The defective kernel mutants we have been working with generally show embryo lethality, but under favorable genetic backgrounds some mutant embryos are viable. Forty-one defective kernel mutants, most of which initially had lethal embryos, but which showed a few good mutant embryos on a segregating ear, were chosen for this study. For each case fifty mutant kernels were removed from the ear and ten with the best looking embryos were selected. Kernels were sterilized and germinated in petri dishes for a germination test. Those that grew were transplanted into flats for seedling observation. At the 2-3 leaf stage, surviving seedlings were transplanted to the field for plant trait observation. Those that matured were selfed and the ears recorded for mutant segregation. At least one kernel germinated in each case. The results (Table 1) were as follows: one mutant, cp*-1430, was embryo lethal (class L), twenty-one mutants had clear seedling phenotypes but were seedling lethal (failed before maturity, class I), three mutants gave mature plants that were sterile (class S), eight mutants were viable and were proven by at least one homozygous mutant ear (class V), and the remaining eight mutants did not get a good test (class X). Among the twenty-one mutants, five had white seedlings, six had pale green seedlings, three had very pale green seedlings, three had small weak seedlings, three had tiny seedlings and one had virescent seedlings. In all these cases the mutant seedlings died early. The four sterile mutants were as follows: Mutant cp*-935 had normal weak plant phenotype, few florets in the tassel, with shriveled anthers; mutant de*-1153 had small weak plants with barren tassels; and mutant fl*-1390B had small plants with non-shedding anthers. Sixteen mutants gave normal mature plants which were selfed. Eight of the sixteen, crp*-888A, smk*890, cp*-936A, o*-948B, o*-1310A, smk*-1373A, o*-1388 and smk*-1484, had one or more homozygous mutant ears, but four of these mutants, crp*-888A, o*-948B, o*-1388 and smk*-1484 also had a high proportion of heterozygous ears. Mutant cp*-936A had one ear which showed an ear sector of all mutant kernels (homozygous) and the remaining part had a 1 to 1 segregation for normal and mutant kernels, indicating a possible reverse mutation in early ear development. Eight of the sixteen mutants had no homozygotes among the selfs. The reasons for getting heterozygous and homozygous normal plants from mutant kernels appear to be several: (1) misclassification in the selection of mutant kernels; (2) heterofertilization which would be enriched by selecting the best kernels; (3) gene dosage, perhaps the mutant was semidominant; (4) mutant instability giving reversion to normal in the embryo; and (5) genetic modification where a combination of other genes gave a phenotype.
Mutant crp*-888A, for example, had eight selfed ears harvested, of which five ears segregated 3 normal to 1 mutant kernel and three ears had all mutant kernels. The parent ear from which the best mutant seeds were taken showed a wide range of mutant kernel expressions, kernels from almost normal to severely collapsed. Kernel separation showed 432 normal kernels vs. 108 combined mutant kernels (64 crumpled and 44 defective or collapsed) or a 3 normal to 1 mutant ratio. The best mutant kernels selected for the experiment were the better developed crumpled kernels. The homozygous ears from surviving mutant plants had fewer crumpled kernels and more collapsed. One selfed homozygous ear had 123 crumpled, 174 defective or collapsed kernels and a scattering of nearly normal kernels. The expression of variability could be caused by a combination of gene dosage of the mutant and genetic modification involving both dominant and recessive modifiers. From these tests, it is clear that genetic modification of mutant expression is common and very effective within certain limits (ex. lethality due to albinism cannot be overcome by modifiers). It is also clear that in certain genetic backgrounds an otherwise recessive mutant may be expressed in a heterozygous m/m/ + endosperm.
Table 1: Summary of mutant kernel germination, plant number and phenotype1, ears harvested and segregation ratio of 41 defective kernel mutants.
M-T. Chang and M. G. Neuffer
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