DEPENDENCE OF THE R-MOTTLED ALEURONE PHENOTYPE IN MAIZE ON MODE OF SEXUAL TRANSMISSION
YPICAL R-mottling, the irregular distribution of anthocyanin in the aleurone Tlayer of maize endosperm, occurs upon crossing appropriate R R plants with r r (colorless) only when R R is the pollen parent (EMERSON 1918; KEMPTON 1919) . When R R is used in the mating as female, the aleurone is solidly colored. From a conventional point of view the mottled and solidly colored phenotypes suggest the dosage effects associated with the unequal numbers of chromosome sets contributed to the triploid
... to the triploid endosperm, two from the female and one from the male parent. This explapation assumes equal pigmenting action of R from the two origins. Furthermore, if parental source were to affect R expression, such an influence ordinarily would be confounded with the effect of dosage. ROMAN'S (1947) discovery that a given chromosome segment translocated to the centromeric portion of an accessory B chromosome in maize is transmitted by the male in duplicate provides an opportunity to vary the dosage of genes in such a segment independently of parental origin. Accordingly, his translocation B-loa, with the point of interchange in chromosome-10 between R and the centromere, permits introduction of two R factors into the endosperm via pollen. Unexpectedly on the R dosage interpretation, R R from the male when combined with r r from the female gave mottled, rather than solidly colored, kernels (ROMAN, cited by SCHWARTZ 1965; KERMICLE 1963) . This observation called into question sufficiency of the dosage explanation and prompted further investigation. The evidence presented here shows the basis of the disparity in aleurone phenotype following reciprocal crosses between R R and r r plants is a pronounced difference in level of action at which R is transmitted to the endosperm through male and female gametophytes. MATERIALS AND METHODS The various alleles of R involved affect anthocyanin pigmentation of the aleurone and vegetative parts of the plant as follows: RV-Pigmented aleurone, red seedling and anthers; representative of those alleles which give darkly mottled aleurone in crosses to rgrg as female but solidly colored aleurone when rgrg is male. The particular Rr employed in experiments involving translocation B-loa was that present in the TB-loa stock obtained from the Maize Genetics Stock Center, University of Illinois, Urbana. A second source of Rr, that designated standard Rr in the Wisconsin collection, was Central nucleus (above) Egg (below) Sperm r O r O / -Colorless r 9 /R' R' r e d FIGURE 1 .-Kernel classes resulting from reciprocal crosses between rsrg and plants carrying Rr on translocation B-loa. As male, the B10 chromosome of TB-loa usually fails to disjoin in the sperm-forming division within the microspore, yielding RrRp and R-deficient male gametes. Double fertilizations involving such sperm pairs are illustrated by the first two categories. If the BIO chromosomes disjoin normally, kernels of the third class are produced. The fourth class of seeds results from the reciprocal cross, where mitotic disjunction is regular.