--Virginia Walbot

In examining somatic instability in the early excision bz2-mu2 line described in an accompanying note, I found null sectors in addition to purple sectors. In material with more than one dose of the reporter allele, null sectors are very likely to represent loss of Mutator activity. In outcross ears with just one dose of the reporter allele, however, null sectors could represent either loss of activity or excision events that fail to restore sufficient Bz2 activity to condition a purple phenotype. Because Mu somatic excision is not very precise, usually resulting in small deletions (A. B. Britt, unpublished data), only one-third of Mu excisions from an exon are expected to restore the reading frame. Seven ears -- outcross progeny of 3 different plants -- were analyzed, and null sectors were found to be about 3 times more common than purple sectors of the same size.
 

Sector Size	Purple Sectors	Null Sectors
		Obs.	Exp.
1/2 kernel	6	23	12
1/4	16	52	32
1/8	34	95	68

The excess of null sectors compared to predictions based solely on restoration of the reading frame (Exp. column) could result from [1] loss of Mutator activity (fewer than expected spotted kernels were recovered in this population), and [2] deletions or alterations that mutate an essential gene element, i.e. loss of the initiator or termination codon, loss of a site required for splicing, or amino acid changes that destroy protein function. The precise location of the Mu1 insertion in bz2-mu2 is not known but by Southern mapping it is in the reading frame near the ATG. Alterations of the reading frame would, therefore, be expected to yield null function.

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