Silk browning, maysin content, and corn earworm resistance
--P. F. Byrne, L. L. Darrah, D. J. Moellenbeck, B. D. Barry, M. E. Snook, B. R. Wiseman and N. W. Widstrom

Silks of some genotypes turn brown after wounding, while those of other genotypes do not change color. The trait is controlled by a factor at the p1 locus (Han and Coe, MNL 60:55, 1986; MNL 61:46, 1987) and is believed to be due to enzymatic oxidation of flavonoid compounds to quinones, which condense with themselves or proteins to produce brown pigments (Levings and Stuber, Genetics 69:491-498, 1971). A previous study found a significant correlation between percentage of plants with browning silks and damage caused by corn earworm, Helicoverpa zea (Boddie) (CEW), in cycles of selection of the population 10LDD, which had been selected for CEW resistance (Byrne et al., Environ. Entomol. 18:356-360, 1989). Subsequently, 10LDD Cycle 0 was divergently selected to form subpopulations with silks that were nearly all browning (10LDD BR) or nearly all nonbrowning (10LDD NBR).

In the summer of 1993, these subpopulations were evaluated along with 10LDD Cycles 0, 2, 4, 6, and 7A1. Cycles 2, 4, and 6 were selected on the basis of CEW ear penetration and husk tightness, and Cycle 7A1 was selected based on silk maysin content and dried silk bioassays. Maysin, a flavone glycoside, is a major factor responsible for antibiotic resistance to CEW (Waiss et al., J. Econ. Entomol. 72:256-258, 1979). Two commercial hybrids, Pioneer Brand 3369A and Pioneer Brand 3184, were included as check entries. The CEW resistant variety 'Zapalote Chico 2451# (P) C3' was delay-planted in an adjacent plot. The trial was arranged in a randomized complete block design with five replications, and was grown at three locations in or near Columbia, MO. Experimental plots were two rows 4.9 m long with 0.91 m between rows.

Primary ear shoots were covered with shoot bags to prevent pollination and subsequent physical and chemical changes in the silks. Approximately three days after silks appeared, 10 primary ears per plot of 10LDD BR, NBR, Cycle 0 and Cycle 6, and Zapalote Chico were harvested, packed in coolers with blue-ice packs, and shipped to the USDA-ARS Phytochemical Research Unit, Athens, GA. All silks from the same plot were bulked, and concentrations of maysin and related compounds were determined by reversed-phase high-performance liquid chromatography (Snook et al., J. Chromatogr. 477:439-447, 1989).

At about the same stage of silk development, four silk masses per plot of each entry were collected, dried at 41 C for 10 days, and bulked across replications for each entry at each location. Dried silks were shipped to the USDA-ARS Insect Biology and Population Management Research Laboratory, Tifton, GA. Silks were ground and mixed with a pinto bean diet, and bioassays carried out as described by Wiseman (Toward insect resistant maize for the third world, p. 94-100, CIMMYT, Mexico, D.F., 1989). Fifteen replications of a split-plot design (whole plots=sampling locations, subplots=entries) were conducted, and data recorded on eight-day larval weight, time to pupation, and pupal weight. Silks of eight to ten ears per plot were evaluated for the browning reaction about two weeks after silking began.

Eight to ten ears per plot were artificially infested with CEW eggs at three to five days after silk emergence. Due to environmental conditions, high predator populations, or other factors, the infestation was not uniformly successful; data for depth of CEW penetration were highly variable and are not presented here.

Selecting solely for browning and nonbrowning silks resulted in subpopulations with high and undetectable levels of silk maysin, respectively (Table 1). In the bioassay, the browning subpopulation resulted in significantly lower (P<0.01) eight-day larval weights and longer (P<0.01) time to pupation than the nonbrowning entry. Little change in the measured parameters was observed from Cycle 0 to Cycle 6, probably because good husk tightness in this population obscured differences in antibiosis. However, after selection criteria were changed in Cycle 7A1, a large decrease in percentage of nonbrowning silks, smaller larval weight, and a longer time to pupation resulted. Pioneer Brand 3184, which had previously demonstrated resistance to European corn borer, Ostrinia nubilalis (Hubner), (B. D. Barry, personal communication), showed a high degree of antibiosis to CEW based on the bioassay.

Because maysin synthesis occurs as part of the flavonoid pathway, a blockage early in the pathway effected by a recessive allele at p1 will result in silks lacking maysin and other compounds contributing to the browning reaction. Although in the 10LDD population variation at the p1 locus (and/or loci controlling nearby steps in the pathway) apparently is the key factor determining maysin content, in other materials (those with all browning silks, for example), other loci along the pathway presumably will be responsible for variation in maysin level. 

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