A lethal a1 mutation

In a large test in 1981 in which a purple aleurone Mutator line was used as a pollen parent and a1 sh2 as the ear parent, several mutable and stable mutants were obtained. These new mutants were planted and self-pollinated. These selfed ears segregated for the new (Mutator-induced?) mutants and a1 sh2 seeds. One of the selfed ears segregated for dark, medium and light pale purple seeds among the Sh2 seeds. There was no sharp demarcation between these classes. Thus it was difficult to determine their actual frequencies. This mutant was given the symbol a1-SR03 (stable a1 mutant #3 produced by Robertson).

This mutant appears to be a leaky a1 allele. The different intensities of pigmentation could be due to the number of doses of a1-SR03 present (i.e., dark pale purple, a1-SR03 a1-SR03 a1-SR03; medium pale purple a1-SR03 a1-SR03 a1; light pale purple a1-SR03 a1 a1). To test this hypothesis, seeds of each class were planted and, where possible, the resulting plants were selfed and crossed to a1 a1 sh2 sh2 plants or they were reciprocally crossed to a1 a1 sh2 sh2. If the hypothesis is correct, the plants from dark pale purple seeds should be homozygous for the new allele and Sh2 while plants from the other two classes should be heterozygous (i.e., a1-SR03 Sh2/a1 sh2).

The medium and light pale purple seeds were, indeed, all heterozygous. However, most of the plants from the 30 dark purple seeds planted died under field conditions. Unfortunately I was confined to bed with a back problem during the early growth stages. By the time I was able to get to the field there were only two very weak runts surviving. Thus, I was not able to determine when and how those plants that died succumbed. Seedlings from dark pale purple seeds germinated in the sand bench give green, rather weak looking seedlings.

Thus, this possible Mutator-induced mutant at the a1 locus appears to be a lethal or semi-lethal mutant.

Donald S. Robertson

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