Science 69: 526-527. (May 17, 1929)
Hetero-Fertilization in Maize
George F. Sprague

1 These investigations were conducted in the department of plant breeding,
Cornell University, and at the North Platte Substation, North Platte, Neb.

There is abundant evidence that the endosperm and the embryo of the maize kernel regularly are genetically identical. Were they not, there would be no reason for selecting on the basis of endosperm characters among the individual kernels in genetic or breeding experiments. In connection with the writer's investigations on the inheritance of scutellum color, however, considerable data have been obtained which show that this usual condition is not universal.1

2. G. F. Sprague, Jour. of Heredity, 18: 41-44, 1927.

"Colored Scutellum" already has been described as a heritable character of maize (Sprague).2 The development of scutellum color is dependent upon several factors, the interaction of which will be described later. Before purple or red color can develop in the scutellum, the fundamental aleurone factors A, C, R, and i must be preent. The Pr pr factor pair which differentiates purple and red aleurone has a similar effect on scutellum color. The scutellum is, of course, a product of the fusion of one sperm and the egg, whereas the aleurone (endosperm) results from the fusion of the second sperm with the polar nuclei.

Ordinarily, kernels with white aleurone have no scutellum color and, when their progeny are selfed, produce either white aleurone kernels or kernels segregating for aleurone color in ratios characteristic of the action of an inhibitory factor. Many aberrant kernels which possessed white aleurone and colored scutellums have been found and tested for breeding behavior. In spite of the colorless aleurone of the parent kernels, the progeny segregated for aleurone color in ratios characteristic of those generally obtained only from hybrid kernels with colored aleurone. This points clearly to a difference in genotype between endosperm (aleurone) and embryo (scutellum).

The term hetero-fertilization has been applied by the writer to the process resulting in those exceptional cases in which the endosperm and embryo differ genetically. These may occur because (a) the egg and polar nuclei are of different genetic constitution and fuse with identical sperms or, conversely, (b) the egg and polar nuclei have the same genotype but fuse with sperms having unlike genotypes during syngamy. Either of these phenomena would give rise to hetero-fertilized kernels.

It is obvious that hetero-fertilized kernels may be produced and escape detection because of phenotypical identity with their normal sibs. Hetero-fertilized kernels of this kind undoubtedly occur unnoticed in much maize material. For the ready identification of hetero-fertilized kernels, the embryo and endosperm must be of different phenotypes. The relation of aleurone and scutellum factors provides an ideal combination for detecting hetero-fertilization. The case of identifying a particular kind of hetero-fertilized kernel, namely, those having a colorless aleurone and colored scutellum, has been taken advantage of in studying this phenomenon genetically. There is some evidence that hetero-fertilization may occur only in the presence of a certain gene or complex of genes. Some strains show no hetero-fertilization among several thousand kernels, whereas other strains show as many a 10 per cent. of hetero-fertilized kernels, and individual ears have shown much higher percentages of this anomaly.

It is conceivable that hetero-fertilization may be brought about in various ways. Dispermy, non-disjunction of one or more chromosome pairs when the generative nucleus divides to form the sperms, the persistence and functioning of the four megaspores, or mutation of one of the aleurone or scutellum factors might result in hetero-fertilized kernels. The genetic tests applied have failed so far to distinguish with certainty between the possible causes of hetero-fertilization. The occurrence of the phenomenon, however, is abundantly proved.

George F. Sprague
Bureau of Plant Industry,
U.S. Department of Agriculture