Functional variants of a maysin QTL - - - Is there a difference?
--Lee, EA, Byrne, PF, McMullen, MD, Snook, ME, Coe, EH

Genetic variation is essential to detect QTLs, but is it as simple as that? The P1 myb-like transcription factor encoded by the p1 locus is responsible for activating C-glycosyl flavone, 3-deoxyanthocyanin, and phlobaphene synthesis in pericarp, cob, and silk tissue (Grotewold et al., PNAS U.S.A., 88:4587-4591, 1991; Grotewold et al., Cell, 76:543-553, 1994: Byrne et al., PNAS U.S.A, 93:8820-8825, 1996). Because of its control over C-glycosyl flavone synthesis, p1 is a major QTL for maysin synthesis and corn earworm (Helicoverpa zea [Boddie]) antibiosis (Byrne et al., 1996; Byrne et al., J. Econ. Entom., 90:1039-1045, 1997). There are five allelic variants of p1 that are distinguished by their tissue specificity: p1-rrb, p1-rww, p1-wrb, p1-wwb, and p1-www (Peterson, Genetics 126:469-476, 1990; Chopra et al., Plant Cell, 8:1149-1158, 1996; Coe, MNL, 59:40, 1985; Coe and Han, MNL, 60:55, 1986; McMullen unpublished data). Three of the variants direct flavone synthesis in silk tissue: p1-rrb (red pericarp, red cob, browning silks), p1-wrb (colorless pericarp, red cob, browning silks) and p1-wwb (colorless pericarp, white cob, browning silks). Silk browning is due to oxidation of accumulated flavones following tissue damage (Levings and Stuber, Genetics 69:491-498, 1971). Regardless of pericarp expression, p1 alleles that confer cob glume pigmentation also confer silk browning. During development, cell layers that give rise to glume tissues are believed to give rise to silk tissues (Emerson. Genetics 2:1-34, 1916; Peterson, Genetics 126:469-476, 1990; McMullen personal observations). While almost all white-cobbed lines lack flavones in silk tissue, a number of exceptions occur (Coe, 1985; Coe and Han, 1986). One exception is NC7A (p1-wwb allele), which lacks both pericarp and cob glume expression, but still confers silk browning.

Is a p1-wwb allele as effective as a p1-wrb allele in directing maysin synthesis? Are the antibiotic properties of p1-wwb silks equivalent to those of p1-wrb silks? We examined these questions in an F2 population developed from a cross between the inbred lines GT114 and NC7A. NC7A has a p1-wwb allele and GT114 has a p1-wrb allele. Both inbred lines accumulate appreciable amounts of maysin. Silk tissue was collected from the 316 individual F2 plants and maysin levels were determined as described in Byrne et al. 1996. Plants were selfed to produce F2:3 families. Ninety F2:3 families were grown out in 2-row plots for silk tissue collection to determine maysin content and antibiotic properties as described in Byrne et al. 1997. F2 individuals were genotyped molecularly using a p1 RFLP probe and 86 other molecular markers (10 linkage groups with a total map length of 1414 cM). Single factor analysis of variance did not detect significant (P<0.01) differences among the p1 genotypic class least-square means for F2 individual maysin concentration, F2:3 family maysin concentration, and F2:3 family larval weights (Table 1). Epistatic interactions between p1 and the other 86 loci were also examined. Again, the p1 region was not involved in any significant (P<0.001) epistatic interactions for maysin. So, even though the p1 alleles from the parents are different in terms of tissue specificities, in terms of maysin synthesis in silk tissues and corn earworm antibiosis they are equivalent to one another. Genetically distinct alleles at a QTL do not necessarily result in different phenotypic expression in all tissues, and thus may not allow detection of the QTL. The genetic variation must be specific to the tissue in question.

Table 1. p1 genotype least-square means of individual F2 and F2:3 family maysin concentrations and F2:3 family insect bioassay larval weights. Maysin and larval weight p1 genotype class least-square means were not significantly (P<0.01) different from one another.
p1 genotype F2 maysin conc. F2:3 maysin conc. F2:3 larval wt.
  (% fresh wt.) (% fresh wt.) (mg.)
p1-wrb / p1-wrb 0.39 0.24 88.7
p1-wrb / p1-wwb 0.36 0.22 105.2
p1-wwb / p1-wwb 0.33 0.14 137.7

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