Early events in Mutator lines: I. Germinal revertants

--Virginia Walbot

Last year (MNL 64:107) I reported on the gradual selection of a bz1-mu1 line that yields early somatic excision events in the aleurone including half-kernel somatic and full function, germinal revertants. The Mu1 allele is inserted just beyond the 3' intron boundary of the gene (A. B. Britt, unpublished data) in precisely the same location as the bz-rcy allele reported by P. S. Schnable et al. (Mol. Gen. Genetics 217:459, 1989). Two germinal revertants from ear sectors in this stock have been verified at the DNA sequence level (A. B. Britt, unpublished data). In 1990, the bz1-mu1 early excision stock was grown in an isolation plot; about 900 ears were recovered, and the majority of these contain one or more germinal revertants.

A detailed analysis is complete for a smaller population of a second early excision line. This line was recovered in 1989, and involves an exceptional bz2-mu2 plant (I216-3). The I216 family was derived from a faintly spotted stock, H210-1 (X), that appeared to be losing Mutator activity; other I216 family members showed near complete loss of Mutator activity on selfing and outcrossing, but I216-3 gave large spots on selfing and outcrossing to bz2 tester. These progeny were verified by Southern mapping to contain a Mu1 insertion near the 5' terminus of the coding region and are not bz2-Ds contaminants (C. Warren, unpublished data). From the 1990 crop, 20 progeny ears of 6 plants (one bz2-mu2 and five bz2-mu2/bz2) crossed onto bz2 tester have been analyzed for sector distribution. The Mutator plants are "turning off" and give about 25% sectored kernels (rather than 100% or 50%).
 
Phenotype Number Percent
purple 149 4.0 
large spots 360 9.7 
fine spots 354 9.5 
bronze 2,853 76.8 

Progeny of 14 selfed ears were also examined and germinal revertants were found. Although the data set is small, germinal reversion events are twice as likely in the pollen outcross test than in selfed ear progeny (1.9% in the ear vs. 4% in the pollen, on a per allele basis). This bias is more striking considering that loss of Mutator activity was more severe in pollen compared to ear progeny. Mu element excision frequency rate may differ significantly in the lineages leading to sperm and eggs. The data are consistent with the possibility that all of the purple kernels in the selfed test represent events transmitted through the pollen.

It is possible that early excision activity depends on a segregating factor. Kernels with the large spotting pattern give rise to progeny with both patterns; similarly kernels with the typical Mutator late spotting pattern yield progeny with both large and small sectors. Thus kernels with both sector types have the factors required to yield sectors of all sizes; the early pattern could depend on the copy number of a factor present in all stocks.


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