Improving a hybrid from within itself

Improving a hybrid by transferring partially dominant alleles from one parent to the other is feasible if the median level of dominance is moderate and prediction accuracy is high. Convergent improvement within a P₁ × P₂ single-cross hybrid attempts to transfer partially dominant favorable alleles from P₁ to P₂ and from P₂ to P₁. My objective in this simulation study was to determine if convergent improvement is a feasible procedure to develop a better single cross. Heterozygote effects (d) and homozygote effects (a) at quantitative trait loci (QTL) were resampled from an empirical distribution of genomewide marker estimates of d and a for maize (Zea mays L.) grain yield. The d values were scaled (d scale = 30%, 45%, and 60%) to account for the known upward bias when d/a ratios at markers are used to estimate d/a ratios at QTL. Convergent improvement led to positive responses to selection when only 30 QTL controlled the trait. When 250 QTL controlled the trait, convergent improvement led to a positive response when the d scale was 30% but not when the d scale was 60%. For the intermediate d scale of 45%, responses were positive in later backcross generations and when prediction accuracy was 0.60 or 0.80. The largest response was achieved with only one cycle of genomewide recurrent selection; additional cycles compromised the gains achieved from the final step of phenotypic selection among single crosses. Overall, the results suggested that success in convergent improvement is a possibility but not a certainty in maize, and that more consistent results would be expected in species such as rice (Oryza sativa L.) that show weaker heterosis than maize.