In the summer crop of 1984, testcrosses were made to screen for revertants of shrunken mutations that were recovered from "Aberrant Ratio" stocks. Among the offspring of plants that were heterozygous for several shrunken mutations, a plump phenotype segregated. Crosses of sh bz individuals as male parents to both sh-5586 Bz/sh-5588 Bz and sh-5586 Bz/sh-bz-x2 heterozygotes generated plump offspring (sh-bz-x2 is a deletion of the shrunken and bronze loci). The pedigrees of the plump individuals are shown in Figure 1. Several observations can be made from the segregation patterns. First, from the cross of sh-5586 Bz/sh-5586 Bz x sh-5588 Bz/sh-bz-x2, plants were produced in a 3:1 ratio, 3 which bore no plump kernels in the following generation and 1 that segregated for plump and shrunken. The number of plants tested was 83, 63 of the first class and 20 of the second. On those ears which contained plump kernels, the ratio was 1 plump:1 shrunken; moreover, the phenotypes of shrunken and bronze assorted independently, indicating that the gene responsible for the plump phenotype is not linked to bronze. A chromosomal aberration with one break between the shrunken and bronze loci could have occurred; however, such an event would generate substantial ovule abortion and none was seen at levels above background.
An explanation which can account for the data is that the minor sucrose synthetase gene (or another gene which codes for the enzyme) has been derepressed. According to the proposed model, a dominant inhibitor was heterozygous in both parents of the first cross (see Figure 2) and plants in the following generation segregated 3:1 for the presence of this suppressor. Plants with the inhibitor bore ears with all shrunken kernels while progeny on ears from those plants lacking the suppressor segregated 1:1 for plump and shrunken. Since the 3:1 segregation for plump-bearing ears occurred in the generation prior to the one in which the plump phenotype was expressed, the genotype of the maternal parent apparently plays a determining role in the phenotype of the offspring. This could be due, for example, to a repressor that is produced in the megaspore mother cell and then carried over to the embryo sac. Or, a regulatory factor could be involved in a presetting or imprinting whereby repression of genes expressed in the endosperm occurs in the previous generation.
The action of a second factor affecting expression must be invoked to explain the 1:1 ratio on ears containing plump and shrunken kernels. Either the mutant heterozygotes (second generation in Figure 1) or the sh bz parent was heterozygous for this gene and the other parent homozygous recessive. This factor could be a second regulatory component or the structural gene.
Figure 1. Pedigrees of the ears on which shrunken and bronze segregate independently.
Figure 2. Proposed inheritance pattern of minor sucrose synthetase inhibitor (Inh) and second factor.
J. P. Mottinger
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