A Review of the Evidence - Search CIMMYT repository

est in most studies, it may be dangerous to conclude that traits such as vigor and yield areexpressed only in additive fashion. Cancellation of inter-locus interactions (epistasis) mayprevent detection of non-additive variation.Unlike most previous researchers who had focused on crosses made among inbred lines,Pollak et al. (1957) studied heterosis and inbreeding depression in crosses made among threeOPVs. In their study, the F1, F2, and backcross populations of the OPVs were used toestablish different levels of heterozygosity. After determining that the mean yield of the F2plants lay in between the mean yield of the parents and the mean yield of the F1 plants,Pollak et al. concluded that their results were consistent with an additive genetic model (withdominance) and that epistatic effects were minimal.Shehata and Dhawan (1975) evaluated inbreeding depression among three sets of diallelcrosses; each set included 10 parents that were either OPVs or synthetics. In comparing theyields of F1 and F2 plants, the average inbreeding depression for each of the three diallelswas estimated to be 10.2%, 11.4%, and 11.2%. The deviation in yield performance of the F2plants from the mean performance of the parental varieties and the F1 variety crosses wassmall in all instances, suggesting largely additive effects among loci, with limited netepistatic␣ef fects.Most studies in the plant breeding literature confirm that the arithmetic model best predictsthe decline in yield performance observed between F1 and F2 plants. In a comprehensivereview, Hallauer and Miranda (1989) conclude that the yield decreases observed when F1plants are self-pollinated (inbred) to produce F2 plants generally are smaller when the F1plants are OPVs, rather than inbred lines. While epistatic effects are generally minimal, theytend to be greater in line-based crosses compared to variety-based crosses. Hallauer andMiranda remark, “This difference seems reasonable because of the nature of the materialincluded in the crosses. For varieties this would involve an array of genotypes included in thecrosses, whereas crosses of inbred lines would involve the expression of two specificgenotypes.” Extrapolating from this observation, one would expect that the inbreedingdepression observed in three-way crosses, double crosses, and non-conventional hybridsformed using one inbred line to fall somewhere in between that observed in single crossesand variety crosses.Mather and Jinks (1971) propose equations derived from Wright’s original equation to predictthe performance of F2, F3, F4, and subsequent generations of plants in self-pollinated crops.For quantitative traits, the mean of F2 generation plants (obtained by selfing or sib-mating F1plants) can be calculated as follows:where:F2 = (1/4) (P1 + P2 + 2F1) (2)F2 = the mean of F2 generation plants,F1 = the cross-mean F1 generation plants,P1 = the mean of the first parent line (assumed to be completely homozygous), andP2 = the mean of the second parent line (assumed to be completely homozygous).35