Genetic Diversity, Heterotic Grouping, and Testers in Hybrid Maize Production

Abstract

Heterosis, the superior performance of a hybrid over the average of its two parents (known as mid-parent heterosis) or over the better parent (known as heterobeltiosis), is an indispensable concept in maize breeding, especially in the hybrid development programs. The level of expression of heterosis is determined by genetic diversity of the parental lines, type of gene action controlling the trait under consideration, type of testers used to evaluate the parental lines, and environmental factors. At the initial stages of genetic improvement of maize, statistical and quantitative determination of heterosis was established. The concept of combining ability was introduced and used to identify parental lines whose hybrids were highly heterotic. Combining ability estimates have also been used to classify maize germplasm into heterotic groups and heterotic patterns to maximize maize productivity in hybrid combinations. Recently, molecular approaches have been used to refine genetic diversity and combining ability studies and this has resulted in higher hybrid maize productivity at relatively faster and cheaper rates. Presently trending are two efficacious models, one of which was developed by IITA maize breeders and their national scientist collaborators for heterotic classification of inbred lines in hybrid production. Details of the different approaches, along with empirical evidence for classification of early and extra-early maize germplasm into heterotic groups in SSA, are presented in this Chapter.