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C-glycosyl flavones in maize silks confer resistance (i.e., antibiosis) to corn earworm (Helicoverpa zea [Boddie]) larvae and are distinguished by their B-ring substitutions, with maysin and apimaysin being the di- and monohydroxy B-ring forms, respectively. Herein, we examine the genetic mechanisms underling the synthesis of maysin and apimaysin and the corresponding effects on corn earworm larval growth. Using an [F.sub.2] population, we found a quantitative trait locus (QTL), rem1, which accounted for 55.3% of the phenotypic variance for maysin, and a QTL, pr1, which explained 64.7% of the phenotypic variance for apimaysin. The maysin QTL did not affect apimaysin synthesis, and the apimaysin QTL did not affect maysin synthesis, suggesting that the synthesis of these closely related compounds occurs independently. The two QTLs, rem1 and pr1, were involved in a significant epistatic interaction for total flavones, suggesting that a ceiling exists governing the total possible amount of C-glycosyl flavone. The maysin and apimaysin QTLs were significant QTLs for corn earworm antibiosis, accounting for 14.1% (rem1) and 14.7% (pr1) of the phenotypic variation. An additional QTL, represented by umc85 on the short arm of chromosome 6, affected antibiosis ([R.sup.2] = 15.2%), but did not affect the synthesis of the C-glycosyl flavones.

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-glycosyi flavone synthesized in silks via a branch of the well characterized flavonoid pathway. Our results using flavone 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 flavone 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.

Growth of the corn earworm (Helicoverpa zea Boddie), a major pest of maize (Zea mays L.), is inhibited by high concentrations of maysin, a C-glycosyl flavone, in maize silks. To advance understanding of the genetic control of silk maysin concentration and antibiosis to corn earworm, we conducted a quantitative trail locus (QTL) study in a population derived from a cross of high-maysin (GE37) and low-maysin (FF8) lines. Both lines have phenotypically identical, functional alleles at the p1 locus, a regulator of the biochemical pathway leading to maysin synthesis. We evaluated 250 (GE37 x FF8)[F.sub.2:3] families for (i) restriction fragment length polymorphisms at 121 loci, (ii) silk maysin concentrations in two locations (in Georgia and Missouri), and (iii) weight of corn earworm larvae grown on artificial diet containing oven-dried silks. For maysin concentration in both locations, composite interval mapping revealed major QTLs in the asg20-whp1 interval of chromosome 2 and near the wx1 locus on chromosome 9; smaller effects were detected on chromosomes 1, 6, and 8. For larval weight, the largest QTL was detected in the asg20-whp1 interval, and major effects were also associated with chromosomes 1 and 6. However, we found no effect on larval weight at wx1. Significant (P [is less than] 0.001) epistasis was observed for A traits. Our results suggest that whp1 on chromosome 2, a gene that encodes chalcone synthase and sm1, a silk-specific gene on chromosome 6, are candidate genes affecting maysin concentration and corn earworm larval weight in this population., The corn earworm damages maize crops in the USA and Latin America by directly consuming kernels and by introducing ear-rotting fungi (Dicke and Guthrie, 1988; Ortega et al., 1980). In [...]

The isolation and identification of maysin and two of its analogues, 3'-methoxymaysin and apimaysin (3'M + A), in the silks of corn, Zea mays L., has given plant breeders a mew tool for reducing damage by larvae of the corn earworm, Helicoverpa zea (Boddie). This study was initiated to examine the inheritance of these compounds so that a logical course for developing germplasm with high concentrations can be determined. Incorporation of the compounds into corn hybrids for the southern USA can then proceed. Twelve inbred Sues were crossed in all combinations and evaluated for maysin and the sure of 3'-methoxymaysin plus apimaysin concentrations in their silks. Analysis of data from 2 yr by Griffing's Method 3 Model I revealed that reciprocal and maternal effects were not significantly different from zero. Subsequent pooling of reciprocal crosses in a Gardner-Eberhart Analysis II gave significant main effects (P [is less than] 0.01) and years X entries interactions (P [is less than] 0.05). Further partitioning of the variation established additive effects as the predominant source of genetic variation among the 66 crosses evaluated. Selection for increased maysin and (3'-methoxymaysin plus apimaysin) concentrations in corn silks should be successful by means of a simple backcrossing procedure or routine recurrent selection within a population. One or two inbreds, among the 12 tested, appear to be good candidates for establishing a selection program because they have both good general and specific combining ability. Maysin concentration and the sum of its analogues 3'-methoxymaysin and apimaysin are closely correlated in corn silks (r = 0.81, P [is less than] 0.01)., The corn earworm is a devastating insect pest on corn and other crops in the southern USA. Since the discovery of maysin, a C-glycosylflavone, in the silks of corn (Waiss [...]

