Browsing by Subject "Hybrid performance"

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    Genetic dissection of hybrid performance and heterosis for yield-related traits in maize
    (2021) Li, Dongdong; Zhou, Zhiqiang; Lu, Xiaohuan; Jiang, Yong; Li, Guoliang; Li, Junhui; Wang, Haoying; Chen, Shaojiang; Li, Xinhai; Würschum, Tobias; Reif, Jochen C.; Xu, Shizhong; Li, Mingshun; Liu, Wenxin
    Heterosis contributes a big proportion to hybrid performance in maize, especially for grain yield. It is attractive to explore the underlying genetic architecture of hybrid performance and heterosis. Considering its complexity, different from former mapping method, we developed a series of linear mixed models incorporating multiple polygenic covariance structures to quantify the contribution of each genetic component (additive, dominance, additive-by-additive, additive-by-dominance, and dominance-by-dominance) to hybrid performance and midparent heterosis variation and to identify significant additive and non-additive (dominance and epistatic) quantitative trait loci (QTL). Here, we developed a North Carolina II population by crossing 339 recombinant inbred lines with two elite lines (Chang7-2 and Mo17), resulting in two populations of hybrids signed as Chang7-2 × recombinant inbred lines and Mo17 × recombinant inbred lines, respectively. The results of a path analysis showed that kernel number per row and hundred grain weight contributed the most to the variation of grain yield. The heritability of midparent heterosis for 10 investigated traits ranged from 0.27 to 0.81. For the 10 traits, 21 main (additive and dominance) QTL for hybrid performance and 17 dominance QTL for midparent heterosis were identified in the pooled hybrid populations with two overlapping QTL. Several of the identified QTL showed pleiotropic effects. Significant epistatic QTL were also identified and were shown to play an important role in ear height variation. Genomic selection was used to assess the influence of QTL on prediction accuracy and to explore the strategy of heterosis utilization in maize breeding. Results showed that treating significant single nucleotide polymorphisms as fixed effects in the linear mixed model could improve the prediction accuracy under prediction schemes 2 and 3. In conclusion, the different analyses all substantiated the different genetic architecture of hybrid performance and midparent heterosis in maize. Dominance contributes the highest proportion to heterosis, especially for grain yield, however, epistasis contributes the highest proportion to hybrid performance of grain yield.
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    Molecular and agronomic assessment of genetic diversity and hybrid breeding in triticale
    (2006) Tams, Swenja H.; Melchinger, Albrecht E.
    Knowledge of the genetic diversity of a species is of paramount importance for the choice of crossing parents in line and hybrid breeding. Genetic distance (GD) estimates based on molecular markers proved to be well suited for direct exploration of the relationship within a germplasm pool. Triticale hybrid breeding and heterosis have received increasing attention in recent years. Hybrid seed production is highly attractive for autogamous species because of the built-in variety protection of hybrids in comparison to line varieties. The main objective was to appraise the prospect of hybrid breeding in European winter triticale and develop time- and cost-reducing strategies. In particular, the main objectives were to (i) assess and compare genetic diversity estimates in European winter triticale elite germplasm based on molecular markers and pedigree data, (ii) determine hybrid performance and heterosis in multiple environments, and (iii) evaluate prediction methods for hybrid performance and heterosis to support future hybrid breeding programs. Average coancestry coefficient between all pairs of the 128 European elite genotypes was low (f = 0.059) due to scanty information available for the majority of the varieties and breeding lines. Better estimates of genetic distance of triticale genotypes were obtained by molecular marker assessment with 93 simple sequence repeat (SSR) markers and 10 PstI/TaqI primer combinations of amplified fragment length polymorphism (AFLP) markers. While SSR markers have been developed in wheat and rye and are mapped in the genome, the location and distribution of AFLP markers is unknown. Both marker systems resulted in reliable genetic diversity estimates. The moderate correlation between genetic distance estimate (GD) of SSR and AFLP marker analyses (GDSSR; GDAFLP) corresponded with other studies. Cluster analysis and principle coordinate analysis revealed no clear separation of germplasm groups. Supported by a bootstrap analysis, it was concluded that both marker systems provide consistent information for germplasm identification. The lack of grouping is in concordance with the breeding history of triticale as a self-pollinator, the wide adaptation of the inter-generic species and the single end-use purpose. Simultaneously to the marker assessment, 209 F1 hybrids were produced by a chemical hybridizing agent. The hybrids and their parents (57 females and five testers) were evaluated in field trials in six environments in Germany during the season 2001-2002. A combined analysis revealed significant heterosis for all eight traits. The level of mid-parent heterosis was positive for grain yield, 1000-kernel weight, number of kernels per spike, test weight and plant height and negative for number of spikes per m², falling number and protein concentration. Forty-six of the hybrids outyielded modern varieties, which were included as checks, by 10% and more. This aspect is important for the success of hybrids on the market for commercial production. Results regarding hybrid performance, heterosis, GCA/SCA relationship, trait correlation in hybrids and parents and aspects regarding cost-effective high quality F1 seed production appear to be sufficiently positive to encourage further work on hybrid breeding. Approaches to reduce time and costs for the identification of superior parental combinations and the prediction of hybrid performance revealed no reliable method yet. Correlations between SCA and GD of parents based on the different marker systems were low for all traits, which hampers prediction. Grouping of germplasm based on GD estimates or on heterotic response of the hybrids could not be discovered in triticale. As a consequence, a first step for an optimum allocation of resources in commercial hybrid breeding programs is the development of heterotic groups. In the present study, several females have been sub-grouped according to their heterotic response and SCA for grain yield with two tester pairs. Following the early history of hybrid breeding in maize, a multi-stage procedure was suggested for triticale to evaluate and expand the sub-grouping and enhance heterosis among groups.
