Estimates of Genetic Variability in F2 Maize Populations

Maize (Zea mays L.) breeders emphasize selection within F2 populations derived from crosses of inbred lines: Studies of the inheritance of quantitative traits in maize have been conducted primarily for generically broad-based populations. Objectives of our study were to estimate the generic variability in F2 populations developed from crosses of related and unrelated lines and to determine the effects of five generations of random intermating of plants within F2 populations on the estimates of genetic variability. Estimates of additive genetic variability were greater in the unrelated line crosses, but the estimates were not significantly different before and after random intermating within both crosses. Estimates of dominance variance decreased with random mating, suggesting that linkage effects were affecting the estimates. For applied breeding programs, it seems that adequate genetic variability was available in both types of crosses and that five generations of random intermating were not effective for increasing genetic variability

Response to Reciprocal Full-sib and Mass Selection in Corn (Zea mays L.)

Responses to mass selection for number of ears per plant and reciprocal full-sib selection for grain yield in 2 synthetic varieties of corn (Zea mays L.) were determined. Ten cycles of mass selection were effective for increasing number of ears per plant, but no correlated response for grain yield improvement was realized. Reciprocal full-sib selection was effective in increasing grain yield of the 2 parental varieties and their cross and in increasing number of ears per plant. No evidence was obtained that indicated genetic variability decreased in the parent varieties after 4 cycles of reciprocal full-sib selection or after 5 cycles of selection for the interpopulation full-sib crosses. Reciprocal full-sib selection was more effective than mass selection for improvement of other agronomic traits

Expression of Heterosis in Leaming and Midland Corn Belt Dent Populations

Successful maize (Zea mays 1.) breeding programs are based on specific heterotic patterns. The Iowa Stiff Stalk Synthetic x Lancaster Sure Crop heterotic pattern is widely exploited in U.S. maize breeding programs. Alternative U.S. heterotic patterns have been suggested, but they have received limited attention and improvement. The objectives of this study were to evaluate cultivar crosses between Leaming and Midland Yellow Dent cultivars before and after inbred progeny selection and to assess their potential as an alternative U.S. heterotic pattern. Grain yield had a midparent heterosis value of 4.9% before selection and 17.8% in the third cycle of selection. The increase of grain yield heterosis (12.9%) through selection was highly significant (P \u3c 0.01), and crosses between S1 generations showed an increased rate of inbreeding depression through selection. Grain moisture of the cross (20.1 %) was significantly (P \u3c 0.05) less than the Midland Yellow Dent (24.5%) cultivar but was not significantly greater than the Leaming (17.7%) cultivar. Root lodging strength was significantly improved through selection with 1.1 % root lodging of the cross after selection (midparent heterosis value of -73.2%). Intrapopulation recurrent selection was successful for increasing the heterotic expression between Leaming and Midland Yellow Dent maize cultivars. An interpopulation recurrent selection program between the improved selection cycles Leaming and Midland Yellow Dent seems desirable

Relation Between S2 and Later Generation Testcrosses of Two Corn Populations

Determination of the relative combining abilities of corn (Zea mays L.) inbred lines is an important feature of applied corn breeding programs. Combining ability is measured by the relative performance of a line in testcrosses to one or more testers. Inbred lines from BS13(S2)Cl and BSCB1(R)C7 corn populations were evaluated at the S2 and later generations of inbreeding. Intense selection was practiced among and within lines during inbreeding to develop the S5-equivalent and S8 generation lines. The objective of this study was to determine if the combining ability of lines in early generations (S2) of inbreeding was similar to the combining ability of lines at later generations of inbreeding. Testcross trials were conducted at four Iowa locations. Genetic correlations between the S2 and later generation testcrosses for gram yield were 0.9.7 for BS13(S)Cl and 0.86 for BSCB1(R)C7. The S2 testcross data were highly predictive of S8 testcross data, suggesting that early testing was effective in discriminating among these lines for relative combining abilities at later generations of inbreeding

Inheritance and Number of Genes Affecting Quantitative Traits Within F2 Maize Populations

Populations derived from crosses of elite genotypes are commonly used in plant breeding programs. Knowledge of the genetic variation among individuals and their progenies within F2 populations is essential for effective selection of important traits. The genetic variation and number of effective factors affecting the expression of quantitative traits within 30 F2 maize (Zea mays L.) populations were determined by different methods suggested for study of F2 populations. Estimates of heritability and number of effective factors varied among F2 populations for each trait, primarily because of the differences in trait expression between parents of crosses and the environmental effects upon individual plants. Average estimates of heritabilities for grain yield ranged from -0.03 to 0.63 among methods of estimation and ranged from 0.02 to 0.75 among methods of estimation for the same F2 population. The range of estimates of heritability and gene number among crosses and methods of estimation suggests that estimates are unique for a specific F2 population and for a specific method of estimation. If the newer techniques of marker-assisted selection are applied to F2 populations, the effectiveness of marker-assisted selection may vary among F2 populations

Mass Selection for Adaptation in Antigua Maize (Zea mays L.) Composite

Tropical maize (Zea mays L.) germplasm has had limited use in temperate areas because of photoperiod sensitivity. Antigua germplasm from the island Antigua was introduced co Iowa because of its resistance to important pests of maize and its heterosis exposed m crosses with ocher tropical varieties. Mass selection procedures were used to adapt the Antigua Composite germplasm co Iowa conditions. After. Six cycles of mass selection for earlier silk emergence, the selected Antigua Composite flowered 17 days earlier than the original composite. Grain yield and ocher agronomic traits of the adapted scram of Antigua were similar co chose of adapted varieties