Crosses between a-mum2 (= A + Mu1) and bz-rcy have shown that a-mum2 stocks contain Cy, as do most Mutator-related stocks (Schnable and Peterson, MGN 59:5). In order to determine whether the Mu1 element triggers mutability at bz-rcy, spotted and non-spotted selections were made from ears from cross 1 that exhibited 1:1:2 ratios of spotted: non-spotted-bronze: colored, suggestive of the segregation of a single Cy. These selections were testcrossed by a.
Cross 1: a-mum2/A, Sh Bz/Sh bz-rcy x A/A, Sh bz-rcy/Sh bz-rcy
If the 1:1:2 ratios from cross 1 were due to the segregation of a single Cy, and this Cy is at or closely linked to a-mum2 (which would be true if the Mu1 at a-mum2 has Cy activity), the genotype of bronze spotted selections should be a-mum2/A, and the genotype of the non-spotted selections would be expected to be A/A. This was not observed. Selection for Cy activity did not select for a-mum2 versus A. However, while all bronze spotted selections with the genotype a-mum2/A exhibited a-mum2 mutability when testcrossed by a, none of the bronze nonspotted selections did so, i.e., this latter group of ears exhibited a 1:1 ratio of colorless non-spotted:colored. Mutability at bz-rcy cosegregated with mutability at a-mum2. Hence, again assuming the 1:1:2 ratios from cross 1 were due to the segregation of a single Cy element, the 1.4 kb Mu1 element at a-mum2 is not capable of triggering mutability at bz-rcy, and a-mum2 is itself a Cy responsive allele.
Another interpretation is possible. The 11:2 ratios from cross 1 may have been due to a general deactivation of all Cy (Mu1) elements in the genomes of half the kernels. Such an interpretation would not require that a-mum2 be a responsive allele, but would still demand that mutability at a-mum2 and bz-rcy be controlled by the same family of elements Mu1). Under this interpretation a-mum2 would be an autonomously controlled allele, while bz-rcy would be independently controlled by Mu1.
Patrick S. Schnable and Peter A. Peterson
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