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 [F.sub.2] 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% of the phenotypic variance and showed additive gene action. The p1 locus is a transcription activator for portions of the flavonoid pathway. A second QTL, represented by marker umc105a near the brown pericarp1 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 p1 allele. The model explaining the greatest proportion of phenotypic variance (75.9%) included p1, umc105a, umc166b (chromosome 1), r1 (chromosome 10), and two epistatic interaction terms, p1 x umc105a and p1 x r1. 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.

Information on heterosis is critical to assimilation of desirable traits into usable maize, Zea mays L., germplasm. Our objective was to identify maize populations with heterosis for desirable traits. Twelve southern-adapted maize populations with resistance to damage by ear and leaf-feeding insects and all 66 possible population crosses were evaluated in a randomized complete-block design in three environments. Population effects, heterosis, and specific combining ability (SCA) effects were estimated for seven plant traits. Population effects were significant (P is less than or equal to 0.01) for all traits except yield, heterosis effects were significant for all traits, and SCA effects were significant for all traits except insect damage. Population and average heterosis effects were less important among genetic effects than SCA, which accounted for more than one-half the heterosis sums of squares for all traits. Population performance as a predictor of relative crossbred performance was least effective for grain yield and quality. Average heterosis, relative to midparent values, varied from 2.2% for lodging to 22.5% for yield. Crosses existed with SCA effects for resistance to one or more insects for all traits. Genotype x environment interactions were significant for all traits except insect damage. Heterosis interactions with environment accounted for more variation than population x environment interactions. Among heterosis interactions with environments, those due to average heterosis and SCA effects were of greatest significance. The information will be useful in the development of germplasm with resistance to insects and improved agronomic performance.

Improvement of resistance to insects while maintaining other desirable agronomic traits in maize (Zea mays L.) populations has been an elusive goal for breeders. This study was conducted to evaluate crosses among 11 insect-resistant breeding populations to identify paired populations for selection or extraction of inbreds. A complete diallel was evaluated for lodging, plant height, ear height, yield, ear damage by insects, and maturity in a complete-block experiment with five replicates at two locations. Locations were combined because errors were homogeneous and the crosses x locations interaction was nonsignificant. General combining ability (GCA) effects were significant (P [less than or equal to] 0.01) for all traits, specific combining ability (SCA) effects were significant (P [less than or equal to] 0.01) for all traits except yield and maturity, reciprocal effects were significant (P [less than or equal to] 0.01) for all traits except yield, and maternal effects were significant (P [less than or equal to] 0.05) for all traits except yield and damage by insects. Hybrid checks were earlier, shorter, higher yielding, and less prone to lodging but also less resistant to insects than were population crosses. Paired crosses resistent to corn earworm (Helicoverpa zea Boddie) and maize weevil (Sitophilus zeamais Motschulsky) that performed well for other traits are available for selection and inbred extraction. Paired crosses with resistance to fall armyworm (Spodoptera frugiperda J.E. Smith) were not identified. The information on GCA, SCA, reciprocal, and maternal effects will assist in identifying critical population crosses., The crossbreeding performances among 11 corn insect-resistant breeding populations were evaluated by the variety cross diallel. Lodging, plant height, ear height, yield, ear damage, and maturity were tested. The results showed that general combining ability effects were significant for all the traits, while specific combining ability effects were significant for all traits except yield and maturity. Reciprocal effects and maternal effects were also significant for most traits except for yield and damage by insects. These and other data may be used to select the best population crosses for insect resistance.

Shrunken Zapalote Chico (ZC-sh2) (Reg. No. GP-361, PI 612343), a maize (Zea mays L.) germplasm population, was released in the autumn of 1999. It was jointly developed by the Florida [...]

Zapalote Chico 2451F (ZC-2451F) (Reg. no. GP-370, PI 618810), a maize (Zea mays L.) germplasm population was released in April 2001 by the Florida Agricultural Experiment Station and the USDA-ARS [...]

Germplasm sources EPM6 and SIM6 (Reg. no. GP-364, PI 614735, and Reg. no. GP-365, PI 614736, respectively) are dent maize (Zea mays L.) populations developed cooperatively by the USDA-ARS Insect [...]

GT-HID9, a maize (Zea mays L.) germplasm population (Reg. no. GP-366, PI 614805) was released by the USDAARS and the University of Georgia Coastal Plain Experiment Station in March 2000. [...]