Comparison of Testcrosses Produced from F₂ and First Backcross Populations in Maize

Abstract

Single crosses and backcrosses from established lines are importeat parental sources for developing new inbred lines in hybrid maize (Zea mays L.) breeding. The objectives of this study were (i) compare, both theoretically and experimentally, the testcross properties of F2 vs. first backcross populations; and (ii) to outline criteria for choosing between these two types of base populations in recycling breeding programs. Two homozygous lines (P1, P2) and individual plants from their F2 and first backcross (BC1, BC2) generations were mated to an unrelated single‐cross tester. A total of 186 testcross progenies were evaluated for grain and forage traits at three and two locations, respectively. The five generations (P1, P2, F2, BC1, BC2) differed significantly (P < 0.05) in their testcross means (X̄) for grain (GY) and forage (FY) yield. A nonepistatic model accounted for 92 to 99% of the variation among testcross generation means for five of the six traits examined, yet the presence of epistasis was indicated in several instances. In accordance with theory, the ratio of BC (the pooled estimate of BC1 and BC2) to F2 genotypic variance among testcross progenies (σ2g) was close to 0.50 for all traits but one. Heterogeneous σ2g estimates in BC1 and BC2 were found for GY, FY, and grain dry matter content, suggesting the presence of epistasis. Predicted selection response for testcross performance (ΔG) was greatest in the F2. Testcross progenies of the F2, BC1, and BC2 generations displayed similar phenotypic and genotypic correlations as expected by theory. The “usefulness” criterion U = x̄ + ΔG was employed to assess the breeding prospects of each generation. With weak selection, the better backcross had superior usefulness for four traits, whereas with strong selection the F2 had highest usefulness for all traits except GY.

Predicting Genetic Variance from Genomewide Marker Effects Estimated from a Diverse Panel of Maize Inbreds

“…The usefulness criterion (U p ), which is the expected genotypic mean of superior progenies in a breeding population, is a logical criterion for identifying promising crosses (Schnell and Utz, 1975;Melchinger et al, 1988). A large number of parental lines leads to an excessively large number of possible breeding populations.…”

Breeding Potential of Intra‐ and Interheterotic Group Crosses in Maize

“…To account for differences in both the mean and V TC among populations, the predicted mean of the best 5% of testcrosses ( x 0.05) in each F 2 population was calculated (Melchinger et al, 1988) where k 0.05 was the standardized selection differential for a selected proportion of 5%. The value of k 0.05 was 2.06 for grain yield and –2.06 (i.e., selection for lower values) for grain moisture and stalk lodging.…”

Doubled haploids in tropical maize: II. Quantitative genetic parameters for testcross performance

“…It combines information on the mean and the predicted selection response in a given population and, thus, depends on properties of the source germplasm and on the selection intensity. While information on the usefulness criterion for different types of source germplasm is scarce in temperate maize germplasm (Melchinger et al 1988;Lamkey et al 1995;Fountain and Hallauer 1996;Seitz 2005), virtually no information exists for tropical germplasm.…”