Expressions of alleles at the B locus

From a variety of sources, including genetic strains, inbred lines and varieties, 31 sources of plant color independent of R (i.e., color expressed with r-g) have been converged four times to a standard strain of K55 background with A C r-g b Pl. All sources have single-factor inheritance of the plant color expression; with Pl, all have glume bar color; all are allelic to B.

Mature plants carrying each allele heterozygous with b were graded (3 to 9 plants each) in randomly planted families. Intensity grades on a scale from 0 to 7 were tallied in three sheaths (basal; lower than ear; upper), leaf blades, auricles, nodal rings, tassel glumes, glume bars, brace roots, culm and husks. The average of these 11 grades for each tissue is arrayed in rank order in the left portion of Fig. 1. The strongest allele I know, B-I (replicated four times in the figure), is the standard allele used in studies of paramutation at the B locus; B'-I is the paramutant form of B-I (among the other alleles only B'-K and B'-V, from genetic strains, are paramutagenic); B-Bolivia and B-Peru are the alleles that confer aleurone color replacing R (the first irregular and faint, the second uniform and intense); B-V and B'-V are-mutable; B-N6, B-W22 and B-38-11 were derived from the respective inbred lines; B-Bolita and so forth were derived from varieties, and the rest from various genetic stocks. The alleles seem to fall generally into about four discrete levels of expression, although they may actually represent a continuous series.

If the intensity of expression is considered for each tissue in the rank order established by the overall average grade, tissue-specific effects that are characteristic of each allele may be identified by variations in a profile line (right portion of Fig. 1). Generally, the grades for these three tissue examples follow the rank order, but some alleles evidently are not coordinate from tissue to tissue; discontinuities for auricle color are especially prominent.

Figure 1.

Assuming that these alleles are a reasonably representative sample of the possible range of expression at this locus, the data suggest that quantitative variation is parallel only roughly from tissue to tissue, and that the determinants for variation may vary non-coordinately with determinants for tissue specificity.

E. H. Coe, Jr.


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