From an original a-m (dense) (dense-very heavy a-m to A1, Peterson, 1961), an exception arose showing a fine pattern that, in addition, displayed a loss of mutability. Both the parental dense and the exceptional fine derivative are autonomously heritable, i.e., En is located at the a locus. In an attempt to determine the extent of the mutability loss, plants containing this allele were crossed as males onto the receptive a-m-1 allele that is linked to sh.
The cross: a-m-1 sh2/a-m-1 Sh2 female x a-m En Sh2/a-m-1 Sh2 male.
If the loss of mutability is beyond the range of the a locus and included the Sh2 gene, colored shrunken sectors would appear (with En, the heterozygote a-m1 sh2/a-m En Sh2 is colorless-round with fine spots). From this cross, kernels with colored shrunken sectors were found among the progeny. The results of several crosses are presented in Table 1.
The frequency and size of colored shrunken sectors on fine-round kernels varied among the progeny. In order to evaluate the heritability of this frequency of colored shrunken sectors, the size of the colored shrunken sectors was measured with a scaled grid in the eyepiece at 10x magnification. Sector size was classified as small (less than or equal to .5 sq mm) or big (more than .5 sq mm).
(1) All the round kernels were spotted, indicating that the En is located at the a-m locus (all of the shrunken kernels were colored).
(2) All the colored shrunken sectors were found on the spotted-round kernels, indicating that the loss phenomenon is also autonomously controlled at the a locus.
(3) The size and frequency of sectors varied on different kernels.
(4) The percentage of the mutable kernels on individual ears varied among ears.
It is evident from these observations that the colored shrunken sector is due to loss of the a-m En Sh2 segment from chromosome 3. Tests of this hypothesis are in progress.
With reference to the heritability of kernel selection (presence or absence of colored shrunken sectors), a-m-1 sh2/a-m-1 sh2 tester plants were crossed by plants from the selected kernels. The results indicated that there was a significant positive correlation (r = .9123*) between the percentage of sectors of parental kernels and that of their corresponding progeny. This shows that selection can be made for the frequency of loss and is thus heritable. Varied size and frequency of the sector on the kernels (see last column of Table 1) probably depends on the state of this En.
Yih Ching Huang and Peter A. Peterson
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