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a USDA-ARS, Crop Genetics and Breeding Research Unit, Tifton, GA 31793
b USDA-ARS (Retired), Richard Russell Research Center, Athens, GA 30604-5677
* Corresponding author (nwidstro{at}tifton.cpes.peachnet.edu)
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 Biology and Population Management Research Laboratory, Tifton, GA and Richard Russell Research Center, Athens, GA. The populations were released in August 1999 as sources of resistance to the corn earworm, Helicoverpa zea Boddie, because of the high concentration of a C-glycosyl flavone, maysin, in their silks that gives them antibiosis resistance to the corn earworm (Snook et al., 1997).
The EPM breeding population was derived by bulking equal numbers of seed from four crosses among a maysin producing inbred, GT114, (Widstrom et al., 1988) and three exotic collections; a flavonoid (maysin = 0.171%) with the intensifier gene (in), a selection of Zapalote Chico (maysin = 0.350%) containing mostly purple plants, and a popcorn plant introduction PI340856 (maysin = 0.823%), (obtained from sources, Dr. Ed Coe, USDA, Columbia, MO, CIMMYT, Mexico D.F. and USDA Plant Introduction Station, Ames, IA, respectively). The SIM breeding population was derived by bulking equal numbers of seed from seven crosses (Akd24 x Ab618, Akd-24 x GT114, Akd24 x H30, GT112 x GT114, GT114 x H30, Tx-501 x Ab618, and Tx501 x Akd24) among six inbred lines with silks having higher than average concentrations of maysin in their silks (Ab618 = 0.204%, Akd24 = 0.305%, GT112 = 0.097%, GT114 = 0.259%, H30 = 0.059%, and Tx501 = 0.215%). The average concentrations of the original composited breeding populations, EPM and SIM were 0.54% and 0.36% on a fresh weight basis.
Each population was independently subjected to six cycles of recurrent selection for high concentration of silk-maysin evaluated by reversed-phase HPLC (Snook et al., 1989). At least one hundred plants were evaluated and selfed in each population for silk maysin concentration within each cycle of selection. Progress was evaluated after completion of six cycles of selection in replicated, randomized complete block experiments in 1997 and 1998. Approximately 10% of the plants with highest silk maysin concentration in each population (10 to 12) were selected for recombination in each cycle. Sixth cycle plants of EPM6 contain 1.6–1.9% silk maysin on a fresh weight basis while those of SIM contain 1.5%–1.9% (Widstrom et al., 1999). Consistent advances of approximately 0.2% maysin per cycle occurred for both populations, resulting in levels that are seven to eight times the concentration necessary to produce a significant impact on first instar larvae of corn earworm (Snook et al., 1989). Trait responses are highly heritable (h2 = 0.65 to 0.68) and will allow plant breeders to introgress resistance to the corn earworm into elite material (Byrne et al., 1998). Both classical and quantitative inheritance procedures may be effectively used to increase or transfer silk maysin concentrations (Widstrom and Snook, 1998).
The sixth cycle populations are designated EPM6 and SIM6, respectively. Plants in the EPM6 population reach pollen shed and silking 52–55 d after planting, while those of SIM6 reach this stage later at 59–62 d. EPM6 has red cobs and purple kernels of flinty or popcorn type, while SIM6 has yellow dent kernels with red cobs. Both have good seed quality. Seed of EPM6 and SIM6 have been deposited in the National Seed Storage Laboratory at Fort Collins, CO, and in the Maize Active Collection at the Plant Introduction Research Unit at Ames, IA. Seed of these populations will also be maintained and distributed by USDA-ARS at Tifton, GA, and can be obtained in 100 g lots by request from USDA, Agricultural Research Service, Crop Genetics and Breeding Research, P.O. Box 748, Tifton, GA 31793-0748. It is requested that appropriate recognition be given if this germplasm contributed to the development of a new breeding line or hybrid.
NOTES
Accepted for publication May 31, 2001.
REFERENCES
This article has been cited by other articles:
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  N.W. Widstrom, B.R. Wiseman, M.E. Snook, G.S. Nuessly, and B.T. Scully Registration of the Maize Population Zapalote Chico 2451F Crop Sci., January 1, 2003; 43(1): 444 - 445. [Full Text]
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