Expressivity of the heterochronic mutation Cg2 and its correlation with gene dose
--N. V. Krivov

In the first report on Cg1 this mutation was referred to as a second "teopod" (Singleton, MNL 21:6, 1947). Subsequently. corngrass was shown to be highly susceptible to genetic modification. The early selections for weak expression of the corngrass gene were similar to the teopod mutant while later selections approached normal corn (Galinat, Amer. Nat. 88:101, 1954).

According to the expression degree, the heterochronic mutation Cg2, like Cg1 segregates for two relatively discrete phenes: "teopod", a moderate expression of Cg2, Cg2(m) and "corngrass", a strong expression of Cg2. This made it possible to study the kind of Cg2 expression in homo- and heterozygotes, or in other words, how expressivity depends on gene dose (Table 1).

Table 1. Correlation between heterochronic mutation Cg2 expressivity and gene dose.
 
Cg2/Cg2
Cg2/+
Lines no.
Phenotypes of plants tested
No. plants
%
No. plants
%
143 Cg2(m) 9 75 48 96
Cg2(s) 3 25 2 4
220 Cg2(m) 12 60 67 95.7
Cg2(s) 8 40 3 4.3

The table shows that among mutant plants which, according to analyses, turned out to be heterozygous for the progeny, 96% had a teopod phene while the remaining 4% belonged to the corngrass phenotype. Among homozygous plants (Cg2/Cg2) tested for the progeny a share of plants with a corngrass phene rose from 25% in the line No. 143 to 40% in No. 220. Thus, the possibility of finding a homozygous plant in the line No. 143 is 6 times, and in No. 220 10 times higher among specimens showing a strong mutant phenotype, Cg2(s). Hence, two doses of the Cg2 gene increase mutant phene expressivity while alleles of the wild type reduce Cg expressivity. Cg2 is likely to encode a product which is antagonistic to normal gene activity. 


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