Doctoral Dissertations

Date of Award

8-1971

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Plant, Soil and Environmental Sciences

Major Professor

L. M. Josephson

Committee Members

Vernon H. Reich, J. W. Hilty, R. R. Shrode, Gordon E. Hunt

Abstract

The purpose of this study was to determine the types of gene action which are important in the inheritance of resistance to maize dwarf mosaic virus (MDMV) in corn (Zea mays L.) inbreds and hybrids. Two experiments were conducted; one on diallel crosses, and the other on generation means involving one cross. A set of 45 F1 crosses derived by diallel mating among five resistant inbreds (Tx601, Ga209, Mo18W, T232, and Ky226) and five susceptible inbreds (CI872, T218, T105, Ky27, and T13) together with the inbreds, formed the populations for the diallel experiment. The parents, F1, F2, F3, and first and second backcrosses to the parents of the cross Ga209 x T105 formed the populations for the generation experiment. These populations were grown under con-ditions of artificial (mechanical inoculation) and natural infections at Knoxville and Waverly, Tennessee, in 1970. A total of 21,140 plants were classified for MDM symptoms at the seedling, tasseling, and maturity stages during the life cycle of the crop using different MDM severity rating scales for each stage. The diallel data were analyzed for combining ability, Vr, Wr graphs, components of variance (D, H1, H2, and F), and heritability. The generation mean data were analyzed for gene number, genetic variances (D and H), and gene effects. In addition, results on correlation and regression involving MDM ratings and agronomic characters were also obtained. Most of the genetic variance from combining ability was of the additive type. Evidence for nonadditive variance also was present. Analyses of Vr, Wr showed largely additive gene action; however, the intercept (a) was indicative of some degree of dominance. Substan-tial evidence for epistasis was not detected. T232 and Ky226 carry mostly dominant genes for resistance, while T218 and T13 carry mostly recessive alleles for susceptibility. There were more recessive genes for susceptibility; but some dominant genes for resistance seemed also to be present in the inbred C1872. The results from components of vari-ance, analysis (D, H1, H2, and F) were similar to those of Vr, Wr graphs. Mostly additive variance and partial dominance were detected for all MDM ratings. Heritability estimates obtained by a regression analysis of diallel data ranged from 60.4 to 81.1 percent. Analysis of MDM ratings for the cross Ga209 x T105 and its progeny, showed dominance in Ga209 for inheritance of resistance to MDMV. Calculated gene numbers ranged from 1 to 3. A minimum of 2 gene pairs interacting either in an inhibitory epistatic (13:3) ratio or more likely duplicate epistatic (15:1) ratio manner was inferred in the cross Ga209 x T105. Heritability estimates derived from the components of variance of F3 progenies ranged from 43.9 to 80.4 percent. Analysis of components of variance (D, H, E1, and E2) revealed largely dominance variance (H) for the cross Ga209 x T105. Estimates of (H/D)½ indicated over-dominance for all MDM ratings. On the other hand, significant amounts of additive, and additive x dominance genetic effects were detected in the analysis of generation means. Based on the signs-of dominance, and dominance x dominance gene effects, duplicate epistasis was inferred in the cross. Significant midparent heterosis and inbreeding depressions obtained for the cross, confirmed the impor-tance of dominance gene action. On the basis of results obtained in this study, mostly additive genetic variance for inheritance of resist-ance to MDMV for the diallel hybrids and dominance for the cross Ga209 x T105 were indicated. A recurrent selection procedure to enhance resistance to MDMV in open pollinated varieties and synthetics is suggested, and a recurrent backcrossing procedure is suggested to transfer resistance to suscep-tible elite inbreds.

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