GLADIOLUS PARTLY WHITE AND PARTLY RED
A chimera caused by a vegetative mutation in a gladiolus corm resulting in the production of red, white and partly red and white flowers on a stalk of normally white flowering variety. (Courtesy of Prof. E. B. Babcock.) (Frontispiece.)

Journal of Heredity 10(1): 3-10 (Jan 1919)
CHIMERAS IN CORN HYBRIDS
Xenia in F1 Corn Hybrids Changed Through Mutation—Chimeras in Flowers—A Case of Chimera in a Fig
J. L. COLLINS
Instructor in Genetics, University of California, Berkeley, Cal.

AMONG the purple starchy F1 dent grains of corn resulting from crossing Extra Early Adams white dent corn with Black Mexican sweet corn one grain was found half of which was white, the other half being the dark purple color characteristic of the male parent (Fig. 1a). The line separating the colored from the uncolored area was sharply defined.

1 Endosperm —The substance stored in a seed adjacent to the embryo, for its early nourishment.
2 Aleurone —The protein granules found in the endosperm of ripe seeds.

In order to determine whether there is any hereditary tendency for the formation of such "mosaics," this F1 grain was planted, and the four ears shown in Fig. 2 were produced. They showed segregation into purple and white colors and starchy and sugary endosperm1 characters in the following proportions: 408 purple starchy, 352 white starchy, 172 purple sweet, 151 white sweet. This approximates the F2 dihybrid ratio to be expected from a cross of white dent with purple sweet corn when starchy endosperm and purple aleurone2 are dominant characters.

Among the 1,083 F2 grains twelve were found showing reappearance of the mosaic in a degree as great or less than was shown by the F1 parent. There is not sufficient evidence in the way of reappearances, however, to lead one to believe that the condition is inherited in a Mendelian fashion. East and Hayes grew a number of such seeds to see whether the tendency was inherited, but without positive results.

3 Italics are mine.

Similar cases in F1 corn hybrids have been noted by Correns and by Webber, both of whom believed that it was caused by the failure of the second pollen nucleus and the endosperm nucleus to fuse, so that each nucleus developed independently and thus formed one-half of the endosperm, which is enclosed by the colorless pericarp composed entirely of maternal tissue. This explanation cannot hold in this case, because, if such were true, the purple half of the seed should also have had the sweet endosperm, inasmuch as the factor for purple aleurone and the factor for sweet endosperm were carried by the same (male) nucleus. Reference to Fig. 1a shows that the entire grain had the starchy endosperm. The F2 results show the embryo to have been a true hybrid for both aleurone color and endosperm texture, with the starchy endosperm, to all appearances, completely dominant in the F1. If we are to believe that each nucleus developed independently to produce the bicolored aleurone effect, how can we harmonize this belief with the fact of uniform dominance of the starchy endosperm? East and Hayes suggest that the phenomenon may be due to "Mendelian segregation" in somatic tissue. That there is no known physical basis for the operation of such segregation is clearly shown by Babcock and Lloyd where they state that "the mechanism by which this (Mendelian) segregation is accomplished is the separation of whole chromosomes in the heterotypic mitosis (meiosis) during maturation of the germ cells."3 Such separation of chromosomes does not normally occur during division of somatic cells.


A PECULIAR GRAIN OF CORN
A chimera in a grain of corn caused by the occurrence of a factor mutation in a single cell at an early stage of aleurone development. Colors purple and white. (Fig. 1a.)
A chimera involving a smaller area of changed aleurone tissue. Colors purple and yellow. (Fig. 1b, at right.)

A more probable explanation of this two-color phenomenon in corn is the occurrence of a vegetative factor mutation in meristematic tissue causing the development of a chimera. The result could be produced by the occurrence of a mutation affecting the aleurone color factor in one of the cells at a very early stage of the development and growth of the zygote, such that all cells descending from the mutated cell would have the white aleurone layer. The application of the mutation idea adequately accounts for the occurrence of areas or stripes of white, which are less than one-half of the surface of the grain, as in Fig. 1b.. Some of the F2 grains from the one shown in Fig. 1a had only a small spot of white aleurone, which was surrounded by purple. These white cells may be considered as the outgrowth of a single cell in which mutation occurred in the color factor much later in the development of the aleurone layer than was the case where approximately one-half of the grain was white. The appearance of these smaller areas cannot be adequately accounted for by the theory of the independent development of the second pollen nucleus and of the endosperm nucleus, even when the additional evidence from endosperm texture is not considered.

