Quantitative Genetics of Inbreeding in a Synthetic Maize Population

genetic values of independent lines with inbreeding have revealed large variability among lines (Bartual and HalThe average effects of inbreeding depression have been measured lauer, 1976; Cornelius and Dudley, 1974, 1976; Obilana extensively in maize (Zea mays L.), but the influence of inbreeding and Hallauer, 1974) and variability in inbreeding depreson genetic variance has not been well studied. Two hundred random S1, sion (Sing et. al., 1967). However, with the exception of S2, S3, and S4 lines were developed from the BS13(S)C0 population by Cornelius and Dudley (1974, 1976), variability attribsingle-seed descent and a set of 200 related half-sib families were deutable to inbreeding in these studies could not be veloped from the S1 lines. The lines and half-sib families were evaldescribed in terms of any quantitative genetic effects. uated in replicated yield trials for six agronomic characters. Under a Cornelius and Dudley (1974, 1976) and Coors (1988) purely additive model, the expected variance among inbred individuals is exactly twice the variance of noninbred individuals. The observed provided some quantitative genetic analysis of the efvariance of inbred individuals in our study was 1.18 times the variance fects of inbreeding on genetic variability in maize. Both of noninbred individuals or less for five of six traits studied. By constudies found that dominance deviations of inbred inditrast, variance of dominance deviations of inbred individuals ranged viduals became negatively correlated with their breedfrom 1.6 to 3.3 times the variance of dominance deviations of nonining values, whereas dominance deviations and breeding bred individuals for five of six traits studied. A negative covariance values are independent in noninbred individuals by defibetween dominance deviations and breeding values in inbred individunition. This finding provides some insight into how inals was found for all six traits. An estimator of the degree of dominance breeding affects inheritance of quantitative traits, but for arbitrary allele frequencies was developed. The estimated average clearly better insights would be useful. The work of Cordegree of dominance in BS13(S)C0 ranged from 1.28 to 2.76, correnelius and Dudley (1974, 1976) and Coors (1988) did sponding to overdominance or pseudo-overdominance. Our results sugnot permit much additional information. Coors (1988) gested that some regions of linked genes have large effects on inbreedhad only a single generation of inbreeding which limited ing depression in this population. the number of estimable quantitative genetic parameters. Cornelius and Dudley’s (1974, 1976) mating design was inadequate to resolve all of the desired genetic efI depression in maize is a ubiquitous phefects (Cornelius, 1988). Shaw et al. (1998) evaluated five nomenon found in all populations and for most meatraits in a natural population of Nemophila menziesii surable traits. Significant inbreeding depression was Hook. & Arn. and also found a trend towards negative association between breeding values and dominance defound for 19 of 22 phenotypic and agronomic characters viations in inbred individuals, although none of the coevaluated in six agronomic studies of inbreeding in maize variance estimates were significantly less than zero. In ad(Benson and Hallauer, 1994; Cornelius and Dudley, 1974; dition to the negative association with breeding values, Good and Hallauer, 1977; Hallauer and Sears, 1973; San Shaw et al. (1988) found that dominance deviations of Vicente and Hallauer, 1993; Walters et al., 1991). All inbred individuals were numerically (no hypothesis test of these studies found the decrease in population means available) larger in magnitude than dominance deviawith inbreeding was a linear function of the inbreeding tions of noninbred individuals for four out of five traits. coefficient. Linear regression on the inbreeding coeffiGallais (1984) concluded in a study of inbreeding and cient accounted for 98% or more of the variation among crossing in alfalfa that nonadditivity was more important inbred generations for grain yield and 90% or more of in inbred relatives than it appeared to be in noninbred the variation among generation means for all traits other relatives. The study of Gallais (1984) did not address spethan grain yield. Non linearity of changes in population cific quantitative genetic components to the degree of means in the inbreeding coefficient is a function of epiother studies. static gene action (Crow and Kimura, 1970, p. 80; KempGenetic effects of interest to breeders, namely breedthorne, 1957). ing values and dominance deviations of individuals, are functions of the action of alleles at individual loci. In Average effects of inbreeding in maize, i.e., changes particular, inbreeding depression is an outcome of direcin the population mean with inbreeding, are well undertional dominance, which the historical literature in maize stood. However, studies of changes in genetic variance has shown to be quite important. Estimates of the dewith inbreeding have been rare. Studies of changes in gree of dominance of genes affecting quantitative traits have nearly always been greater than one, correspondJ.W. Edwards, Monsanto Company, 101 Tomaras Ave., Savoy, IL ing to overdominance, in biparental F2 populations (Gard61874; K.R. Lamkey, USDA-ARS, Dep. of Agronomy, Iowa State ner et al., 1953; Gardner and Lonnquist, 1959; Han and Univ., Ames, IA 50011-1010. Joint contribution from the Corn Insects Hallauer, 1989; Moll et al., 1964; Robinson et al., 1949). and Crop Genetics Research Unit, USDA-ARS, Dep. of Agronomy, Iowa State Univ. and Journal Paper No. J-18342 of the Iowa Agric. Random mating of F2 populations to reduce linkage and Home Econ. Exp. Stn., Ames, IA. Project No. 3495, and supported disequilibrium, however, generally has reduced the estiby Hatch Act and State of Iowa funds. Part of a dissertation submitted mate of the degree of dominance to approximately one by J.W. Edwards in partial fulfillment of requirements of the Ph.D. or less, corresponding to partial or complete dominance degree at Iowa State University. Received 1 Mar. 2001. *Correspond(Gardner and Lonnquist, 1959; Han and Hallauer, 1989; ing author ([email protected]). Moll et al., 1964). The objectives of our study were to dissect genetically Published in Crop Sci. 42:1094–1104 (2002).