Structure of the R' Tandem Duplication in Maize1

is the symbol used to describe an allele at the R locus that controls-together with certain complementary factors-anthocyanin pigmentation in both plant and seed. R factors that lack the capacity to elicit pigment formation either in plant or seed tissues, or in both, are designated as follows: Rg = colored seed, green plant; I ' = colorless seed, colored plant; 19 = colorless seed. green plant. STADLER (1946, 1948) characterized the mutational behavior of R' alleles of different geographic origin and carried out extensive experiments with R': Cornell, an allele that conditions strong piamentation of certain seedling tissues and of anthers in the mature plant. Evidence obtained by STADLER (1942) suggested that R' behaved as a compound allele comprising a seed-pigmenting determiner and a plant-pigmenting determiner which mutated independently of each other. The presence at the R locus of two elements that have retained synaptic homology, and are therefore capable of displaced pairing, was suggested by STADLER and NUFFER (1953) from the observation that among the offspring of R': Cornell homozygotes, loss of the seed-pigmenting determiner (I' mutation) was often associated with recombination of outside markers. The frequent association of R' mutants with a particular recombinant class in R'/Rg heterozygotes enabled STADLER and NUFFER to place the plant-pigmenting determiner (labeled P ) in the proximal member of the duplication, and the seed-pigmenting determiner (labeled S) in the distal member (see Figure 1 ). Additional data that bear on the relation of R' mutation to recombination are presented in this report. The data are used in (1) establishing the relative position of the genetic determiners ( P ) and (S) within the corresponding members of the duplication, (2) measuring the genetic length of the duplication, and ( 3 ) estimating the relative frequency of displaced pairing in duplication homozygotes.