New induced heterochronic Cg2 mutation
--N. V. Krivov and V. N. Lysikov
In 1962 the Cg2 macromutation was obtained in M3 after irradiation of VIR-44 strain pollen at the rate of 1500R. Like the well-known Cg1, Tp1, Tp2 and Tp3 mutations, the Cg2 macromutation has a strong pleiotropic effect and causes (1) a higher number of vegetative buds, (2) a higher number of buds producing ears, stollons, brace roots, (3) smaller leaf and internode sizes, (4) smaller ears and tassel size, and (5) transformation of reproductive structures into vegetative ones. Such phenotypic resemblance of the Cg2 with already known macromutations leads to the conclusion that the Cg2 is a heterochronic mutation with prolonged expression of the juvenile stage in the vegetative development programme, which is superimposed with a reproductive development programme.
As is obvious now, the Cg2 variable expression described earlier generally falls into three discrete phenotypes: strong Cg2-s similar to the Cg1 macromutation in its expression, moderate Cg2-m similar to Tp1, and weak Cg2-w, Cg2 allele carriers differing from normal maize by the lack of lateral branches in a tassel. Single maize plants show Cg2 mosaic expression when the ratio of corngrass to phenotypically normal shoots in a plant may vary strongly. Generally, a progeny is mutant following homozygote Cg2/Cg2 self-pollination, and heterozygote self-pollination results in the segregation ratio close to 3:1.
However, an analysis of individuals in a progeny of Cg2/Cg2 and Cg2+/Cg2+ carried out for 9-11 generations showed that individual maize plants with normal phenotype occurred in the former, and Cg2 carriers appeared in the latter. While the appearance of phenotypically normal plants in the first case can be explained as an error in parent plant genotype determination, such an explanation is not feasible in the second case.
In our opinion, the appearance of phenotypically normal plants following self-pollination of Cg2/Cg2 and of individual corngrass-type plants or a small number of these upon self-pollination of Cg2+/Cg2+ can be due to either 1) random segregation of suppressor recessive genes, which suppress expression of the Cg2 dominant mutation in a homozygote or 2) Cg2 gene instability. Therefore, generative cells of a plant will be mosaic due to frequent Cg2 -> Cg2+ mutations, and both homozygous and heterozygous initial cells will occur. Such mosaicism will result in the appearance of Cg2+/Cg2+ plants, whose number will vary (depending on the degree of mosaicism).
In fact, the appearance of mutants among the Cg2+/Cg2+ progeny and phenotypically normal segregants in the Cg2/Cg2 resulted from Cg2 gene instability. Macromutation instability is also indicated by: 1) genotype differences between ear progenies from the same plant, 2) high frequency of mosaic plants, and results of the genotype analysis of normal and mutant shoots in mosaic plants, and 3) high mutation activity, evidenced by the rate and spectrum of mutations occurring in the progeny of the unstable Cg2 macromutation.
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