Interaction of Cg2+ revertants with the cytoplasm genome and between themselves
--N. V. Krivov

It has been shown earlier that up to 50% of plants with the Cg2 phenotype are possible in F1, if reciprocally crossed stocks isolated in the progeny of the heterochronic Cg2 mutation carry the Cg2+ alleles resulting from the reversion Cg2 ---> Cg2+. Moreover, in some cases such mutation events are observed even with hybridization of the pollen parent, carrier of the revertant Cg2+ allele, on the marker stock gs bm2 (Krivov, MNL 66:56, 1992). Not only the induction of the Cg2+ ---> Cg2 mutation, but the suppression of the mutant Cg2 depends on the genotype of the cytoplasm (Krivov, MNL 68:48, 1994).

This communication analyzes the case of the Cg2+ allele mutation from Cg2+ ---> Cg2 with the hybridization Cg2+ ys*-9196 br*-220 x Cg2+. Both partners participating in the cross were isolated in the heterochronic Cg2 mutation progeny; i.e. they are carriers of the revertant Cg2+ alleles and have the wild-type phenotype. These alleles mutate to Cg2 with a frequency of 10%-15% in the pollen parent, whereas in the maternal stock, that also contains two recessive mutations originated in the heterochronic Cg2 mutation progeny, it is stable since during the 4 years from isolating, no plants with the Cg2 phenotype have been discovered in its progeny.

This hybridization produced 17 maize plants possessing the Cg2 phenotype. In 4 plants, despite the fact that the central shoot ended in a tassel, numerous side shoots were Cg2 and only one plant was confirmed to be wildtype. Such a progeny may be expected only from parents homozygous for the Cg2 allele, i.e., both Cg2+ alleles have become Cg2 alleles.

The individual analysis of the progeny of 8 hybrid plants among 22 indicates that they are all heterozygous Cg2 /Cg2+ (Table 1). Even the plant with the wt phenotype was Cg2 /Cg2+. Only family No. 45 characterizes the parent plant as a putative homozygote Cg2/Cg2 . In order to know how frequently the mutation Cg2+ ---> Cg2 occurs at the hybridization with such an effect when nearly 100% of the offspring have the Cg2 phenotype, repeated reciprocal crossings were performed with the same source of the Cg2 alleles. This time, along with the families in which Cg2+ ---> Cg2 occurred, the families that were carriers of stable alleles Cg2 were taken as well. In all, 13 families were checked. Five of them produced plants with the Cg2 phenotype, and only in one of the 5 was the share of plants with the Cg2 phenotype significant.

The appearance of more than 50% and sometimes even 100% of plants having the Cg2 phenotype in the Cg2+/Cg2+ compounds in the F1 showed that the nature of these mutation transitions is quite different from the mutation events that are observed in marker loci due to Ac-Ds insertions and excisions or any other well-known transposition elements. Moreover, the Cg2 mutation is caused by the cytoplasm genotype. Therefore, the instability of the heterochronic Cg2 mutation is likely to be related to the transpositions of an episome-like genetic element. Spontaneous reversions to male fertility in maize stocks with cms-S are explained by the presence of such episomal genetic elements that may be fixed either in the cytoplasm or nucleus (Kemble et al., Nature 304:744-747, 1983). The Cg2 ---> Cg2+ reversions are likely to occur in the cytoplasm and not in the nucleus, as has been observed in the cytoplasmic revertants towards fertility in which rearrangement of the R-sequence takes place regardless of the nuclear genetic background (Zabala et al., MNL 66: 110, 1992). Hence, these reversions do not affect the Cg2 locus and are pseudoreversions. At hybridization, if the Cg2 alleles hit the corresponding cytoplasm genotype, the mutant heterochronic phenotype Cg2 is restored.

The event of the Cg2+ allele mutation from Cg2+ ---> Cg2 , described here when nearly 100% of the offspring possess the Cg2 phenotype, indicates that the earlier stable Cg2+ alleles, which is a contribution of the maternal partner of Cg2+ ys*-9196 br*-220 participating in this cross, have also become mutable. When the stable allele becomes mutable, the Cg2+ alleles may be supposed to be subject to paramutation. As has been known for a long time, these are the r and b genes of anthocyanin biosynthesis in maize that are subject to paramutation (Brink, Ann. Rev. Genet. 7:129-152, 1973; Coe, PNAS 45:828-832, 1959). However, paramutable R and B alleles dominate over their recessive alleles while the paramutable Cg2+ allele is recessive and it can demonstrate its paramutagenic properties only on a specific type of cytoplasm.

Thus, this report proposes a new model of the unstable heterochronic Cg2 mutation behavior. This model will be further checked during the course of tests with the stocks, carriers of Cg2+ and Cg2 allele derivatives.

Table 1. Identification of parent plants Cg2 /Cg2+ or Cg2 /Cg2 for the genotype in progeny.
 
Offspring phenotype
Fam. No. Phenotype of self-pollinated parent plant wt Cg2 Total X2 1:3
39 M (Cg2) self; stolon wt 12 21 33 2.3
40 " 12 23 35 1.6
41 " 9 21 30 0.4
42 wt 13 25 38 1.7
43 Cg2 m 13 17 30 5.4*
44 " 8 24 32 0
45 " 3 30 33 4.5*
46 " 9 16 25 1.6
Total 79 177 256 4.7*


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