KISHINEV, MOLDOVA
Institute of Genetics of Academy of Sciences RM

Cg2 macromutation linkage data
--N. V. Krivov and V. N. Lysikov

Four semidominant heterochronic mutations, the localization of which is well known, Cg1 (Singleton, MNL23:7, 1949; Galinat, W, MNL26:51, 1952), Tp1 (Lindstrom, Hered. 16:135, 1925; Weatherwax, Hered. 20:325, 1929), Tp2 (Peterson, PA, MNL33:41, 1959; Horovitz, MNL37:148, 1963), and Tp3 (Beckett, JB, J. Hered. 69:27, 1978; Poethig, RS, Genetics 119:959, 1988) have originated spontaneously. They are responsible for the ancestral phenotype of the shoot vegetative morphology and the transformation of reproductive structures into leaves. The analyses of the phenotypes of Tp1, Tp2 and Tp3 (Poethig, 1988) demonstrate that the traits displayed during the juvenile phase are a part of the genetically controlled developmental programme and support the hypothesis (Galinat, MNL40:102, 1966) that this phenotype presents a defect in the transition from the juvenile to mature phase during the shoot development.

Like already known heterochronic mutations, the Cg2 macromutation was supposed to be located in the 3rd, 7th and 10th chromosome. With this aim crossings with Mangelsdorf, PC's marker, as well as with d1 at 3S-44, ra2 at 3S-49, lg2 at 3L-101, o2 at 7S-16, ra1 at 7S-24, gl1 at 7L-32, v5 at 7L-36 and g at 10L-50, were carried out. Only ra2 and lg2 showed linkage with Cg2 (Table 1).

However, due to the significant excess of phenotypically normal plants, + +, + ra2 and + lg2, which was, apparently, caused by the presence of a recessive gene that suppresses Cg2 in the marker line ra2 lg2, we doubted whether the linkage observed refers only to Cg2-ra2 and Cg2-lg2. It is not less probable that the linkage observed can refer both to the pairs marker-gene suppressor and Cg2-suppressor. Moreover, the excess of ra2 carriers, which is probably connected with the ra2 display increases on the background of this macromutation, confusing the picture even more. Nevertheless, a chance occurrence has helped to answer this question unequivocally. A marker line carrying ra2 and containing no suppressor gene was found. Now it became obvious that the linkage marked earlier, undoubtedly, referred to Cg2-ra2 and Cg2-lg2 (Table 2).

Table 1. Cg2 + +/+ ra2 lg2 self-pollination.
 
Parental
Crossover
Total
% recombination
Cg2 +
+ ra2
Cg2 m2
+ +
338 261 73 470 1142 39.2±2.7
Cg2 + + lg2 Cg2 lg2 + +
335 203 76 528 46.5±2.9

Table 2. Cg2 +/+ ra2 self-pollination.
 
Parental
Crossover
Total
% recombination
Cg2 +
+ ra2
Cg2 ra2
+ +
   
239
65
61
55
420
29.9±2.8



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