Search

In the United States, insecticide is used extensively in the production of sweet corn due to consumer demand for zero damage to ears and to a sweet corn genetic base with little or no resistance to ear-feeding insects. Growers in the southern United States depend on scheduled pesticide applications to control ear-feeding insects. In a study of quantitative genetic control over silk maysin, AM-maysin (apimaysin and methoxymaysin), and chlorogenic acid contents in an F2 population derived from GE37 (dent corn, P1A1) and 565 (sh2 sweet corn, p1a1), we demonstrate that the P1 allele from field corn, which was selected against in the development of sweet corn, has a strong epistatic interaction with the a1 allele in sh2 sweet corn. We detected that the p1 gene has significant effects (P < 0.0001) not only on silk maysin concentrations but also on AM-maysin, and chlorogenic acid concentrations. The a1 gene also has significant (P < 0.0005) effects on these silk antibiotic chemicals. Successful selection from the fourth and fifth selfed backcrosses for high-maysin individuals of sweet corn homozygous for the recessive a1 allele (tightly linked to sh2) and the dominant P1 allele has been demonstrated. These selected lines have much higher (2 to 3 times) concentrations of silk maysin and other chemicals (AM-maysin and chlorogenic acid) than the donor parent GE37 and could enhance sweet corn resistance to corn earworm and reduce the number of applications of insecticide required to produce sweet corn.

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 Crop Genetics and Breeding Research Unit. This population was released as a source of improved resistance to silk and ear feeding by larvae of the corn silk fly [Euxesta stigmatias Loew. (Diptera: Otitidae)], the fall armyworm [Spodoptera frugiperda (J.E. Smith)], and the corn earworm [Heliocoverpa zea (Boddie) (Lepidoptera: Noctuidae)]. Zapalote Chico 2451F is distinct from Shrunken Zapalote Chico (ZC-sh2) (PI 612343), and the Zapalote Chico land race collected in the state of Oaxaca, Mexico, in the late 1940s, and first acceded to the National Seed Storage Laboratory as PI 217413 (Scully et al., 2000; Anderson, 1959; Straub and Fairchild, 1970). PI 217413 was one of the earliest Zapalote Chico populations identified as a source of natural compounds with insecticidal properties (Wais et al., 1979;Wilson and Wiseman, 1988). Resistance in Zapalote Chico 2451F is also due to elevated levels of the flavone glycoside maysin that is found in fresh silk (Ellinger et al., 1980; Snook et al., development of new germplasms or parental lines. 1993, 1995). Maysin is synthesized in the flavonoid pathway W.L. Rooney and known to specifically confer antibiosis-based resistance to silk feeding (Byrne et al., 1996).

Several resistance sources and resistance mechanisms to aflatoxin formation and corn earworm (Helicoverpazea Boddie) damage to maize (Zeamays L.) have been identified. Based on this knowledge, experiments were initiated toward achievement of the following objectives: (1) to confirm earlier determinations on resistance traits of germplasm sources and to identify quantitative trait loci (QTL) associated with each of the traits, and (2) upon estimation of the degree of QTL effects on each trait, to generate a maize population, with chemical and physical resistance to Aspergillus spp. and ear-feeding insects, for inbred development. A 2-year field experiment to evaluate selected genotypes inoculated with A. flavus and infested with corn earworm revealed that significant variation exists among the genotypes for aflatoxin contamination and corn earworm damage. The protection of maize ears against aflatoxin contamination was primarily dependent on resistance to fungal infection and ear-feeding insects, and excellent husk coverage and tightness. A major QTL (p1) identified on chromosome 1S had effects of 54.0, 42.1, and 28.3% on the phenotypic variability for concentrations of silk maysin, 3′-methoxymaysin+apimaysin, and chlorogenic acid, respectively. Markers/QTLs for husk phenotypic traits and total aflatoxin concentrations have been determined, but more detailed mapping of these chromosomic regions will be necessary to locate precise markers/QTLs for husk traits and aflatoxin production. Realizing the complexity of the Aspergillus–aflatoxin-maize system and the factors affecting aflatoxin contamination, we are directing our program toward marker-assisted breeding to enhance or improve general genetic resistance to ear-feeding insects and invasion by Aspergillus spp.

