Studies on R-nj

Variability in the expression of the navajo gene in the R-nj:Illinois isolate in stock 2 background was examined. There were wide variations in the extent of anthocyanin pigmentation and eleven different types of expression ranging from "silk scar color only" to "almost whole kernel colored" were recovered. Progeny testing of the isolated phenotypes revealed that the variations in the R-nj expression are not heritable and are due to developmental conditions. A kernel with the minimum amount of aleurone pigmentation may, in the next generation, yield the whole spectrum of variations.

The R-nj gene showed dosage effects on aleurone pigmentation. Homozygous R-nj stock with three doses of R-nj in the triploid aleurone is uniformly and more intensely pigmented than kernels with two and one dose of the gene in heterozygous combination with r. Extent of pigmented area in the aleurone was found to be extremely variable in all three R-nj dosages. The effect of the heterochromatic knob (K) on the same arm of the chromosome, on the intensity of color and extent of pigmentation was also studied, but no detectable difference was observed for the presence or absence of K.

When R-nj was crossed reciprocally with marbled (R-mb), another pattern allele, and the F1 testcrossed to r, an interesting situation was noticed. There is no dominance relationship between the two and both the phenotypes are expressed on the kernel. However, when the female carried the R-nj allele in homozygous or heterozygous condition, the majority of the kernels on crossed ears showed R-nj phenotype regardless of whether R-nj was present in the endosperm tissue or not. The situation suggests a pre-setting effect of the R-nj allele implying that the developmental information is pre-programmed in the mother and passed on to the offspring.

Recombinational analysis of the R-nj allele carried out with the help of a proximal marker, g (golden plant color), and a distal marker, K (heterochromatic knob), and one contamination marker, y (white kernel), revealed that R-nj represents a compound form with two closely linked but separable components, the self-color (Sc) component responsible for pigmentation of the whole kernel, and the navajo (Nj) responsible for restriction of aleurone color to the crown region of the kernel. Nj gene is most probably regulatory in nature controlling onset, termination, or period of Sc action. Sixty-four putative self-color recombinants were obtained from screening of 65,119 kernels from 430 ears of the testcross G R-nj K/g r-g k. Progeny testing of these suspected cases revealed that four were genuine self-color recombinants involving crossing over between G and K and also between silk and anther color and seed color. The probable gene order of this region was G Sc Nj K. In another testcross involving the mutable R-nj (R-nj Mp), two genuine self-colored exceptions were isolated from among 4,246 kernels. This further confirms the compoundness of the R-nj allele.

D. Kumar and K. R. Sarkar


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