An Analysis of Heterosis vs. Inbreeding Effects Medicago Satna L. with an Autotetraploid Cross-fertilized Plant:

Self-fertilization and crossing were combined to produce a large number of levels of inbreeding and of degrees of kinship. The inbreeding effect increases with the complexity of the character and with its supposed relationship with fitness. A certain amount of heterozygosity appears to be necessary for the expression of variability. With crossing of unrelated noninbred plants, genetic variance is mainly additive, but with inbreeding its major part is nonadditive. High additivity in crossing, therefore, coexists with strong inbreeding depression. However, even in inbreeding the genetic coefficient of covariation among relatives appears to be strongly and linearly related to the classical coefficient of kinship. This means that deviations from the additive model with inbreeding could be partly due to an effect of inbreeding on variances through an effect on means. An attempt to analyze genetic effects from a theoretical model, based upon the identity by descent relationship at the level of means and of covariances between relatives, tends to show that allelic interactions are more important and nonallelic interactions are less important for a character closely related to fitness. For a complex character, these results lead to the conception of a genome organized in polygenic complementary blocks integrating epistasis and dominance. Some consequences for plant breeding are also discussed. H E T E R O S I S , or its corollary of inbreeding depression, plays as important a role in natural selection as it does in artificial selection. In spite of molecular genetic advances, however, the explanation of heterosis at the whole organism level has not progressed much since the first studies (SHULL 1914). The study of heterosis has relevance for population and quantitative genetics and for biochemical, physiological and ecological genetics. In this paper I use a biometrical approach for studying the phenomenon at the level of a single population, and I examine the influence of mating system (inbreeding followed by the crossing of unrelated individuals) on the means and variances of several quantitative characters. I hope that such studies will lead to testable hypotheses regarding the organization of the genome caused by natural selection, with concomitant implications for plant breeding. Although the effects of heterosis or inbreeding are often studied for means, Present address: I.N.A. P.-G., G.I.S. Moulon, 91 190 Cif sur Yvette, France. Genetics 106: 123-137, January, 1984.