination in somatic cells are known. This can be detected during the manifestation in places of such S. m. of recessive genes of the remaining homologous chromosome. In those cases when such S. m. occurs at the very early stage of development (during cleavage of the egg), bilateral mosaics can arise, in which one half of the body manifests dominant traits, the other - recessive [see separate table (to the article Streptococci), fig. 6]. If such an early elimination is carried out with respect to the sex chromosome, gynandromorphs will arise, in which one half of the body will have the traits of the female sex, the other of the male sex (see in more detail Gynandromorphism). If a somatic mutation took place at such an early moment that it preceded the complete separation of the germ line, then it would affect a certain part of the germ cells and therefore would be detected in a part of the offspring already in the form of an ordinary germ line mutation; such cases are described in Drosophila. Essentially, only in these cases can we speak with absolute certainty about a somatic mutation. In the vast majority of cases of somatic mutation, of course, we will not detect the corresponding mutants in the offspring. If spontaneous S. m. are usually observed extremely rarely, then experimentally it is possible to significantly strengthen this process by the same factors as in germ line mutations, namely X-rays. In a number of special cases, S. m. arise without any influence. This occurs in relation to special so-called mutable or labile genes. Such mutable genes have been detected both in plants (Zea, Antirrhinum, Delphinium, etc.) and in animals (Drosophila virilis). It turns out that if some mutable genes mutate only in germ cells, then others mutate only in somatic cells, and a third both in those and in others. The mutability of genes in somatic cells explains a whole series of phenomena of variegation, variegated leafiness, and coloration of other parts in plants. In humans, reliable cases of S. m. are unknown. However, it is very probable that such an explanation accounts for certain cases of mosaic and asymmetric manifestation of traits (multicolored and spotted eyes, certain cases of naevus and pigmentation, etc.). In recent times, the view has gained great currency that malignant tumors arise as a result of S. m. (Boveri, 1914; Bauer, 1928). It is assumed that chromosomal aberration takes place in this case: according to some, an increase in the number of chromosomes, according to others, elimination of chromosomes. However, numerous cytological studies have not yet given a clear and unified picture.