The D8 locus, which is on the long arm of chromosome 1, has an additive effect on the stature of plants. Selfing a known heterozygote produces a ratio of 1 normal (+/+): 2 dwarf (D8/+) : 1 mini-dwarf (D8/D8). The D8/ + dwarfs are generally less than 1/4 the height of normal (+/+) sibs. Likewise, mini-dwarfs are about 2/3 the size of dwarfs. Unfortunately, mini-dwarfs fail to set seed or produce viable pollen.
In order to study the nature of the D8 allele, D8/+ plants were pollinated by TB-1La hyperploid translocation heterozygotes. Table 1 shows the expected genotypes and chromosome numbers of the progeny of this cross. Half of the progeny are expected to contain the D8 allele in the presence of 0, 1, or 2 doses of the normal (+) allele.
All progeny kernels were from one ear and when planted were separated into two groups. One group consisted of very small kernels and should contain mainly hyperploid embryos (Roman and Ullstrup, Agron. J. 43:450-454, 1951). Approximately half of these should be D8/+/+ genotype and will provide the opportunity to study the influence of the D8 allele in the presence of two normal alleles. The other group consisted of the largest kernels and should contain some hypoploid plants, half of which should be D8/- genotype. The +/- hypoploids also are reduced in stature (about half the size of +/+ sibs) and are easily distinguishable from D8 dwarfs, not only due to their greater height, but also due to their narrow, stiff, pointed leaves.
All kernels were planted in early spring in peat pellets in the greenhouse in order to facilitate root-tip collection. They were subsequently transplanted to the field. Table 2 lists the phenotypes and probable genotypes of all plants for which root-tips were successfully analyzed cytologically
It is interesting to note that two expected classes are conspicuously missing. Normal (+/+) plants with 20 normal chromosomes and normal (+/+) plants with 21 chromosomes (1 1B B1) do not appear in the progeny. This may be due to the kernel size selection that was performed. An earlier planting of kernels from a different ear of the same cross (D8/+ x TB-1La hyperploid) on which the kernels were not as easy to separate into large and small classes produced several normal plants with 20 or 21 chromosomes.
Most importantly, the data show that D8 is a mutation which causes a gain in function. Nine plants were D8/+/+ and had a dwarf phenotype indistinguishable from D8/+ dwarfs. Also two plants with 20 chromosomes exhibited a very interesting phenotype. They displayed the hypoploid phenotype (stiff, narrow leaves) yet were only 6 inches tall due to very short internodes. In other words, they were essentially dwarf hypoploids (D8/-). Thus, it appears that the D8 allele has a dwarfing effect in the presence of 0, 1, or 2 normal (+) alleles.
Rick W. Staub and Patricia M. Laurenson
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