Analysis Of Combining Ability And Heterosis For Development Of Hybrid Maize Breeding Strategies Using Diverse Germplasm Resources

Maize(Zea mays L.)is one of the most important crops for staple food,livestock feed,edible oil and biofuel.Utilization of heterosis is of great importance for agricultural production and one of the most successful examples in crops is from maize.Therefore,the better understanding of the genetic basis of heterosis and combining ability we can get,the more effective maize improvement programs and hybrid performance prediction can be achieved.Diverse germplasm from different ecotypes,including temperate and tropical maize,should have been used for identifying genetic variation for both basic research and commercial breeding.Considering great genetic diversity existing in tropical maize germplasm that could contribute to further genetic improvement,more studies are required by using between-ecotype hybrids.Therefore,large-scale analysis of heterosis and combining ability using diverse germplasm resources will improve our understanding of hybrid performance significantly,thus contributing to increased genetic gain in maize hybrid breeding.Understanding combining ability and heterosis among diverse maize germplasm resources is important for breeding hybrid maize.Using 28 temperate and 23 tropical maize inbreds that represent different ecotypes and worldwide diversity of maize germplasm,we first developed a large-scale multiple-hybrid population(MHP)with 724 hybrids,which could be divided into three subsets,325 temperate diallel hybrids and 136 tropical diallel hybrids generated in Griffing IV,and263 temperate by tropical hybrids generated in NCD II.All the parental lines and hybrids were evaluated for 11 traits in replicated tests across two locations and three years by combining multimethods of genetic analysis,including genome-wide association study and genomic selection.The results are as follows:(1)Potential application of multiple-hybrid populations The MHP used in this study,with a complex genetic background and rich genetic diversity,was developed with Griffing IV diallel and NCD II designs using temperate and tropical elite maize inbreds as parents,which is suitable for analysis of combining ability and heterosis and can be used for breeding different ecotypes by taking the advantages of diverse maize germplasm resources.Both diallel and NCD II designs can provide detailed genetic information,including dominance-recessiveness relationships and genetic interactions.In this study,the MHP was used to reveal useful information about combining ability,heterosis,hybrid performance and genotype × environment interaction.The results of the present study not only contribute to developing breeding strategies,but also improve breeding efficiency by using both temperate and tropical maize to broaden genetic basis.Large sets of parental lines with available genotypic information can be shared and used in worldwide hybrid breeding programs through an open-source breeding strategy.(2)Utilization of combining ability and heterosis in hybrid maize breedingSeveral widely used inbreds showed strong general combing ability(GCA),and their derived hybrids showed strong specific combining ability(SCA).Heterosis is quantifiable,trait-dependent and environment-specific,and the responses of parental lines and their hybrids to environments resulted in various levels of heterosis.For all the tested traits,except plant height and hundred grain weight,temperate by tropical hybrids showed a higher level of average heterosis than the temperate and tropical diallel hybrids,with higher hybrid performance for ear length,ear diameter and hundred grain weight.Tropical maize germplasm can be used to improve the yield potential for temperate maize.Grain number per row and grain number per ear were two most important traits that determined yield heterosis,which can be used as direct selection criteria for yield heterosis.The hybrids from heterotic groups,Reid × SPT,Reid × LRC and SPT × PA,contributed highly significant positive SCA effects and strong heterosis to yield-related traits,and the desirable heterotic patterns identified in this study were potentially useful for commercial maize breeding.(3)Prediction of heterosis and hybrid performance using combining abilityHeterosis can be treated as a single trait for genome prediction.SCA was significantly correlated with MPH and HPH for all the tested traits,supporting that heterosis was largely controlled by non-additive effects in maize.Strong correlation between GCA effects and hybrid performance per se for most traits suggests that parental GCA may be a good indicator for predicting hybrid performance.Hybrid performance was dependent on the effects of GCA and SCA,while heterosis was dependent on SCA effects.Heterosis was more significantly and positively correlated with SCA than GCA,indicating that SCA can be used in heterosis prediction to develop potential hybrids in commercial maize breeding.(4)Genome-wide association analysis of phenotype,combining ability,and heterosisA total of 11 candidate genes for phenotypic traits,17 for MPH and 1 for HPH were identified using Farm CPU model.The candidate genes for flowering were involved in tryptophan synthesis,which play an important role in the regulation of flowering time in plants.The candidate genes for yield-related traits contributed to the regulation of maize growth and development,yield and stress resistance.The candidate genes for heterosis related traits were MYB family transcription factors,succinate dehydrogenase SUDH7,CLE family,and AUX /IAA transcription factors,which contribute to the improvement of plant type and the formation of yield heterosis.(5)Genomic selection with significantly associated markersSix genome-wide selection prediction models were compared,among which the Bayes Lasso and GBLUP models were slightly better than others.Heterosis prediction results were trait specific,and the prediction accuracy was trait-dependent.It could be significantly improved by using the markers significantly associated with traits.The prediction accuracy for agronomic traits was affected by the number of markers,and it increased with the increase of the number of markers.Only significant markers with p=0.1 or above needed to be included to achieve the prediction accuracy using all markers.Different from agronomic traits,the prediction accuracy for MPH and HPH was affected by using significantly associated markers,and using a few high-effect markers could achieve a higher prediction accuracy.