Quantitative trait loci and metabolic pathways

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.

Candidate gene analysis of organ pigmentation loci in the Solanaceae

“…In maize, the only case to date where the complete set of genes from a pathway has been examined, the maysin content of corn silks was associated with alleles of both regulatory and structural loci (1)(2)(3)(4). Other results from both maize (5,6) and Arabidopsis thaliana (7) have shown that variability in quantitative measurements of overall enzymatic activity can be attributed to structural loci or closely linked elements.…”

Candidate Genes, Quantitative Trait Loci, and Functional Trait Evolution in Plants

“…Several QTLs for glycolytic enzyme levels and glucosinolate production in Arabidopsis map to the locations of genes that encode the responsible enzymes (Mitchell-Olds and Pedersen 1998;Kliebenstein et al 2001). In maize, QTLs for concentration of maysin, an important contributor to earworm resistance, have been mapped in several crosses (McMullen et al 1998 Note. Genes that have been identified as flowering time genes on the basis of mutant analysis are described as functional candidates, regardless of whether a candidate gene approach was used to isolate the trait locus.…”

A Segmental Deletion Series Generated by Sister-Chromatid Transposition of Ac Transposable Elements in Maize

“…The region in the vicinity of p1 has been identified as a major QTL for the control of levels of silk maysin, a C-glycosyl flavone that deters feeding by corn earworm (Byrne et al 1996;Lee et al 1998;McMullen et al 1998). Maysin accumulation is correlated with a phenotype termed silk browning, in which the cut ends of silks turn brown due to the oxidation of flavones (Byrne et al 1996;Lee et al 1998;McMullen et al 1998;Guo et al 2001;Rector et al 2003).…”

“…Maysin accumulation is correlated with a phenotype termed silk browning, in which the cut ends of silks turn brown due to the oxidation of flavones (Byrne et al 1996;Lee et al 1998;McMullen et al 1998;Guo et al 2001;Rector et al 2003). Both p1 and p2 genes are expressed in maize silk (Zhang et al 2000), and both encode highly similar R2R3-Myb proteins with a similar potential to activate flavonoid biosynthesis in transgenic cell lines (P. .…”