The American Naturalist 56: 64-79 (1922)
The Nature of Bud Variations as Indicated by Their Mode of Inheritance (excerpt)
Professor R. A. Emerson

Cornell University

Somatic Mutation of Genes

Recent investigations of variegated maize by Eyster and Anderson have established the fact that somatic mutations affecting small areas occur much more frequently than those affecting large areas. Since a mutation arising in a single cell late in development obviously could not affect so large an area as one originating earlier, it follows that mutations in variegated maize occur with increasing frequency in the later stages of ontogeny. It is true, as pointed out by Muller (1920), that given a constant rate of mutation throughout all stages of ontogeny and granting that one cell is as likely as another to mutate, mutations should appear more frequently in the later stages of development because of the fact that there are then many more cells in which mutations may arise. But Eyster and Anderson have found that the increase in the frequency of occurrence of mutations during the progress of development is accelerated far beyond expectation based on the increase in number of cells

This behavior is strongly suggestive of a progressive acceleration in the mutability of the variegation gene as development proceeds. It is much too early to say whether this progressive change, if such it be, is inherent in the organization of the gene itself, as suggested by Anderson and Demerec, or whether it is a response to progressive changes in physiological and environmental relations. Perhaps the assumption of an equal chance of mutation as between any two cells is without sufficient warrant. Possibly there is a time element to be taken into account, as noted by Muller (1920). As cell division becomes progressively retarded in the late growth stages, may not each cell be exposed for an increasingly longer period of time to the chance of mutation? Perhaps it may be possible to test this assumption in favorable material by a comparison of the frequency of mutation in the very early slow-growth, the later rapid-growth, and the final slow-growth periods of the life cycle; but the relatively few cells present in the very early growth period seems likely to place serious limitations on the practicability of such a test. An observation of possible importance in connection with the question of a time element in mutation and with the problem of environmental and physiological influences is that made by Eyster and Anderson concerning the greater frequency of the nonheritable (epidermal) mutations than of the heritable (sub-epidermal) ones in variegated pericarp of maize.

I have recently obtained results bearing on another phase of the somatic-mutation problem as related to variegated maize pericarp, namely, the relative frequency of mutation of homozygous, VV, and of heterozygous, VW, material. It has been shown above that the W gene for colorless (white) pericarp does not mutate, so far as known, when paired either with itself, WW, with the variegation gene, VW, or with the self-color gene, SW. It will be recalled further that only one of the two homologous genes in homozygous variegated, VV, material mutates at any one time. If it could be assumed that the mutability of either allelomorph is uninfluenced by the presence of the other, it should follow that somatic mutations will occur with approximately twice the frequency in homozygous, VV, as in heterozygous, VW, material. But this expectation has not been realized. On the contrary, both heritable (self-color) and non-heritable (dark-crown) mutations have appeared throughout all my cultures with somewhat greater frequency in heterozygous than in homozygous variegated ears. The difference has been especially pronounced in very light variegated strains, where mutations have appeared about two and one half times as often in heterozygous as in homozygous material. Even if mutations appeared with equal frequency in heterozygous and in homozygous ears, the simplex gene of the former must have a mutability of about twice that of either of the duplex genes of the latter. In the very light variegated strains, therefore, a simplex gene must have a mutability of about five times that of a duplex gene.

What appears to be a similar result in Mirabilis has been reported by Correns (1903, 1904). Crosses of a supposedly pure white race with several self-colored pink, yellow, and pale yellow races resulted in every case in plants with strongly red-striped flowers and with numerous self-red flowers or even whole branches of such flowers. Intercrosses of the pink and yellow races gave only self-colored progeny, from which fact it was concluded that the white-flowered race carried a latent factor for striping. It was later discovered that about three per cent. of the flowers of the white race showed minute flecks of red. It was evidently an extremely light, variegated race, rarely if ever throwing somatic self-color mutations when the variegation gene was duplex (homozygous material) but producing such mutations with considerable frequency when that gene was simplex (heterozygous material). Correns concluded that red variegation of Mirabilis flowers is a character that, with self-fertilization or inbreeding, remains almost completely latent, but which, through the entrance of foreign germ plasms, is brought to full expression.

If the mutability of a gene can be increased through the influence of some modifying factor or factors brought into combination with it by crossing, as suggested by Correns, it should be possible to discover crosses that would not produce the effects so far observed in Zea and Mirabilis. While the problem deserves much more study from this viewpoint, it seems unlikely that results with maize can be explained on any such basis, unless the postulated modifying factor is the allelomorph of the variegation gene or some factor very closely linked with it. It must be noted in this connection that the comparison in maize was made between homozygous and heterozygous variegated ears of the same F2 progenies grown from self-pollinated F1 heterozygotes—a circumstance that would afford abundant opportunity for recombinations of independently inherited modifying factors. That the differences in mutability noted in maize may be due to differences in the interaction of like as contrasted with that of unlike allelomorphs, as suggested by Anderson and Demerec, is a somewhat novel conception worth careful consideration if means can be devised for subjecting it to a crucial test.

Before the topic of somatic mutation is dismissed, it should be noted that the phenomenon is not limited to plants. Among animals, Drosophila (Morgan and Bridges, 1919) has furnished several examples of undoubted somatic mutation resulting in mosaic individuals other than gynandromorphs.

CybeRose note: It is interesting to see, so many years after this paper was written, how experts of the time made unnecessary assumptions about the existence of "modifiers" to explain every deviation from the Mendelianist dogma. Why should all the yellow, pink and pale yellow Mirabilis strains carry specialized modifiers that apparently had nothing to do but interfere with the expression of whiteness in a separate strain? And if the occasional self-colored flowers produced on variegated-flowered plants did not produce self-colored offspring, why should we regard the change as a gene mutation? Correns thought (as Emerson noted earlier in this paper) that "their failure to transmit the self-color character being due presumably to the accident that the mutation occurs in epidermal cells from which no gametes arise." Of course, this does not explain why such mutations occur only in epidermal cells.

A more plausible explanation, in modern terms, is that the suppression of red pigment in the white-flowered Mirabilis involves gene silencing. The "genomic shock" that results from crossing with a different strain releases (or partially releases) the suppression, allowing red pigment to be expressed in some cell-lines. Something similar occurs when the Black Mexican sweet corn is crossed with other varieties. Ordinarily the silks, glumes and anthers of Black Mexican are white or pale green; the seeds become fully pigmented only when mature. Cross-bred progeny of Black Mexican express the red pigment in all or most of the silks, glumes and anthers, but not in the seeds — except for occasional black or purple spots.

Bazavluk found some odd results when pollinating white Mirabilis jalapa with pollen mixtures that suggest a cross influence between neighboring embryos.