Mutation system inducing instability in the Cg2 locus
--N. V. Krivov
The analysis of individual plants from the progeny of the Cg2 unstable macromutation shows that this monogenic dominant mutation, exhibiting 100% penetrance in the genetic background of specially selected strains, mutates with high frequency in somatic and generative cells. The degree of the Cg2/Cg2 homozygote instability is as high as 55% in the Cg2+ -> Cg2 mutation, with the rate of Cg2+ -> Cg2 mutation of Cg2+/Cg2+ being 2 to 2.5% and 5.5% in generative and somatic cells respectively. Mutability at the Cg2 locus can be maintained indefinitely affecting other loci unlinked to Cg2 such as sr*-220, br*-220, ys*-253, P-rr and ys*-143. All these mutants, isolated from the Cg2 macromutation progeny and occurring at a high frequency, were tested for allelism with phenotypically similar mutations from the VIR collection in order to determine their chromosomal locations.
Tests of allelism showed that the new mutant br*-220 was not allelic to br1, br2 and br3. The new variant sr was not allelic to sr1 or sr2 and the new mutation ra was not allelic to ra1. Thus it became obvious that recessive mutations isolated from the Cg2 macromutation progeny reflect the features of mutation activity at the unstable Cg2 locus or those of the genetic background of the strain carrying Cg2. Moreover, crosses between phenotypically normal maize strains carrying Cg2+ alleles as well as ys*-253 and ys*-143 mutations non-allelic to one another, can produce up to 51% F1 plants having Cg2-m and Cg2-w phenotypes as well as Cg2 mosaic plants.
Tests for the ability to induce Cg2+ -> Cg2 mutations upon crossing strains homozygous for the Cg2+ allele and carrying recessive mutations have revealed that Cg2 macromutations occur at a frequency of 44.1-51.5% in ys*-253 x ys*-143, ys*-143 x ys*-253 P-rr and ys*-253 P-rr x br*-220 combinations. In the latter case, the appearance of Cg2 plants appears to depend on cytoplasm genotype since no Cg2 mutant plants appeared when br*-220 strain was used as a female parent.
The Cg2 locus instability persisting for many generations, induction of Cg2+ -> Cg2 mutations in following hybridization, and high frequency of recessive mutations unlinked to the Cg2 locus all suggest the existence of a Fpj (factor prolonged juvenile) mutation system, similar to the well-known Ac, Spm/En, Uq, Mrh, etc. mutation systems. Tests of five marker strains: wx sh, lg gl2 b v4 gs2 Ch sh/+ fl, gs bm2, bm bv bt and j v16 ms8 showed the latter three carried a Fpj, since 25-51.7% of plants from crosses of these marker strains had the corngrass phenotype. Testing F7, Co125, MK01, BIP-44, 092 strains, belonging to commercial hybrids, for the Fpj presence showed that among 135 plants tested a single Cg2 mutant plant was found in the 092 x ys*-253 P-rr combination. Thus the newly discovered Fpj mutation system, while widely occurring in maize marker strains, seems to be found rarely in the strains of cultivated hybrids.
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