Sectorial analysis of dek1 mutant expression --William F. Sheridan The accompanying report on x-ray treatment of early proembryo stage embryos describes the timing and dosage of x-ray treatments used in an effort to induce dek mutant sectors in +/dek1 embryos. Among the kernels analyzed to date a total of 534 were produced with pollen shed by plants heterozygous for the dek1 mutation (+/dek1). Both the male and female parents were homozygous for all of the aleurone color factors. They were also homozygous for the R-scm2 allele. A normal embryo carrying all of these factors (with at least one dose of the dominant allele at each locus) is pigmented with anthocyanin in the scutellum. The embryonic axis is yellowish-white except for a region of purple pigmentation at the tip of the coleoptile.

Each of the 534 kernels was examined visually (with an 8-power magnification hand lens when needed) in a search for colorless sectors since it is known (M. G. Neuffer and W. F. Sheridan, 1980) that dek1 mutant embryos do not synthesize anthocyanin. The dek1 mutation segregates in a normal three to one fashion on self-pollinated ears. Therefore about half or about 267 of the kernels examined should be heterozygous for the dek1 mutation. In their study, Poethig, Coe, and Johri (1986) irradiated plants carrying the R-sc allele (which conditions anthocyanin synthesis only at the periphery of the scutellum, and is therefore less favorable material than that containing the R-scm2 allele) in searching for colorless sectors against a colored scutellum background. These workers used x-rays to break chromosome arm 9S bearing the C-I allele, the loss of which allowed pigment formation to occur. Poethig and coworkers found six sectored embryos out of a total of 815 kernels examined of material given 500R of hard x-rays at 56 hours after pollination. This is a frequency of 0.7 percent. If the same frequency of sectors were expected in the +/dek1 embryos then two embryos of the 267 presumed heterozygotes or two among the 534 total embryos examined should have displayed a colorless sector of dek1 mutant tissue. No embryos have been found that contain colorless sectors. These results are limited by the small sample size but are suggestive either that mutant sectors cannot survive in the competition for nutrients during early kernel development or that the mutant sectors are not cell autonomous. It seems most likely that the former suggestion or small sample size applies to these results.

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