Selection-induced differences among Iowa Stiff Stalk Synthetic strains
Relative performance of nine 'Iowa Stiff Stalk Synthetic' (BSSS) strains of maize (Zea mays L.) was determined by evaluating the strains per se and diallel crosses among the strains. The study also included the S(,1) generation of each strain and each strain cross. Yield differences among the strains and the level of heterosis exhibited by their crosses indicated genetic diversity among the different strains due to selection. Generally, the strains that had the greatest number of cycles of selection performed better and were more genetically divergent. Estimates of inbreeding depression effects were considerably lower for the highly selected strains and for crosses involving the selected strains. Magnitude of estimates of additive effects for yield tended to reflect the selection history of each strain. Estimates of dominance effects indicated that intrapopulation dominance effects were important in all the strains. A reduction in yield improvement due to reduced effective population numbers was detected. Selection for resistance to Ostrinia nubilalis Hubner did not show a significant effect on yield, but a negative correlated response on yield was observed after two cycles of selection for resistance to Diabrotica virgifera Le Conte;Grain yield responses of BSSS strains and strain crosses were accompanied by changes in ability to respond to environmental variations. Recurrent selection generally was effective for increasing yield in all types of environments, particularly in less favorable ones. Selection for pest resistance in BSSS(CB) and BSSS(RW) caused important changes in both yielding capacity and ability to respond to environmental changes.