After treatment of dry seeds of W70 o2/o2 gl/gl line with 0.582 mM solution of N-nitroso-N-methylurea kernels having a flint semi-transparent endosperm pattern were selected in M3. From normal corn the mutant thus obtained differed in a distinct pattern of a floury layer in the lower part of the kernel.
The genetic analysis showed that the character was controlled with a dominant gene which was not linked with an opaque-2 allele. At first sight the total data of the segregation account presented in Tables 1 and 3 contradict this conclusion and almost agree well with a semi-dominant type of the character inheritance. However, in F2 and testcrosses involving W64A line there was a perfect agreement between the actual observed segregation and theoretically expected one. All reciprocal A' x AM testcross combinations segregated modified kernels. Therefore, the numerous deviations from the expected ideal ratio of kernel classes may only be interpreted proceeding from the hypothesis of dominance.
Careful analysis of the experimental data proves that the abnormalities in segregation are attributed to incomplete penetrance of a dominant allele (Table 2). The probability of gene penetrance is maximum at three doses and minimum at one dose. The good agreement between the observed segregation in F2 and the expected one calculated with the account of the degree of gene penetrance in testcrosses (c2 = 0.02, p = 0.99, Table 3) is convincing evidence of the adequacy of our interpretation.
Instability of the gene manifestation is especially evident in W70 line. In F2 of this line there were some ears with a kernel ratio near to 12:2:2, 12:3:1 and 12:1:3. Some homozygous plants segregated kernels (on an average 4.0%) possessing a floury endosperm pattern. The test of the floury kernel progeny selected from such families indicated that the allele was not actually affected with irreversible changes and did not disappear. In the background of three other genotypes the phenotypical instability of the gene penetrance was less expressed against that of W70 line (Table 2). The reasons for this are not clear.
The share of a floury part per endosperm depends on gene dose, genetic background and environments, and ranges from 5.0 to 90-95%. The partial restoration of a flint pattern in an opaque-2 endosperm results in a considerable improvement of some characters of high lysine corn (Table 4). On an average, 100-kernel weight and specific gravity of the modified kernels are 8% higher. When infecting artificially and in natural conditions the resistance to Fusarium doubles. In lysine content and nutritional value the modified kernels are similar to the standard opaque-2 counterparts.
Thus, the dominant mutation partially suppresses a morphological and biochemical effect of an opaque-2 allele. The new gene is called a suppressor and is symbolized as Sup1-W70 o2/o2.
M. I. Hadjinov and A. S. Mashnenkov
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