Genetic characterization of a mutable allele,R-marbled (R-mb), in maize (Zea mays L.)

TheR-marbled (R-mb) allele in maize confers a distinct pattern of anthocyanin pigmentation in the aleurone. We investigated the genetic mechanism involved in the formation and variability of this pattern. Wide variation was observed in the extent of anthocyanin pigmentation in marbled kernels. Progeny testing of different expressions resulted in distinct segregation profiles, indicating that the somatic variegation has a genetic basis. Drastic reduction in penetrance and expressivity was noticed whenR-mb was transmitted in a single dose through the pollen parent. Analysis of the colored kernels fromR-mb, including discordant endosperm-embryo phenotypes, showed that only germinal reversions fromR-mb toR-sc (self-colored) were transmissible. Unlike other pattern alleles at theR locus, viz.R-stippled (R-st) andR-Navajo (R-nj),R-mb reverts toR-sc at a higher frequency. No dominance-recessiveness relation was found among the three pattern alleles. Reciprocal-cross differences occurred whenR-mb was crossed withR-nj orR-st, but the interaction ofR-mb withR-st was not entirely similar to that withR-nj. The characteristic variegation pattern due toR-mb is attributed to the action of a transposable genetic element on the basis of somatic and germinal instability, occurrence of discordant endosperm-embryo phenotypes, and genetic analysis ofR-mb/R-st andR-mb/R-nj heterozygotes. The distinct genetic behaviour ofR-mb, in comparison withR-st andR-nj, indicates specificity of the controlling element operating at this allele.