Epistasis for agronomic traits in elite maize hybrids
Certain maize (Zea mays L.) inbred lines are more successful than others in forming elite hybrids. This study was conducted to determine whether epistatic interactions play a significant role in hybrid performance, whether an epistatic or non-epistatic model satisfactorily explains the variation observed, and whether the selected models are appropriate for detecting epistasis. Statistical epistasis was measured with a modified generation means model using testcrosses. The non-epistatic model included components for the main effect and an additive effect. In addition to these terms, the epistatic model included an additive-by-additive epistatic component. Six progeny generations (P1, P2, F1, F2, and a backcross from the F1 to each parent) were produced for all possible hybrids of a five-parent diallel in both the Iowa Stiff Stalk Synthetic (BSSS) heterotic group and the non-BSSS heterotic group. Two testers were hybridized to each of the 10 possible hybrid progeny sets in the BSSS group while two different testers were hybridized to the non-BSSS group. Each testcross progeny set was evaluated in both 1999 and 2000 at five locations. The non-epistatic model accounted for a large amount of the variation in generation means and generally fit the data well as most Chi-square tests were not significant. Epistasis was not detected as frequently as expected based on previous studies. Of the 40 maize testcross progeny sets studied, five resulted in a significant epistatic effect for grain yield. Four of the significant epistatic effects showed evidence of linkage, while the remaining significant case was due to unlinked epistatic effects. Our results suggest that detection of epistasis using testcross generation means models depends on the relatedness of the parents in the hybrid cross and the tester used. In addition, lines not highly selected for specific combining ability may not be expected to express epistasis as a fitness advantage. Epistasis in specific crosses, however, can lead to superior productivity and may be enough cause for modifying breeding programs to accommodate accumulation of favorable epistatic effects.