Principles underlying genetic improvement for high and stable crop yield potential

This paper explores the genetic basis of high and stable crop yield, and delineates the conditions that provide the link between genotypic and phenotypic superiority and, consequently, improve selection efficiency in plant breeding. Response to selection is viewed as the result of the conversion, through intralocus recombination, of two heterotic nonadditive allelic loci into a single super additive locus, and fixation of the latter. This concept connects the important phenomena of vigor, degeneration, and response to selection through a unifying genetic basis. Heterozygosity of nonadditive allelic loci leads to the nonheritable repulsion-phase vigor. Heterozygosity of a super additive locus leads to the partially heritable coupling-phase vigor. Homozygosity of the super additive locus leads to the fully heritable inbred vigor, which is responsible for genetic gain. A reliable explanation of heterosis is the one that considers repulsion- and coupling-phase vigor as being mutually essential and inter-convertible. Response to selection is the outcome of the evolution of hybrid to inbred vigor. The superiority of inbred vigor is due to the improved additive allelic complementation, that at the same time enhances epistatic interactions. The full exploitation of the above concept leads to maximization of homeostatic crop yield and can be successfully achieved using the principles of honeycomb breeding. These include clarification of the negative role of competition on: (1) crop yield, and (2) selection efficiency, and explain why crop yields are maximized when cultivars are monogenotypic. The unit of evaluation and selection becomes the individual plant grown at the critical distance, where the range of phenotypic expression is enlarged, and the negative effect of competition on selection efficiency is eliminated. Honeycomb breeding uncouples the reliable selection for yield and stability from the visual evaluation that predominates during the critical early generations of selection, and offers the transition from single-trait evaluation to whole-genome phenotypic evaluation. The concept of whole-genome phenotypic evaluation recognizes that genes controlling crop yield concern the genome as a whole and belong to three categories: (1) genes that control yield potential per plant and expand the lower limit of the optimal plant density range; (2) genes that confer tolerance to biotic and abiotic stresses and expand the upper limit of the optimal plant density range; (3) genes that control cultivar responsiveness to inputs. The outcome of selection, based on whole-genome phenotypic evaluation during all generations of a breeding program, is high yielding, stable, and density-independent cultivars. (C) 2002 Elsevier Science B.V. All rights reserved.