opaque12 (o12) is located on 4S, and conditions aleurone mosaicism
--Philip S. Stinard and Patrick Schnable

In the course of our studies of putative Mutator-induced mutants with sugary/etched endosperm phenotypes, we collected mutants from other sources with similar phenotypes for the purpose of allele tests. One of these mutants, o12, first described by Oliver Nelson (MNL55:68), was obtained from the Maize Stock Center. Kernels homozygous for o12 were originally described as having opaque, etched/scarred endosperms of variable size, giving rise to chlorophyll-deficient seedlings. When we propagated the colorless aleurone o12 stock, we noted its similarity in phenotype to the mutants dek5, dek7, and cp2, which in addition to the above phenotypes, produce aleurone mosaicism in a colored aleurone background. In order to study the relationships between these mutants, we outcrossed the colorless o12 line to an M14/W22 colored aleurone line and selfed the F1. The F2 segregated for aleurone color, and the colored o12 mutant kernels did indeed show aleurone mosaicism as evidenced by colorless, depressed areas of endosperm cells interspersed with areas of normal appearing colored aleurone cells.

Since o12 had not yet been placed to chromosome, we crossed plants heterozygous for o12 by a series of B-A translocations covering 19 of the 20 chromosome arms. Only crosses by TB-4Sa uncovered the mutant endosperm phenotype. Seedling tests of nonmutant kernels from the positive TB test ears showed segregation for pale green seedlings (putative hypoploids). Mutant kernels from the same ears gave rise to green seedlings (putative hyperploids) as expected. We thus conclude that o12 is located on the short arm of chromosome 4 distal to the TB-4Sa breakpoint. Allele tests of o12 with dek7, also located on 4S, are in progress. 

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