ORGANIZATION OF PARAMUTAGENICITY IN R-STIPPLED MAIZE

In heterozygotes, R-stippled (R-st) reduces the pigmenting potential o f sensitive r alleles heritably (paramutation). R-st is comprised of f our r genes arranged in direct orientation. Unequal crossing over with in R-st generates deletion products retaining from one to three r gene s. Paramutagenic strength decreased in parallel with copy number, both among internal and distal deletions. Single-gene R-st derivatives wer e nonparamutagenic. This was so whether or not the single gene retaine d the transposable element (I-R) responsible for seed spotting. Adding back r genes by intragenic recombination increased paramutagenicity i n proportion to total gene number. Each member of a set of overlapping deletions retained moderately strong activity, showing that no single r gene or intragenic region is required for paramutagenicity. Proxima l and distal loss R-st derivatives, each retaining two r genes, were l ess paramutagenic in trans than the corresponding four copy cis combin ation, indicating R-st's paramutagenic determinants function as a cis- interdependent unit in bringing about modification of a sensitive alle le.