Interpretation of quantitative trait locus (QTL) studies of agronomic traits is limited by lack of knowledge of biochemical pathways leading to trait expression. To more fully elucidate the biological significance of detected QTL, we chose a trait that is the product of a well-characterized pathway, namely the concentration of maysin, a C-glycosyl flavone, in silks of maize, Zea mays L. Maysin is a host-plant resistance factor against the corn earworm, Helicoverpa zea (Boddie). We determined silk maysin concentrations and restriction fragment length polymorphism genotypes at flavonoid pathway loci or linked markers for 285 F2 plants derived from the cross of lines GT114 and GT119.Single-factor analysis of variance indicated that the p1 region on chromosome 1 accounted for 58.0%o of the phenotypic variance and showed additive gene action. The pi locus is a transcription activator for portions of the flavonoid pathway. A second QTL, represented by marker umclO5a near the brown pericarpi locus on chromosome 9, accounted for 10.8% of the variance. Gene action of this region was dominant for low maysin, but was only expressed in the presence of a functional pi allele. The model explaining the greatest proportion of phenotypic variance (75.9%) included pi, umclOSa, umcl66b (chromosome 1), ri (chromosome 10), and two epistatic interaction terms, pi x umclO5a and pi x ri. Our results provide evidence that regulatory loci have a central role and that there is a complex interplay among different branches of the flavonoid pathway in the expression of this trait.Development of molecular-marker linkage maps in many species facilitates the identification of chromosome regions associated with variation in quantitative traits (1). By dissecting the continuous phenotypic variation typical of many traits into contributions from discrete genetic factors, quantitative trait locus (QTL) studies provide insights into trait inheritance and genome organization, and often are sufficient to initiate marker-assisted selection. However, the biological interpretation of QTL data is generally limited by lack of knowledge of the genetics, biochemistry, and physiology underlying trait expression. To advance the level of QTL interpretation, we analyzed variation in an economically important trait that is determined by a well-characterized genetic and biochemical pathway.The corn earworm (CEW) is a major silk-and kernelfeeding insect pest of maize in the United States and parts of Latin America (2, 3). Host-plant resistance to CEW results from both antibiosis due to chemical factors in silks (stylar/ stigmatic tissue), and morphological features such as tight covering of the ear by husk leaves (4). Understanding of the nature of antibiosis to CEW was advanced when maysin (Fig. 1), a C-glycosyl flavone that inhibits CEW larval growth, was isolated from silks of the Mexican maize landrace "Zapalote Chico" (5). Later, Wiseman et al. (6) found a highly significant relationship between increased silk maysin concentration and reduced earworm ...

No abstract

The interpretation of quantitative trait locus (QTL) studies is limited by the lack of information on metabolic pathways leading to most economic traits. Inferences about the roles of the underlying genes with a pathway or the nature of their interaction with other loci are generally not possible. An exception is resistance to the corn earworm Helicoverpa zea (Boddie) in maize (Zea mays L.) because of maysin, a C-glycosyl f lavone synthesized in silks via a branch of the well characterized f lavonoid pathway. Our results using f lavone synthesis as a model QTL system indicate: (i) the importance of regulatory loci as QTLs, (ii) the importance of interconnecting biochemical pathways on product levels, (iii) evidence for ''channeling'' of intermediates, allowing independent synthesis of related compounds, (iv) the utility of QTL analysis in clarifying the role of specific genes in a biochemical pathway, and (v) identification of a previously unknown locus on chromosome 9S affecting f lavone level. A greater understanding of the genetic basis of maysin synthesis and associated corn earworm resistance should lead to improved breeding strategies. More broadly, the insights gained in relating a defined genetic and biochemical pathway affecting a quantitative trait should enhance interpretation of the biological basis of variation for other quantitative traits.The past decade has seen an explosion of information on the structure, organization, and functions of the maize (Zea mays L.) genome, including the development of high density molecular marker maps. One application of new mapping technologies has been the genetic dissection of quantitative traits with much greater precision than was previously possible (1, 2). Still, the quantitative trait loci (QTLs) detected are generally rather poorly defined regions, and the size of a QTL's phenotypic effect is sometimes confounded with its location relative to the nearest marker or to a nearby QTL. For most traits, genetic and biochemical information on metabolic pathways is extremely limited, and, therefore, it is difficult to interpret QTL results in terms of regulatory and structural genes, duplicate function loci, feedback inhibition, branched pathways, or other phenomena affecting trait expression. Our goal in this research project is to analyze the genetic control of a quantitative trait of economic importance [antibiosis to the corn earworm (CEW)] and to interpret the results in terms of the well characterized flavonoid pathway.The CEW Helicoverpa zea (Boddie) is a major insect pest of maize and other crops (cotton, soybeans, peanuts) in the United States and elsewhere in the Western Hemisphere (3, 4). Corn earworm eggs are laid on the silks, and the larvae access the ear by feeding through the silk channel. Host-plant resistance to CEW by antibiosis is caused by the presence of the C-glycosyl flavones maysin, apimaysin, and methoxymaysin and related compounds (Fig.