A critical evaluation of the HOPE breeding system as a means for broadening the deployed germplasm base in maize.

摘要

he heirarchical, open-ended, population enrichment (HOPE) maize (Zea mays L.) breeding system was developed with two major purposes: (i) to serve as a base for developing commercially useful inbred lines, and (ii) to diversify to germplasm used in maize breeding programs. The system consists of two population sets (A and B) with contrasting heterotic patterns. Within each set, four populations are arranged in a hierarchical manner based on performance: low (L), intermediate (I), high (H), and elite (E). New germplasm is continually added to the lower levels of HOPE; as germplasm is improved, it is advanced to higher levels in the hierarchy. More stringent recurrent selection procedures are used at each consecutively higher level to shape the diversity existent at the lower levels into amenable germplasm at the E level.;The response to selection in HOPE populations was evaluated following 15 years of selection. The rate of improvement for major agronomic traits in the HOPE populations was comparable to that reported for closed populations, with random drift occurring only at the H level. In most cases, comparisons among different levels did not reveal significant differences. Regression and genetic analyses indicated there was little heterosis between sets in the lower three levels, and more heteorsis at the E level.;Diversity in the HOPE populations was evaluated by using random amplified polymorphic DNA (RAPD). Germplasm at the L level was more diverse than at the higher three levels. Diversity of the populations was enhanced due to introgressions and advancements of material in the hierarchy. The two E populations showed greater divergence from each other than the populations of the lower levels. The HOPE populations were found to contain RAPD polymorphism that was not detected in commercial material.;A revision of the HOPE system was proposed. The new version would have three hierarchical levels