The interaction between genes suppressing heterochronic mutant Cg2 manifestation and the cytoplasm genome
--N. V. Krivov

The presence of genes modifying Cg1 penetrance was known in self-pollinated corn lines long ago (Galinat, MNL 26:52, 1952). Heterochronic mutations Tp1, Tp2 and Tp3 are also sensitive to genes that modify their penetrance and expressivity (Poethig, Genetics 119:959, 1988). The heterochronic mutation Cg2 reacts to a foreign genetic background like the four dominant mutations (Krivov and Lysikov, Buletinul AS RM, No. 2:20, 1988).

During corn domestication, which took place 4 thousand years ago (Mangelsdorf, Sci. Amer. 255:72, 1986), a lot of dominant and recessive genes maintaining the apical dominance of the main stem have accumulated in the process of corn breeding (Iltis, Science 222:886, 1983). Genes suppressing Cg2 manifestation are supposed to occur more frequently in lines with a high combining ability than in lines with a low one. In order to test this supposition crossings between Cg2 gs bm2 and self-pollinated lines D5, Pr01a, F2, F7, O92, Co125, 346, VIR-44, N384, MK01, Rad. dwarf USA, kindly presented by S. T. Chalyk and A. J. Dukhovniy, have been conducted. All these lines, with the exception of D5 and Pr01a, possess a high combining ability and contain genes that reduce the expression and inhibit the manifestation of the heterochronic Cg2 mutation. In all cases the impact of foreign genetic backgrounds is generally displayed through the reduction of expression of the whole complex of mutant traits characteristic of the heterochronic Cg2 mutation, and through the F1 uniformity disturbance as many plants must be referred to the wild type corn.

Reciprocal crossings have shown that the manifestation of the heterochronic mutation Cg2 depends on the line which is taken as mother, i.e. on the cytoplasm genotype (Table 1). Only the cytoplasm of the 346 line turned out to be quite indifferent and, perhaps genetically similar to the Cg2 gs bm2 line. Two cases of Cg suppression should be mentioned particularly: (1) when the suppressor gene, which is contained in MK01, suppresses Cg2 manifestation completely at the interaction with the Cg2 gs bm2 cytoplasm genome while the suppression of the Cg2 manifestation is incomplete at the interaction with its own cytoplasm genome, and (2) when Cg2 manifestation is suppressed completely by the suppressor gene at the interaction with the cytoplasm genome of the Co125 and VIR-44 lines.

Table 1. Cg2 suppression in F1 when Cg2 gs bm2/Cg2 gs bm2 is hybridized with the lines contained in recognized hybrids.
 
Phenotypic classes
Combinations
wt
Cg2w
Cg2m
Cg2s
m(Cg2)
Cg2 %
Total
Cg2 gs bm2 gl x D5 2 29 3 94.1 34
Cg2 gs bm2 x Pr01a 10 30 75.0 40
Cg2 gs bm2 x F2 11 27 71.0 38
O92 x Cg2 gs bm2 6 13 2 71.4 21
O92 x Cg2 gs bm2 15 17 1 54.5 33
Cg2 gs bm2 x F7 16 18 1 54.3 35
F7 x Cg2 gs bm2 gl 18 12 40.0 30
Cg2 gs bm2 x Co125 14 20 1 60.0 35
Co125 x Cg2 gs bm2 14 0.0 14
Cg2 gs bm2 x 346 19 1 100.0 20
346 x Cg2 gs bm2 39 100.0 39
Cg2 gs bm2 x VIR-44 14 20 2 61.1 36
VIR-44 x Cg2 gs bm2 37 0.0 37
Cg2 gs bm2 x N384 37 1 2.6 38
N384 x Cg2 gs bm2 31 1 4 13.9 36
Cg2 gs bm2 x MK01 37 0.0 37
MK01 x Cg2 gs bm2 7 7 3 58.8 17
Cg2 gs bm2 x Rad. dwarf 22 16 1 43.6 39
Rad. dwarf x Cg2 gs bm2 18 14 6 52.6 38
wt - wild type; Cg2w - weak expression of the mutant phene; Cg2m - moderate expression of the mutant phene; Cg2s - a strong expression of the mutant phene; m(Cg2) - a mosaic expression of the Cg2 phene.

