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Variability of Response to Low Levels of Inbreeding in a Population of Maize¹

Variability of inbreeding response was studied in four sets of eight inbred lines chosen at random from the maize (Zea mays L.) cultivar, ‘Indian Chief’. The eight lines in a set were mated in balanced pedigrees to generate groups of material having expected inbreeding coefficients of 0, 2/16, 3/16, 4/16, and 5/16. (The pedigrees were balanced with respect to the relative contribution of each line). In each pedigree, 20 samples were taken from the generation in which genotypes began segregating, so that 20 subpopulationsw ered erived for each level of inbreeding within each set. This material was evaluated in four environments with respect to grain yield, ear number,and ear height.

All four sets showed significant inbreeding depression in yield. None of the sets showed significant inbreeding depression in ear number, and only two sets showed significant depression in ear height. Inbreeding depression, when significant, was linearly rdated to the increase in F. Quadratic effects were not significant. The linear rate of inbreeding depression in yield was similar in all sets and averaged –0.36 q/ha per 1% increase in F. In all sets, there was much variability in response to inbreeding among the 20 subpopulations of a particular pedigree. The ranges of the 20 subpopulation means about each pedigree varied between 15 and 63% of the pedigree mean. Similar results were obtained with ear number and ear height. The variability was thought to be due to differences in genetic potential for inbreeding depression among the original genotypes sampled and genetic sampling error with inbreeding.

Key Words: Pedigree mating • Inbreeding depression • Yield • Ear number • Ear height

² Research agronomist, ARS-USDA, and assistant professor of crop science; research geneticist, ARS-USDA, and professor of genetics; and professor of genetics; North Carolina State Univ., respectively.

Received for publication September 15, 1976.