A c2-m::En-low and two modifiers of mutability

--Michael G. Muszynski and Peter A. Peterson

The original phenotype of the c2-m881058P allele (MNL 64:9) was medium spotted (2-6 aa-b, Reddy and Peterson MGG 194:124, 1984), but upon outcrossing three mutable phenotypes segregated: high (6-8 aa-d), medium (2-6 aa-b) and low (1-3 aa-a) in approximately equal ratios. Kernels (c2-m/c2) were selected from each class and crossed by and on c2/c2. Each class segregated as follows:
 
Phenotype Segregation on Outcross Ear
Planted High Medium Low cl
High 1/4 - 1/4 1/2
Medium 1/8 1/8 1/8 5/8
Low - - 1/2 1/2
colorless - - 1/4 3/4

The only class which bred true was the low spotted class and also segregated 50% on an outcross, thus the c2-m allele contains a regulatory element which conditions low mutability. Two factors which modify this low spotting are independently segregating in this mutant. One factor (Stimulator) stimulates the c2-m low spotting to high spotting and segregates upon outcrossing plants from high spotted kernels. Another factor (Restrainer), which restrains the high spotting to medium and the low spotting to colorless, is also segregating. Both factors are unlinked to the c2-m allele and to each other.

F1 kernels (C2/c2-m A1/a1-o Wx/wx + Stim.) were backcrossed to c2/c2 and a1-m(r)/a1-m(r) tester lines to test for En(Spm) relation. The results, on a per plant basis, are summarized as follows:
 
Cross to c2 Indicates Cross to a1-m(r) Indicates 
1. c2-m 1 En-low
2. c2-m + 1 Stim. 1 En-low and 1 En-std
3. c2-m + 2 Stim. 1 En-low and 2 En-std
4. C2 + 1 Stim. 1 En-std

These data indicate that the element at c2-m881058P is a weak or low En. The Stimulator is a standard En, which can compensate for the En-low and boost the mutability to a high spotting pattern. The Restrainer is probably a suppressor of mutability similar to the I element of a1-m(r)102 (Cuypers et al., EMBO J. 7:2953, 1988), which restrains the high mutability to a medium pattern and the low c2-m::En-low spotting to colorless. System tests to other element systems were negative.


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