A significant excess of the wild type, and Cg2 deficit are observed in the progeny of self-pollinated heterozygotes (Table 2). Accordingly, among 174 plants in the MK01 x Cg2 gs bm2 combination only one had a weak expression of the Cg2 phenotype in F2, whereas in the VIR-44 x Cg2 gs bm2 and Co125 x Cg2 gs bm2 combinations not a single plant with the Cg2 phenotype was found among progenies. The Cg2 gs bm2 x D5 combination is an exception as the deviation from the theoretically expected ratio 3:1 is not significant (X2=2.7). As the Cg2 phene suppression in VIR-44 x Cg2 gs bm2 and Co125 x Cg2 gs bm2 hybrids is supposed to be related to the interaction between the suppressor gene and cytoplasm genome of the Co125 and VIR-44 lines, the Cg2 gene should be transferred to another cytoplasm through hybridization in order to receive segregants with the Cg2 phenotype.

Table 2. Segregation for Cg2 and wt in F2 when Cg2 gs bm2/Cg2 gs bm2 is hybridized with the lines contained in recognized hybrids.
 
Phenotypic classes
Combinations
Phenotype in F1
wt
Cg2w
Cg2m
Cg2s
Cg2 %
Total
Cg2 gs bm2 x VIR-44 Cg2w 57 14 16 34.5 87
VIR-44 x Cg2 gs bm2 wt 168 168
Cg2 gs bm2 x Co125 Cg2w 99 14 3 14.7 116
Co125 x Cg2 gs bm2 wt 230 230
Cg2 gs bm2 x D5 Cg2m 16 20 9 64.4 45
Cg2 gs bm2 x Pr01a Cg2m 34 23 10 49.3 67
Cg2 gs bm2 x F2 Cg2w 84 4 71 4 48.5 163
Cg2 gs bm2 x 346 Cg2w 23 15 18 4 61.7 60
346 x Cg2 gs bm2 Cg2w 37 10 15 1 41.3 63
Cg2 gs bm2 x N384 wt 120 14 40 5 33.0 179
N384 x Cg2 gs bm2 wt 127 6 27 5 23.0 165
MK01 x Cg2 gs bm2 wt 173 1 0.6 174
Cg2 gs bm2 x MK01 wt 106 1 13 2 13.1 122

Table 3. Crossing test data of Cg2/+ heterozygote transferences to the cytoplasm of Rad. dwarf USA, F2 and F7 lines.
 
Plant tested Crossing Progeny phenotype wt
N1 Rad. dwarf USA x (VIR-44 x Cg2 gs bm2) 21
N2 Rad. dwarf USA x (VIR-44 x Cg2 gs bm2) 39
N4a Rad. dwarf USA x (VIR-44 x Cg2 gs bm2) 32
N4b Rad. dwarf USA x (VIR-44 x Cg2 gs bm2) 34
N7 Rad. dwarf USA x (VIR-44 x Cg2 gs bm2) 35
N1 Rad. dwarf USA x (Co125 x Cg2 gs bm2) 38
N2 Rad. dwarf USA x (Co125 x Cg2 gs bm2) 21
N2 F2 x (Co125 x Cg2 gs bm2) 40
N2 F7 x (Co125 x Cg2 gs bm2) 25
N4 F7 x (Co125 x Cg2 gs bm2) 26
N4 F7 x (Co125 x Cg2 gs bm2) 35
N4 F2 x (Co125 x Cg2 gs bm2) 39
N5 Rad. dwarf USA x (Co125 x Cg2 gs bm2) 32
N6 Rad. dwarf USA x (Co125 x Cg2 gs bm2) 39
N9 Rad. dwarf USA x (Co125 x Cg2 gs bm2) 27
N9 F2 x (Co125 x Cg2 gs bm2) 38
N9 F7 x (Co125 x Cg2 gs bm2) 32

With this aim crossings between the VIR-44 x Cg2 gs bm2 and Co125 x Cg2 gs bm2 hybrids and Rad. dwarf USA, F2 and F7 used as mother were carried out (Table 3). However, plants with the expected Cg2 phenotype have not been obtained. Thus, self-pollinated MK01, VIR-44 and Co125 lines having a very high combining ability contain dominant genes unique for their potentials which maintain corn (Zea mays L.) habitus, while in lines with a low combining ability D5 and Pr01a are very weak recessive genes. 

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