If objection is raised to the application of the mutation hypothesis here because of the appearance of a number of similar mutated individuals in the same or the immediately succeeding generation, attention only need be called to similar cases in other plants where this sort of phenomenon reoccurs many times, and has been explained as due to mutation in vegetative tissue. Perhaps the best known case is that of citrous fruits containing one or more sections differing in color or texture from the remainder of the fruit. Babcock and Clausen attribute the appearance of partly red and partly white colored gladiolus flowers on a plant of the white variety known as "The Bride" (Frontispiece) to a factor mutation in meristematic tissue. Their account of its appearance is as follows: "In 1915 there appeared in a row of 'The Bride' a single stalk bearing partly red and partly white flowers. That this grew from a corm which was an offshoot from a typical white flowering corm is certain. Furthermore, that the mutation occurred very early in the development of this corm and not some time during the growth of the flower stalk is proved by the following observations: In the autumn following the discovery of the mutant stalk it was carefully lifted and the corm from which it grew was separated from the cluster of white flowering corms. It was observed that there were smaller corms located very close to the mutant corm. The following spring one flower stalk bore red and white and the other only red flowers. In gladiolus the young corms push out from near the base of the old one. Hence the original mutant corm must have consisted partly of cells capable of producing red pigment in the flowers. That the cells having this altered chemical constitution comprised about one-half of the corm is indicated by the position of the red and white flowers on the stalk." A like condition in the canna lily is shown in Fig. 5, where the change envolves foliage leaves, flowering stalk and flowers.

CORN GROWN FROM A GRAIN CHIMERA
Four F2 ears of corn produced by selfing the F2 plant grown from the grain chimera shown in Fig. 1a. (Fig. 2.)

A COLOR CHIMERA IN A FIG
The cell in which the change took place was not in the developing young fruit, but in a cell of the young shoot on which the fruit grew. (Photo by courtesy of Professor I. J. Condit.) (Fig. 3.)

4 J. C. Whitten, Division of Pomology, University of California.
5 E. B. Babcock, Division of Genetics, University of California.
6 I. J. Condit. Division of Citriculture. University of California.
Similar kinds of vegetative mutations in fruits, causing the appearance of sharply defined color and texture areas, have been reported in apples, peaches, cherries, prunes,4 tomatoes,5 olives and figs.6

The fig-chimera here reported (see Fig. 3) appears to be of a slightly different type of origin from the other chimeras mentioned, and also from the corn grains under discussion, in that the cell in which the change took place was not in the developing of young fruit itself, but in a cell of the young shoot on which the fruit grew. A few of the leaves growing on the tree which produced the fig-chimera were characterized by white areas or sections, as shown in Fig. 4. Many examples of this kind of chimera have been demonstrated. Norton was able to trace a foliage color difference down the stalk as a different colored stripe in a whole branch of a tomato plant, narrowing until its origin was apparently located in a single cell. Buds pushing out at the point of union between the stripe and the normal colored portion of the stem bore two-colored leaves similar to the fig leaf in Fig. 4.


PARTLY WHITE LEAVES FROM A FIG TREE
Leaf chimera from the tree which produced the fig shown in Fig. 3. Notice the chimera condition in the branch. (Photograph by courtesy of Professor I. J. Condit.) (Fig. 4.)

 

CHIMERAS IN GRAINS OF MAIZE.
Three grains of maize showing chimeras in which the endosperm characters sugary and starchy are involved. (Fig. 5a, b, c.) A color chimera in an F2 grain of sweet maize. (Fig. 5d.)

7 Furnished by Dr. R. E. Clausen.
If we use the mutation hypothesis to account for a change in color in a portion of a corn grain, it would appear reasonable to expect that mutations might occur in factors affecting other chemical constituents of the endosperm. Grains of corn showing such changes in endosperm have been found7 (Fig. 5a, b, c) among the hybrid progeny of two other different strains. The cross between the varieties Country Gentleman (sweet) with U. S. White Dent (starchy) produced among the F2 progeny seven grains with sweet patches in the starchy endosperm (Fig. 5a,b,c).

Among the F2 grains derived from the one shown in Fig. 1a, a half purple and half white sweet grain was found, to which a great deal of interest is attached, because in this grain lies the possibility of our being furnished with the proof that the mutation hypothesis here given is the correct one. If the progeny from this grain (Fig. 5d)  gives evidence that the embryo is homozygous for the purple color, then the change from purple to white in the aleurone can only have come about by somatic mutation in the manner herein described, while on the other hand, if evidence is furnished that this F2 grain is heterozygous for purple and for nonpurple (white), no violence will be done to our mutation conception of the origin of the chimera.


CHIMERA IN A CANNA LILY
A canna lily of the purple or bronze leaved red flowering variety in which a change took place apparently in a cell or cells of the root at the point where the bud pushed out, causing the chimera in the stem, leaves and flowers. Areas of the bronze leaves changed to pure green and flower color changed from red to yellow. The open flower on the left side of the stalk is half red and half yellow. (Photograph by courtesy of Professor W. J. Pope, University Farm, Davis, Calif.) (Fig. 6.)

SUMMARY

Grains of hybrid corn are here reported in which xenia occurs only in a portion of the aleurone layer, others having sweet patches in the starchy endosperm. Evidence is furnished disproving the theory of independent development of the second pollen tube nucleus and the endosperm nucleus. A factor mutation occurring in a single somatic cell, producing a chimera, is offered as the more probable explanation of the phenomenon. A certain F2 grain may furnish conclusive proof of the hypothesis advanced. Instances are cited of the occurrence of similar chimeras in several plant genera.

LIST OF LITERATURE CITED

Black Mexican Sweet Corn