Stanford University

Germinal revertants of bz2-mu1, an allele with a Mu1 element in a background with a typical, late pattern of somatic reversion

--Virginia Walbot

The bz2-mu1 allele contains a Mu1 element in the second exon (J. Nash et al., Plant Cell 2:1039, 1990). Like other Mu-induced mutable alleles, bz2-mu1 yields a fine spotting pattern in the aleurone and other tissues. Only one germinal revertant allele has been verified: purple kernels in a sector encompassing about one-eighth of the ear (A. A. Levy et al., 1989 Dev. Genetics 10: 520) were shown to contain an extra proline residue in the Bz2 sequence. Given the late sectoring pattern typical of the allele, occasional (est. 1/104) individual purple kernels were ascribed to rare contamination and not analyzed further.

To determine the reversion rate more precisely, a large test was made in 1989. bz2-mu1 homozygous and bz2-mu1/bz2 plants were pollinated by bz2. Progeny ears were obtained from 339 plants; 83 of the ears had no spotted kernels indicating that 24% (83/339) of the individuals had lost Mutator activity. One ear had a small sector of 5 purple kernels. Of the total population of 38,000 spotted kernels, there were 9 additional purple kernels. By Southern blotting, three of these were demonstrated to be contaminants, leaving 7 putative, independent revertants. All of these transmitted normally, although one revertant (I124-8) gives a dark pink rather than full purple color. Using PCR we plan to determine the nature of the putative revertant alleles, and will accept alterations at the site of Mu1 insertion as proof that these are revertants rather than contaminants. If all 7 events are verified to contain novel bases, the reversion frequency to a functional Bz2 allele is 1.8 x 10-4 for this population.

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