The R-mb kernels usually display a spotted phenotype with three important features: (i) no restriction on the number of colored sectors, and different spotting types (very light, light, medium and heavy) with varying extents of pigmentation can be obtained; (ii) pigmented spots (results of random excision events) can occur anywhere on the aleurone, with no specificity in arrangement, and (iii) colored spots have well-defined boundaries, but are irregular in their shape.
We came across some exceptional kernels with altered spatial pigmentation, on R-mb selfed or testcross cobs, with the following features: (i) the colored spots on a colorless aleurone background were arranged in an orderly and precise manner, originating from the germinal side and extending to the crown and the abgerminal side of the kernel; (ii) colored sectors were in the form of concentric rings or stripes and seldom irregular in shape; (iii) spots appeared in a symmetrical manner from either side of the scutellum or restricted to one-half of the kernel, and (iv) the flow region on the abgerminal side of the kernel might show irregularly shaped spots (as in R-mb), with the rest of the kernel showing the characteristic spotting arrangement. For convenience in presentation, these kernels will be hereafter referred to as R-mb:cc (marbled in concentric circles).
The genetic behavior of R-mb:cc was largely identical to that of R-mb, evidenced by a drastic dosage effect on aleurone pigmenting potential when transmitted in a single dose through the pollen parent. Homozygous R-mb:cc kernels with three doses of R-mb:cc gave rise to kernels with concentric colored spots in high proportion (92.5%), followed by the colorless kernel class. When transmitted through the pollen parent, a very low percent (1.3%) of R-mb:cc phenotypic class was recorded.
Four generations of selfing the R-mb:cc kernels with plantings at different locations (New Delhi and Hyderabad) showed consistency in the spatial pattern. The R-mb:cc ears frequently showed completely colorless kernels as well as fully colored revertants. Discordant endosperm-embryo phenotypes, like mb:cc endosperm with colored scutellum and colored endosperm with colorless scutellum, were also noticed in frequencies higher than the spontaneous mutation rate. Preliminary observations indicated that further categorization is possible within R-mb:cc. Ears with homogeneous expression of light, medium or heavily striped kernels could be obtained. We have studied the pigmentation onset and progression in R-mb:cc. As in R-mb and R-st, visual manifestation of anthocyanin pigmentation first occurred on the 11th day after pollination (DAP). Later on, pigmentation became more intense with little change in the basic pattern. Therefore, in R-mb:cc, the spatial pigmentation pattern appears to be determined at an early stage in the aleurone formation.
R-mb:cc faithfully transmitted its characteristic phenotype with concentric arrangement of spots, when crossed with R-nj or R-st. As in the case of R-mb, there were no dominance-recessive relationships among the three alleles and there was a preponderance of Navajo phenotype in the R-mb:cc/R-nj and R-nj/R-mb:cc genotypes. Reciprocal cross differences also confirmed the dosage effect of R-mb:cc on aleurone pigmentation.
The study showed that the R-mb:cc was germinally transmissible and had similarities in genetic behavior with R-mb from which it was derived. Mutations from recessive to dominant (self-colored revertants) occurred at high frequency, variation within the basic pattern (both in the degree of spotting and the intensity of pigmented spots) and discordant endosperm-embryo phenotypes warrant the basic assumption that the R-mb:cc phenotype is under the control of a transposable genetic element. Although we do not yet have any clearcut evidence as to how R-mb:cc originated and how the striking regularity in the arrangement of colored spots occurs, it might be instructive to consider the plausible mechanisms, using the information from the well-studied transposable element systems in maize.
In maize, there were no earlier reports of pigmentation pattern in a concentric manner from the germinal side of the kernel, either in the unstable alleles at R or other anthocyanin biosynthetic loci. Analogous, however, is the case of two En alleles, En-crown and En-flow (Peterson, PA, MNL40:64, 1966) which respond quite specifically to different parts of the aleurone tissue. Similarly, Doerschug (Theor. Appl. Genet. 43:182-189, 1973) found a Dt element that caused restriction of element activity in the kernel. A Uq-flow phenotype was recovered by Peterson (in Gene Structure and Function in Higher Plants, eds. G. M. Reddy, E. H. Coe, 1983), where the transaction of the element was restricted to the basal portion of the developing endosperm. These examples typify only restriction of the element activity to specific regions, but no restriction is placed on the arrangement or shape of a colored spot within a region. In contrast, studies carried out on mutable pericarp, characterized by the presence of red stripes on a white pericarp background, revealed that the phenotype is due to transposition of Mp (=Ac) at the P locus (Fedoroff, in Developmental Genetics of Higher Organisms, ed. G. M. Malacinski, pp. 97-125). The stripes are wider at the base of the kernel and come to a narrow point at the silk scar region of the crown. Fedoroff (1988) described that the differences in the shape of the characteristic stripes in the pericarp (a maternal tissue with a growth pattern different from that of the endosperm) are determined by the pattern of cell divisions within the tissue. It should be noted here that the stripes in the Ac-influenced variegation pattern 'radiate' from the silk scar region of the kernel, unlike the concentric spots in R-mb:cc. It would be interesting to consider, in this context, certain pallida alleles in snapdragon. The pal-33, pal-32, pal-15 and pal-41 cause altered patterns of spatial pigmentation in the flower, when compared to other pal alleles. To explain this, Coen et al. (in Temporal and Spatial Regulation of Plant Genes, eds. D. P. S. Verma, R. B. Goldberg, pp. 632-82, 1988) proposed that the wildtype pal promoter contains a set of sequences that respond to diverse regulatory signals spatially arranged as a pre-pattern in the flower to generate specific patterns. They hypothesized that novel patterns are produced by mutations that change the interpretations of pre-pattern by modifying the affinity of the pal promoter for different regulatory molecules. If the colored spots in R-mb:cc are results of excision events during kernel ontogeny, the pattern of excision of the controlling element in only certain cell lineages originating from the germinal side might be under the influence of a developmental signal or host factor. Levy and Walbot (Science 248:1534-37, 1990) demonstrated that the timing of transposable element excision can be controlled by the host. We cannot, at present, rule out other plausible mechanisms by which R-mb:cc pattern is regulated, like DNA methylation.
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