History and Modern Synthesis of Heterosis

George Shull coined the term heterosis in 1952 and defined the concept independently of genetic mechanism. Subsequently, heterosis has been defined formally on the basis of the difference in phenotype between a hybrid cross and its parents. The underlying genetic and molecular basis of heterosis remains a very challenging and important problem. Quantitative genetic models that clearly delineate underlying genetic contributions to heterosis and inbreeding depression enable clear genetic inferences to be drawn from experiments on heterosis. In this talk, genetic models for heterosis and inbreeding depression will be presented in an integrated framework to help clarify relationships among genetic effects, heterosis, and inbreeding depression. An argument is made that the study of heterosis and inbreeding depression can enhance our understanding of the genetics, and subsequently molecular basis, of heterosis. It is also suggested that focusing ongoing molecular research on the molecular basis of nonadditive gene action, dominance and epistasis, and inferring molecular mechanisms of heterosis from the molecular basis of nonadditive gene action may provide greater generality to heterosis research, especially with different population structures.