Organization of paramutagenicity in R-stippled maize.

In heterozygotes, R-stippled (R-st) reduces the pigmenting potential of sensitive r alleles heritably (paramutation). R-st is comprised of four r genes arranged in direct orientation. Unequal crossing over within R-st generates deletion products retaining from one to three r genes. Paramutagenic strength decreased in parallel with copy number, both among internal and distal deletions. Single-gene R-st derivatives were nonparamutagenic. This was so whether or not the single gene retained the transposable element (I-R) responsible for seed spotting. Adding back r genes by intragenic recombination increased paramutagenicity in 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. Proximal and distal loss R-st derivatives, each retaining two r genes, were less paramutagenic in trans than the corresponding four copy cis combination, indicating R-st's paramutagenic determinants function as a cis-interdependent unit in bringing about modification of a sensitive allele.