Results of tests of an alternative explanation for 1:1 ratios involving Mutator-induced mutable a1 mutants

--Donald S. Robertson

We first reported the occurrence of 1:1 ratios (i.e., mutable : stable mutant phenotypes) in some outcross progeny of a1-Mum2 and a1-Mum3 in MNL 61:11-13. Last year (MNL 62:20-21) we suggested that such ratios could be expected if these stocks and the a1 sh2 stocks used in outcrosses had a second aleurone gene (e.g., r) that was segregating. Last winter ('87-'88), we tested plants from yellow stable kernels from 1:1 ears from each of ten families included in our original report (MNL 61:10-13) for their genotype with respect to the aleurone genes a2, c, c2, and r (Table 1).

Some, but by no means all, 1:1 ears were segregating for r in addition to a1. In several crosses, the segregation for r could have been responsible for the 1:1 ratio. Because r is involved, heterozygous kernels of the genotype a1-Mum a1 a1 R r r would be given lower mutability scores due to R mottling, which would result in less pigmented tissue in which reversion sectors could be observed. Thus, the mutability counts in all crosses segregating for r should be suspect and have been dropped from further study. Analyses, however, have continued for those lines in which the aleurone genes are under control, and in newly occurring 1:1 stocks.

It is interesting that none of the other aleurone genes were found to be segregating. Why is r the only one causing trouble? The answer involves a longstanding problem we have faced with our summer nursery. We have our plots adjacent to a wildlife preserve, which serves as a safe haven for raccoons that constantly visit our field during pollinating season. To keep them out of our experimental material, we have provided a very generous border of Illini XTRA Sweet corn around our field. Tests have shown that this line is a1 a1 r r. It is obvious that at some time in the past there has been some contamination of some of our a1 sh2 tester stocks by Illini XTRA Sweet. We have crossed our a1 sh2 stocks to purple aleurone and reextracted new tester lines with all genes under control. These tester stocks will be continually monitored to avoid contamination in the future.

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