We plan to study the expression of lethal mutant tissue in chimeras produced by loss of the normal allele in a heterozygote (+/m). Losses may be produced in a number of ways, including x-radiation, marked ring chromosomes, and chromosome-breaking Ds elements. We have selected the latter for extensive testing. To do this we have produced a heterozygous Ac, homozygous chromosome-breaking Ds stock (actually P-vv/P-wr, Ds-2 R-sc from materials generously supplied by Dr. Jerry Kermicle). We used this stock as a pollen parent in an isolated detasseled plot where the ear stocks were marked for each of the 16 chromosome arms which carry a usable marker. Marked stocks with chromosome-breaking Ds on 9S (McClintock) and 10L (Kermicle) are already available. The ears produced were examined for two types of events: (1) Transposition of Ds between the marker and the centromere to produce mosaics of recessive mutant tissue on a non-mutant background caused by chromosome breakage and, (2) transposition of Ds to the marker site to give a recessive mutant case with dominant revertant sectors (or without revertant sectors in the absence of Ac).

For ear stocks with endosperm markers the examination was for mosaic kernels and for recessive mutable or non-mutable kernels. For stocks with seedling markers all the kernels were planted in sand benches and examined for seedlings with multiple recessive sectors and for whole seedling mutant cases with and without sectors (Ac and no Ac).

The ear stocks used are listed below according to chromosome arms:

To demonstrate the success of this approach preliminary results for two arms are listed below. For the kernel mutant dek1 (clf on chromosome IS) the results are described in the next MNL item. For oy, a seedling mutant on chromosome 10S, the results were as follows: a total of 92,000 kernels from the cross +/oy x pollen stock was planted in sand benches and produced 25 oy seedlings, of which 23 were non-sectored and 2 were clearly oy-m, and 9 green seedlings with frequent tiny oy sectors. These potentially good (oy, oy-m, and Oy Ds cases, respectively, will need confirmation. The frequencies cannot be used because the original events that produced them took place in the male sometime before gamete formation. They may represent many single events or many copies of a single event.

M.G. Neuffer
 
 

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