Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Arnel R. Hallauer


The objective of inbreeding in maize (Zea mays L.) is to develop pure lines that are used to repeatedly produce an elite hybrid. Inbreeding in maize is accompanied by a reduction in the mean phenotypic value for most traits. The reduction in the mean is caused by an increased number of loci in an individual becoming identical by descent from a common ancestor. This genetic phenomena is known as inbreeding depression;Limited empirical information was available to determine how inbreeding depression rates have changed by recurrent selection in maize populations. The population per se and seven bulked inbred subpopulations (S[subscript]1 to S[subscript]7) of the source populations (BSSSC0, BSCB1(R)C0, and BS26) and the improved populations of BSSSC0 and BSCB1C0 (BS13(S)C3, BSCB1(R)C9, and BSSS(R)C9) were evaluated at five Iowa environments in 1989 and 1990 to determine how selection had changed the inbreeding depression rate for 16 plant and ear traits;The rate of inbreeding depression decreased for all traits in BS13(S)C3, BSSS(R)C9, and BSCB1(R)C9 except for yield in grams per plant, 300-kernel weight, and days-to-anthesis in BS13(S)C3, yield in Mg ha[superscript]-1 in BSSS(R)C9, and yield in grams per plant, number of ears per plot, 300-kernel weight, and ear-leaf width in BSCB1(R)C9. Highly significant differences (P ≤ 0.01) in the rate of inbreeding depression for BS13(S)C3 and BSSS(R)C9 were detected for cob diameter, number of ears per plot, 300-kernel weight, kernel-row number, and days-to-anthesis. Differences in the inbreeding depression rates for BS13(S)C3 and BSSS(R)C9 were significant (P ≤ 0.05) for stand, stalk lodging (%), and plant height. The rate of inbreeding depression for grain yield in BSSS(R)C9 (3.06 Mg ha[superscript]-1) and BS13(S)C3 (2.95 Mg ha[superscript]-1) were similar and basically unchanged from the 3.14 Mg ha[superscript]-1 rate for BSSSC0. The rate of inbreeding depression for grain yield declined significantly (P ≤ 0.01) from 3.24 Mg ha[superscript]-1 in BSCB1(R)C0 to 2.96 Mg ha[superscript]-1 in BSCB1(R)C7;The reduced rates of inbreeding depression for most traits suggest that the improved populations of BSSS and BSCB1 are segregating at fewer loci, or selection has increased the allele frequencies beyond 0.5.



Digital Repository @ Iowa State University,

Copyright Owner

David Lee Benson



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235 pages