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    Prediction of hybrid performance in maize using molecular markers
    (2008) Schrag, Tobias; Melchinger, Albrecht E.
    Maize breeders develop a large number of inbred lines in each breeding cycle, but, owing to resource constraints, evaluate only a small proportion of all possible crosses among these lines in field trials. Therefore, predicting the performance of hybrids by utilising the data available from related crosses to identify untested but promising hybrids is extremely important. The objectives of this thesis research were to develop and evaluate methods for marker-based prediction of hybrid performance (HP) in unbalanced data as typically generated in commercial maize hybrid breeding programs. For HP prediction, a promising approach uses the sum of effects across quantitative trait loci (QTL) as predictor. However, comparison of this approach with established prediction methods based on general combining ability (GCA) was lacking. In addition, prediction of specific combining ability (SCA) is also possible with this approach, but was so far not used for HP prediction. The objectives of the first study in this thesis were to identify QTL for grain yield and grain dry matter content, combine GCA with marker-based SCA estimates for HP prediction, and compare marker-based prediction with established methods. Hybrids from four Dent × Flint factorial mating experiments were evaluated in field trials and their parental inbreds were genotyped with amplified fragment length polymorphism (AFLP) markers. Efficiency for prediction of hybrids, of which both parents were testcross evaluated (Type 2), was assessed by leave-one-out cross-validation. The established GCA-based method predicted HP better than the approach exclusively based on markers. However, with greater relevance of SCA, combining GCA with marker-based SCA estimates was superior compared with HP prediction based on GCA only. Linkage disequilibrium between markers was expected to reduce the prediction efficiency due to inflated QTL effects and reduced power. Thus, in the second study, multiple linear regression (MLR) with forward selection was employed for HP prediction. In addition, adjacent markers in strong linkage disequilibrium were combined into haplotype blocks. An approach based on total effects of associated markers (TEAM) was developed for multi-allelic haplotype blocks. Genome scans to search for significant QTL involve multiple testing of many markers, which increases the rate of false-positive associations. Thus, the TEAM approach was enhanced by controlling the false discovery rate. Considerable loss of marker information can be caused by few missing observations, if the prediction method depends on complete marker data. Therefore, the TEAM approach was improved to cope with missing marker observations. Modification of the cross-validation procedure reflected, that often only a subset of parental lines is crossed with all lines from the opposite heterotic group in a factorial mating design. The prediction approaches were evaluated with the same field data as in the previous study. The results suggested that with haplotype blocks instead of original marker data, similar or higher efficiencies for HP prediction can be achieved. Marker-based HP prediction of inter-group crosses between lines, which were marker genotyped but not testcross evaluated, was not investigated hitherto. Heterosis, which considerably contributes to maize grain yield, was so far not incorporated into marker-based HP prediction. Combined analyses of field trials from multiple experiments of a breeding program provide valuable data for HP prediction. With a mixed linear model analysis of such unbalanced data from nine factorial mating experiments, best linear unbiased prediction (BLUP) values for HP, GCA, SCA, line per se performance, and heterosis of 400 hybrids were obtained in the third study. The prediction efficiency was assessed in cross-validation for prediction of hybrids, of which none (Type 0) or one (Type 1) parental inbred was testcross evaluated. An extension of the established HP prediction method based on BLUP of GCA and SCA, but not using marker data, resulted in prediction efficiency intermediate for Type 1 and very low for Type 0 hybrids. Combining line per se with marker-based heterosis estimates (TEAM-LM) mostly resulted in the highest prediction efficiencies of grain yield and grain dry matter content for both Type 0 and Type 1 hybrids. For the heterotic trait grain yield, the highest prediction efficiencies were generally obtained with marker-based TEAM approaches. In conclusion, this thesis research provided methods for the marker-based prediction of HP. The experimental results suggested that marker-based HP prediction is an efficient tool which supports the selection of superior hybrids and has great potential to accelerate commercial hybrid breeding programs in a very cost-effective manner. The significance of marker-based HP prediction is further enhanced by recent advances in production of doubled haploid lines and high-throughput technologies for rapid and inexpensive marker assays.