|Holland, Jim - Jim|
Submitted to: Maydica
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/11/2003
Publication Date: 5/1/2003
Citation: HOLLAND, J.B., GOODMAN, M.M. COMBINING ABILITY OF TROPICAL MAIZE WITH ISOGENIC BT AND CONVENTIONAL TESTERS.. MAYDICA. 2003. Interpretive Summary: Genetically modified maize containing Bt genes are engineered to be resistant to lepidopteran insects, including corn borers. Farmers in the U.S. rapidly adopted Bt corn hybrids, with 19% of the U.S. corn acreage planted to Bt hybrids in the year 2000. Corn breeders develop new inbred lines designed to be used to produce hybrid cultivars. Breeders need to know if the value of their inbred lines as hybrid parents depends on whether they are crossed with conventional or Bt lines. This question was investigated by crossing 97 new tropical maize breeding lines to both conventional and Bt hybrids. The Bt and conventional hybrids used in this study differed only by the presence or absence of the Bt gene. The crosses were evaluated in seven environments, and average yields did not differ if lines were crossed to the Bt or conventional hybrid. The Bt hybrid did not have higher yield than the conventional hybrid, but the tropical corn crosses had significantly greater yield than the commercial hybrids, suggesting that tropical corn is a more important source of yield improvement genes than are insect resistance transgenes.
Technical Abstract: Bt transgenes, which confer protection against lepidopteran pests, have been introduced widely into commercial breeding populations. If inbred lines exhibit specific combining ability interactions between conventional and Bt testers, separate breeding programs for inbred lines to be used in conventional and transgenic hybrids may be warranted, complicating current hybrid breeding procedures. This possibility was investigated by testcrossing 97 F2 plants from the cross of two tropical-derived inbred lines, NC296 and NC298, to both isogenic Bt and conventional (non-Bt) hybrids. The tester hybrids are either identical to or closely related to hybrids commercialized in the U.S.A. The 97 Bt testcrosses and 97 non-Bt testcrosses were evaluated in replicated yield trials in seven North Carolina environments along with four commercial checks and the two hybrids per se. The Bt testcrosses had slightly lower mean grain moisture (0.3%) and plant height (0.03 m) than the non-Bt testcrosses, but otherwise did not differ from them. No F2-by-tester interactions were observed for any trait, and estimates of genetic correlations between the same trait measured on different testers were high (rg = 0.85 for grain yield measured on Bt and non-Bt testers). These results suggest that use of Bt and conventional testers will give similar results, simplifying the use of Bt transgenes in maize breeding programs. Whereas Bt transgenes did not improve yield, tropical F2 testcrosses yielded greater than commercial hybrids, suggesting that greater increases in maize yield potential are likely to result from exploitation of exotic maize germplasm than from single gene transformations.