In 1977, an attempt was made to isolate Spm-controlled bz-mutable genes. The female parent was homozygous Spm c-m5 Sh Bz wx-m8 while the male parent was W22 (c sh bz Wx). The total number of F1 kernels was 61518. All variant kernels were saved for testing in 1978. No variegated kernel proved to be a heritable bz-mutable.
Among the variant kernels were five that resembled bz kernels with no hint of mutability. The plants from these kernels were tested by crossing reciprocally with the sh bz Wx (W22) tester. Four proved not to be changes at the bz locus since the backcrosses in both directions produced progeny that had ca. 0.5 bronze and 0.5 nonbronze kernels. The fifth plant on being crossed by the (sh bz Wx) tester produced ca. 0.5 colorless kernels and 0.5 bronze kernels. The counts were 190 colorless, nonshrunken; 183 bronze, shrunken; 3 colorless, shrunken; 9 purple, non-shrunken; and 6 purple, shrunken. The distribution is asymmetrical since one complementary crossover class (Region II, between Sh and Bz) was not distinguishable from other colorless, nonshrunken kernels.
The data suggest that C-I was generated in the c-m5 stock. The C-I allele could not have been brought in as a contaminant since it is on the chromosome contributed by the female parent, and the markers from the male parent sh bz are present in some of the testcross progeny.
The c-m5 stock came originally from Barbara McClintock in whose nursery it originated. She has kindly traced its origin as follows:
It is not possible to decide on genetic grounds whether the c-m5 arose from a locus that was originally C or one that was C-I. If C, then the departure of Spm is capable in some instances of generating C-I from a functional C. If C-I, the association of Spm with C-I is capable of producing a mutable c just as when a receptor associates with C.
Although I don't have precise data on the point, it appears that c-m5 produces both c and C derivatives far more often than C-I derivatives which may argue, although not strongly, for c-m5's origin from a functional C.
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