A test to separate housekeeping mutants from those unique to kernel development --Donald S. Robertson and Mike Scanlon We have been trying to map and clone Mutator-induced defective kernel mutants (dek's). These mutants undoubtedly are of two classes: 1) Those that result from the mutation of housekeeping genes essential in all stages of development, and 2) those that affect some step unique to kernel development but not essential to complete other stages of the life cycle.

One phase of our mapping work involved crossing the dek stocks by the series of B-A translocations. One of the dek's placed in these tests was dek*-1339. The defective endosperm of this mutant is always underdeveloped. Sometimes the kernel can be almost an empty pericarp without a germ, while some kernels can be about one-third the size of normal. The endosperm of the latter mutant kernels is opaque, and etched. Occasionally the larger dek kernel will have a small germ. Some of the homozygous mutant kernels may be germless because the defective endosperm does not develop sufficiently to support embryo development.

This mutant was placed to the long arm of chromosome 10 with TB-10L19. The cross producing the positive test was +/dek x +/TB. Some of the plump kernels from this cross should have dek/- (hypoploid) germs, if the dek mutation does not affect a housekeeping locus and if its gene product is not essential for embryo development. Seventeen plants from plump kernels were scored for pollen sterility. Seven had semisterile pollen, expected of hypoploid plants. All semisterile ears were self-pollinated and scored for the presence of the dek phenotype. Half of the semisterile plants are expected to carry only the wild type allele and indeed three did. If a hypoploid plant is dek/-, only dek kernels would be expected. The other four semisterile plants gave ears completely devoid of any kernels. If a single dose of the dek allele will not permit kernel development, such ears would be expected. An alternative explanation for such empty ears would be that the dek mutants depend on the presence of normal kernels to augment the partial development seen in the dek's on segregating ears. This could be by some manner of cross feeding by the adjacent normal kernels or could be due to cob changes necessary for kernel development that the dek kernels can not mobilize. The latter seems a likely explanation since the cobs were quite small and non-lignified.

The hypoploid test for kernel specific dek's can only be utilized for those B-A translocations that produce hypoploid plants with sufficient vigor to permit self-pollination. Thus, probably most of the shorter translocations involving chromosomes 4 and shorter chromosomes will work, while most of the balance of the B-A translocations would not be usable in such tests.

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