SN, A LIGHT-DEPENDENT AND TISSUE-SPECIFIC GENE OF MAIZE: THE GENETIC BASIS OF ITS INSTABILITY

The genetic system under investigation is defined by three major components: a gene, Sn, conferring tissue specific anthocyanin accumulation in different plant regions, light, required for color development in competent tissues, and another gene, Pl, substituting for light in its capacity to elicit pigment production. Attention is given in this paper to an Sn allele, symbolized Sn:bol3, capable of some constitutive pigmentation in seedlings and seed integunents. Sn:bol 3 confers a higher pigment potential than the other alleles and is unstable. Its instability relates to its frequent changes from an original condition, indicated as Sn-s, to Sn-w, where -s and -w stand for strong and weak and refer to the two levels of seedling pigmentation. Weak derivatives arise spontaneously at a high frequency in homo- and heterozygous Sn:bol3 genotypes. In the latter, weak derivatives are also recovered on the chromosome originally devoid of Sn as if the heterozygous association had promoted 'contamination' of one chromosome (recipient) with Sn coming from the other (donor). If the two chromosomes in the heterozygote are marked with contrasting alleles of R, a gene lying about two crossover units proximal to Sn, it appears that the R constitution of the recipient chromosome affects their constitution. Presence of R-r in fact leads to changes of both chromosomes in terms of Sn constitution, resulting in a majority of nonparental chromosomes, R-r Sn and r Sn-w or r sn, while replacement of R-r with R-g, a mutant derivative of R-r, leads to a drastic reduction in the yield of nonparental chromosomes. This unexpected result could suggest a transactive effect of R-r upon the rate of passage of Sn from one chromosome to the other or, alternatively, it could be explained by assuming that in heterozygous Sn/sn plants Sn activates a cryptic Sn residing on the other homologue and that the presence of R-r is instrumental in inciting this activation process.