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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