A Review of the Evidence - Search CIMMYT repository

(2)␣envir onmental conditions likely to impede the physical dispersal of pollen, e.g., lack ofwind, heavy rainfall, and high relative humidity. 5Third, deviations from the simulation results would be expected when epistatic effects aresignificant. Although experimental evidence suggests that epistatic effects are ofteninconsequential, they appear to be more important in hybrids (and especially single-crosshybrids) formed using highly inbred lines (Hallauer and Miranda, 1989).Fourth, deviations from the simulation results could also result from strong G x E effects.Experimental evidence shows that because of buffering effects associated withheterozygosity, maize hybrids typically show smaller G x E interactions than inbred lines(Shank and Adams, 1960). For this reason, the performance of heterozygotic materials tendsnot only to be superior to that of homozygous materials, but in the presence of high levelsof environmental variability, it also tends to be more stable. The longer farmers recyclemaize seed, the more inbred their material is likely to become – and hence the moreunstable. To the extent that recycled conventional hybrids based on highly inbred linesshow greater interactions with environmental factors than recycled non-conventionalhybrids based on more heterozygous parents, the simulation results could diverge from theperformance observed in farmers’ fields.Empirical evidence on the effects of segregation in farmers’ fieldsTo what extent are the predicted levels of inbreeding depression in different types of maizehybrids (calculated on the basis of quantitative genetics theory) borne out by empiricalevidence? A considerable amount of research has focused on genetic changes in advancedgenerationhybrids. The topic is of obvious practical interest, given the rising popularity ofhybrids around the world. Evidence from a number of developing countries is presentedbelow. An effort has been made to distinguish between results obtained from on-farm trialsinvolving farmers’ management and results obtained from on-station trials conductedunder controlled conditions. For simplicity, we have chosen to concentrate on yield effects,although results are frequently reported for other characteristics as well.5 The amount of unavoidable inbreeding that occurs in a randomly mating population of maize plants is determined by thenumber (N) of unrelated individuals in the population whose gametes unite at random in every generation. Li (1974) showsthat the probability that a given gamete will unite with a gamete from the same individual is 1 / N, while the probabilitythat it will unite with a gamete from a different individual is (N-1) / N. Building on Li’s formula, Sprague and Eberhart(1977) propose the following formula for effective population size:Ne = 2N / (1 + Fp) (6)where:Ne = effective population sizeN = number of plantsFp = the inbreeding coefficient of parent plants (empirical investigation revealed that Fp ranges from 0 for non-inbredparents to 1 for highly inbred lines).Thus, the greater the number of individual plants and the lower the levels of inbreeding in those plants, the larger theeffective population size. Hallauer and Miranda (1981) use this formula to show the effect of population size on inbreedingand resulting performance. For our application, smaller effective population sizes would be more likely with conventionalhybrids, which tend to be based on highly inbred lines and typically have a narrower genetic base than non-conventionalhybrids.39