Plant resistance offers a good alternative to pesticides for protecting corn (Zea mays L.) from attack by corn earworm, Helicoverpa zea (Boddie). The GT-CEW-RS8 synthetic is a breeding population with some resistance to corn earworm, but based on variability for this trait, further improvement was possible in this population. S1 recurrent selection for corn earworm resistance was conducted to improve three synthetics derived from GT-CEW-RS8. The objective of this research was to study the effectiveness of these selection programs to increase insect resistance and maintain agronomic performance. Cycles of selection for each population were evaluated under four environments. Significant gains in resistance were realized, but all three synthetics derived from GT-CEW-RS8 showed a negative response for yield. However, yield losses were not very dramatic for 66RM4 and inbreeding depression would not affect the yield performance in hybrid combination. Selection for yield done simultaneously with selection for reducing ear damage by corn earworm and husk tightness could assist in avoiding decreased yield.

Aflatoxin, produced by Aspergillus flavus, is one of the most toxic and carcinogenic substances known and contaminates many agricultural commodities such as corn, peanuts, cottonseed, and tree nuts. The challenge to breeders/plant pathologists is to identify lines that have resistance to aflatoxin production. Maize population GT-MAS:gk has been identified and released as a germplasm with resistance to aflatoxin contamination. In the present study, we assessed genetic divergence in the GTMAS:gk population using restriction fragment length polymorphism (RFLP) DNA markers to survey 11 selfed inbred lines and conducted field evaluations for the dissimilarities in aflatoxin production among these inbred lines in comparison with a sister population, GT-MAS:pw,nf. The 11 selfed inbred lines were assayed for DNA polymorphism using 113 RFLP markers in 10 linkage groups covering 1,518.2 centimorgans (cM; unit of gene or chromosome size). Considerable variation among the inbreds was detected with RFLP markers, of which 42 probe-enzyme combinations gave 102 polymorphic bands. Cluster analysis based on genetic similarities revealed associations and variations among the tested lines. Three polymorphic groups were distinguished by cluster analysis. Two years of field evaluation data showed that aflatoxin concentrations among the lines were significantly different in both years (P < 0.001). Maturity data were also different. Thus, this study demonstrates that the maize population GT-MAS:gk is heterogeneous and that individuals may be different in resistance to A. flavus infection and aflatoxin production. Therefore, the most resistant lines should be inbred to increase homogeneity, and resistance should be confirmed through progeny testing.

Maysin and related compounds, such as apimaysin, 3 ' -methoxymaysin, and chlorogenic acid, have been determined to be important maize (Zea mays L.) produced antibiotic compounds against corn earworm (Helicoverpa zea Boddie), but, to be effective under field conditions, silk antibiotics should be accompanied by good husk coverage. The objective of this work was to identify molecular markers associated with synthesis of maysin and related compounds in a maize Population and determine if the markers could be linked to the genes which affect husk tightness. A total of 113 probes were used to screen for RFLP polymorphisms and the 53 probes that were polymorphic between the parents were used as codominant markers to genotype 205 F-2 individuals. Silks and husks of F-2:3 families were evaluated. Two major QTL were identified for the synthesis of maysin and related compounds, the already known pl, on the short arm of chromosome 1, and a novel one on the interval csu1066-umcl76 on genomic region 2C-2L. A QTL for husk tightness was located near pl. The functional allele for pi and the favorable allele for husk tightness were in repulsion linkage. in a marker-assisted selection program for increasing resistance to corn earworm, markers for silk antibiotic synthesis should be accompanied by markers for husk tightness. Efforts Should be made to convert the RFLP-markers into PCR-based markers for user friendly application in marker-assisted breeding.

Corn (Zea mays L.) grown in the Southeastern U. S. is often severely damaged by corn earworm (Helicoverpa zea Boddie), which feed on developing kernels on the ear. Natural resistance to this insect due to husk cover and chemicals in the silks has been identified. we studied the improvement in the resistance to ear injury by the corn earworm in the population 10LDDSR after 10 cycles of S1 recurrent selection and in the population cross DDSA x DDSB after seven cycles of reciprocal recurrent selection. We evaluated progress for increased resistance over a two-year period. The linear response to selection for improved resistance to corn earworm in the 10LDDSR population and DDSA x DDSB population cross was 0.06 cm of damage per cycle. Only three cycles of S1 and reciprocal recurrent selection for ear damage and husk tightness to improve resistance reduced ear damage by the corn earworm while maintaining other desirable agronomic traits. Therefore, future efforts to reduce ear damage in 10LDDSR(C3) should probably be focused on increasing silk maysin content. Antibiosis is also proposed as a potential trait to improve the resistance of breeding populations DDSA(C3) and DDSB(C3).