Revertants of the putative Mutator-induced dominant amylose-extender allele, Ae-5180 --Philip S. Stinard The availability of germinal revertants of transposable element induced mutations aids in the cloning and characterization of the mutant and wild type alleles. This report describes our preliminary efforts to obtain revertant alleles of the putative Mutator-induced dominant amylose-extender allele, Ae-5180. Our approach was to cross Ae-5180 to germinally active Mutator lines, sib the F1's to achieve homozygosity for Ae-5180 while maintaining Mutator activity, backcross the homozygous Ae-5180 Mutator plants by homozygous ae testers, and select the starchy (nonmutant) Ae-5180 to Ae-Rev revertants.

During the summer of 1987, germinally inactive stocks of Ae-5180 were crossed to germinally active Mu2 lines. The F1's were sib-pollinated in 1988, and mutant kernels were planted in winter 1988-89. Of the plants growing from these kernels, two thirds would be expected to be heterozygous for Ae-5180 (Ae-5180 Ae), and one third homozygous (Ae-5180 Ae-5180). These plants were pollinated by pollen from homozygous standard ae (ae ae) plants. The results of these crosses are presented in Table 1. Of 17 crosses made, only 6 had ratios of nonmutant to mutant kernels that did not differ significantly from the 1:1 ratio expected of heterozygous plants. Only one ear was completely mutant. Seven ears deviated from a 1:1 ratio, having a surplus of nonmutant kernels. The surplus of nonmutant kernels on these ears could be due to a transmission anomaly (although this has rarely been seen in female outcrosses of Ae-5180 in a non-Mu background) or a high reversion rate of Ae-5180 in heterozygous (or even homozygous) Ae-5180 plants. Of the remaining ears, one had only nonmutant kernels (possibly the result of a heterofertilization event in the previous generation or an early reversion event encompassing the entire ear in a heterozygous Ae-5180 plant), one was nonmutant with an ear sector of mutant kernels, and one was mutant with a few nonmutant kernels. Of the 17 ears, the latter two provide the greatest evidence for reversion of Ae-5180. Ear 88-89-8591-8, mapped in Figure 1 (left), is predominantly nonmutant with an ear sector of 25 mutant kernels. This most likely represents an early reversion encompassing most of the ear of a heterozygous Ae-5180 plant. Ear 88-89-8592-4 (Figure 1, right), is mostly mutant, with four putative revertant kernels. Two of the revertants are in a sector, and the other two occur separately on the ear.

Table 1. Counts of nonmutant and mutant kernels on ears of [(Mu2/Ae-5180) X (Mu2/Ae-5180)] X ae ae.

Figure 1. Ear maps of sectored ears 88-89-8591-8 (left) and 88-89-8592-4 (right). Revertant kernels from 88-89-8592-4 that were planted under numbers 89-3035.0-1 and 89-3035.1-1 are circled and boxed, respectively. Symbols: + nonmutant kernel, m mutant kernel, x aborted kernel or ovule.

Two of the putative revertant kernels from ear 88-89-8592-4 (of the presumable genotype Ae-rev ae) were planted in summer of 1989 under the field numbers 89-3035.0-1 (revertant from two-seeded sector, see Figure 1) and 89-3035.1-1 (single revertant). The resultant plants were backcrossed by ae testers, and the ears scored for nonmutant (Ae-rev ae) and mutant (ae ae) kernels (Table 2). The ears from both plants had 1:1 ratios of nonmutant to mutant kernels indicating that the two reversions were heritable.

Table 2. Counts of nonmutant and mutant kernels on ears of Ae-rev ae X ae ae.

We are currently in the process of developing stocks of homozygous Ae-5180 in Mutator and non-Mutator backgrounds for isolation plots to check the reversion rate of Ae-5180 in these two backgrounds. We are also crossing Ae-5180 into Spm and Ac-Ds stocks in order to determine whether reversion of Ae-5180 is triggered by either of these two controlling elements.


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