Date of Award
Doctor of Philosophy
Kendall R. Lamkey
Second cycle breeding is characterized by the improvement of an elite line through the addition of favorable alleles from a complimentary line. Second cycle breeding, utilizing backcross and F[subscript]2 generations, has become the predominant type of line development program in the U.S. maize industry. Epistasis and linkage have been identified as possible limits to progress from selection in F[subscript]2 and backcross populations;Objectives of this study were: (1) to provide empirical data comparing the testcross progenies of the F[subscript]2, F[subscript]2-Syn 8, and first backcross generations for means, genotypic variances, heritabilities, frequency distributions, selection responses, and trait correlations, (2) to examine the importance of epistasis in the generation testcrosses derived from inbred lines B73, B84, and Mo17, (3) to examine changes in the testcross population derived from the F[subscript]2 generation following eight generations of random mating, and (4) to recommend the superior type of segregating population to develop from the cross of elite inbred lines B73 and B84;Significant positive digenic epistatic effects were detected for grain yield. The estimate of epistasis obtained by weighted least squares regression of generation means was 0.20 Mg ha[superscript]-1. Ear height also contained significant digenic epistatic effects and evidence for higher order epistasis was detected. Epistatic effects were not significant contributors to genetic variation for any other traits. Due to a lack of significant genetic variance in the superior backcross generation, epistasis was concluded to be of minor importance in the choice of segregating population to develop for selection of superior yielding lines;The F[subscript]2 population had significantly greater mean grain yield (0.27 Mg ha[superscript]-1) while significantly lower genetic variance from 5.80 x 10[superscript]-2 to 9.02 x 10[superscript]-2 in the F[subscript]2-Syn 8 population. Random mating was considered of little value, however, considering the increased time to produce progeny for evaluation when selection is for individuals with increased grain yield;Applied breeding programs generally utilize a high selection intensity for grain yield in elite by elite segregating populations. The superior type of segregating population to develop from the cross of inbreds B73 by B84 under high selection intensity for grain yield was the F[subscript]2 generation. The backcross to B84 was the superior type of segregating population under low selection intensities.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Bruce J. Schnicker
Schnicker, Bruce J., "Comparison of genetic variances in F2 and backcross populations of maize " (1992). Retrospective Theses and Dissertations. 9950.