Re-examining the relationship between degree of relatedness, genetic effects, and heterosis in maize

The dominance hypothesis is one of two major genetic hypotheses that have been proposed regarding heterosis in maize (Zea mays L.). This study examines two underlying tenets of the dominance hypothesis: (i) Dominant gene action must occur at many loci in order for heterosis to be expressed; and (ii) genetic diversity is a good predictor of heterosis (i.e., differences in gene frequency are required for the expression of heterosis). To examine these tenets, we used a unique set of genetic materials, sister-line inbred lines. Sister-line inbred lines are highly related inbred lines that are derived from a common parental cross. Three sets of six sister lines were used in this study, ranging between 47 and 77% identical-by-descent (IBD), creating a series of lines that potentially vary in gene frequency. The sister lines were mated using a partial diallel to form sister-line hybrids. The sister-line hybrids and the parental inbred lines were evaluated in replicated yield trials for grain yield, grain moisture, broken stalks, and test weight in five environments. The genotypic variance was partitioned using Gardner and Eberhart's Analysis III to examine additive and nonadditive genetic effects. Three relevant findings regarding heterosis for grain yield can be drawn from our results: Substantial genome-wide heterozygosity is not a requirement for the expression of heterosis, there is not a consistent relationship between degree of relatedness and the magnitude of heterosis, and the presence of nonadditive genetic effects is not a requirement for the manifestation of heterosis.