The a-m61138-3 allele is autonomously mutable with En. Since this is a unique En, it will be identified as En-61138-3. This En shows a consistent low frequency of coincident loss of the a En Sh2 chromosome segment from chromosome 3 (1983 MNL). The remote possibility arose that this loss event, the first among maize mobile elements outside of the Ac Ds system, might be from the insertion of an Ac element within the a locus. To test this possibility the following experiments were executed: En-61138-3 was crossed to a C Ds sh-m stock (see Table 1). Half the F1 plants have both C Ds sh1-m and En-61138-3. If Ac was present, crosses of the F1 onto a c sh wx tester would express Ac.
Two different shrunken 1 genotypes are expected, one with En and the other with a-m-1 sh2. If Ac is present in En-61138-3, one-half of the colored shrunken kernels will be expected to show colorless sectors and sh1 to Sh1 sectors. However, in no case were colorless sectors found on colored shrunken kernels. The data from the selfs of these F1 plants are also shown in Table 1. In six out of the twelve selfed progeny, the ratios with respect to total number of kernels were 3/16 colorless round spotted and 1/16 colorless shrunken spotted. The other 6 of the 12 tested plants giving rise to selfed progenies should be without En. This confirms that En-61138-3 is present in our male parents. At the same time, these same plants do not show C Ds losses when used as outcrosses onto the c sh wx
tester. Thus, it is evident that Ac is not a component of the En-61138-3 complex. The multiple loss mutability of En-61138-3 that includes a1, sh2 and En is due to En.
This En-61138-3 a1 allele arose as an individual event from the original a-m(dense) (Peterson, Genetics 1961). This is the only allele of the En systems to display chromosome breakage events leading to the loss of chromosome segments.
Yih Ching Huang and Peter A. Peterson
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