Reconstitution of the R compound allele in maize

The R(r):standard allele in maize, which conditions anthocyanin pigmentation in plant and seed tissues in the presence of appropriate complementary factors, is associated with a tandem duplication. The proximal member of the duplication carries P, the plant pigmenting determiner and the distal member member carries S, the seed pigmenting determiner. Derivatives from R(r) that have lost S function are designated r(r). They represent either losses of the distal member of the duplication (P derivatives) or mutations of S to s (P s). Derivatives that have lost P function are designated R(g), and represent either losses of the proximal member of the duplication (S derivatives) or mutations of P to p (p S).-All four possible types of r(r)/R(g) heterozygotes were tested for their capacity to yield R(r) reconstitution by crossing over. No R(r) derivatives were obtained from P/S heterozygotes, a result consistent with the view that P and S occupy corresponding positions in homologous chromosome segments. R(r) reconstitution was detected in both tandem duplication heterozygotes P s/S and P/p S, and was found to be about ten times more frequent in the latter. The ratio of R(r) reconstitution in the two heterozygotes is a function of position of the anthocyanin marker within the duplicated segment. The data from these heterozygotes allow one to measure the distance between P and S, that is to say, the genetic length of the duplicated segment. This distance was found to be 0.16 map units. The highest frequency of R(r) reconstitution was obtained from P s/p S heterozygotes, since direct pairing (see PDF) as well as the p//s type of displaced pairing have the potential to produce R(r) derivatives. One of the R(g) derivatives used in this study, R(g) (6), was found to back-mutate in some sublines to R(r). The basis for this instability remains unknown.