MOLECULAR-ORGANIZATION AND GERMINAL INSTABILITY OF R-STIPPLED MAIZE

The spotted seed allele R-stippled (R-st) is comprised of the followin g genetic components: strong seed color (Sc), inhibitor-of-R (I-R) and near-colorless seed (Nc). I-R is a mobile element that represses (Sc) expression irregularly. Germinal I-R losses produce progeny with full y colored seed. Southern blot analysis revealed four r-hybridizing seg ments in R-st and three, two or one in two sets of unequal crossover d eletion products. By comparison to published reports of r gene structu re, we maintain that each segment contains at least one r gene. The pr oximal r gene, Sc, confers strong seed color; the three distal r genes together produce near-colorless seed. R-st's seed spotting phenotype is correlated with the presence of a 3.3-kb insert in Sc identified as I-R The level of the near-colorless phenotype is inversely correlated with the number of r genes present, suggesting involvement of a multi ple copy silencing mechanism in their regulation. Phenotypic changes i n R-st occurred primarily by unequal exchange between r genes. The loc ations of exchange positions showed a strong polarity, nearly all occu rring in the 3' portions of the identified r genes.