Heterosis And Quantitative Genetic Dissection Of Yield Of Hybrid Soybean

The exploitation of heterosis was one of few milestones in the revolution of agricultural sciences and technologies leading to big jump in crop improvement in 20th century. Utilization of heterosis is one of the most effective ways to increase yield and improve quality in several major crops, including maize, sorghum and rice. In order to apply heterosis more efficiently, scientists have made every effort for a long time to probe into the genetic basis and the method of prediction for heterosis. The development of molecular markers and its application in biological researches had provided technological base for dissecting heterosis in crops.Soybean [Glycine max (L.) Merr.] is the leading oilseed crop produced in the world, and one of the most important sources of vegetable protein and edible oil worldwide, however the yield is relatively low compared to other important field crops such as maize, rice etc. The use of hybrid could not only increase the yield but also accelerate the genetic gain per year. Soybean breeders keep trying to find ways to use heterosis. Even through the first hybrid "HybSoyl" was released in 2002, its commercialization has not been achieved in large scale yet. In recent years, breeding for hybrid cultivars of soybean for utilization of heterosis has been paid great attention, but there are few reports published on the fundamental aspects regarding the heterosis in soybean. In fact, for a real utilization of hybrid soybean, the important prerequisite is high heterosis. Therefore, a fundamental effort in hybrid breeding is the choice of parents and identification of superior hybridized combinations.In this study,8 summer soybean cultivars (lines) from different origins from Huang-Huai Valley of China and US in maturity groupⅡ-Ⅳ, were used to develop 28 crosses according to a GriffingⅣmating design in 3 years (2002-2004). These crosses, together with their parents, were used to test for yield and quality traits in 3 years (2003-2005) in Huaian, Jiangsu, China. The genotyping data of 300 SSR polymorphic markers on 8 accessions were obtained. The present study was aimed at evaluating the heterosis performed in F1 generation of 14 yield and quality traits as to provide guidelines of parental selection in breeding for hybrid cultivars. In the paper, combining ability of yield-related traits was analyzed, and relationships between F1 yield heterosis and its pedigree-based and SSR-based genetic distances were investigated. Furthermore, the analysis of major-minor locus groups of yield based on major gene plus polygene mixed inheritance model was used to explore the genetic structure of hybrids among a group of soybean materials, additive and dominance effects of major-minor locus groups were estimated. Finally, the molecular data of 300 SSR markers on 8 parental materials were analyzed for association between SSR markers and hybrid yield using the single marker regression analysis. The hybrids were dissected into their allele constitution and the effects of alleles and genotypic value of each locus was estimated. The main objectives of this study were to understand the genetic basis of heterosis and lay a foundation for hybrid soybean breeding by design. The main results were as follows:1 Study on heterosis of agronomic and quality traits in hybrid soybean in Huang-Huai ValleyThere appeared mid-parent heterosis among 8 soybean parents. Heterosis of yield, pods and seeds per plant was relatively larger, while no obvious heterosis for 100-seed-weight, and no obvious heterosis in days to flowering and maturity and quality (protein and oil contents) traits. There were heterobeltiosis in yield among 8 soybean parents with average of 20.39%, and a big difference among hybridized combinations with a range from-5.34% to 76.88%. Among the combinations, Yudou 22 X Jindou 27, Huaidou 4 X Jindou 27, Youbian 30 X Meng 90-24 and Hedou 12 X Jindou 27 had the heterobeltiosis in yield of 76.88%,29.90%,34.42% and 43.16%, respectively.2 Analysis on combining ability of yield-related traits among key parental materials in soybean in Huang-Huai ValleyThere were significant differences among the parents for general combining ability (GCA) and crosses for specific combining ability (SCA) for yield-related traits studied. The GCA variance was significant, and larger than SCA variance, which indicated that yield traits were controlled by additive and non-additive gene effects. The interaction of year by GCA and SCA was significant, and the interaction of year by GCA was larger than the interaction of year by SCA. Among the parents, Jindou 27, Youbian 30 and Huaidou 4 were the best general combiners for yield. The best specific crosses for yield were Yudou 22 X Jindou 27 and Youbian 30×Meng 90-24. Yield heterosis among parents was related to GCA and SCA. One of soybean parents has high GCA or both have high GCA and high SCA in high-yield crosses. Combining ability of pods and seeds per plant was relatively in accord with combining ability for yield.3 Relationship between parental genetic distance measured by SSR markers and pedigree with heterosis in soybean in Huang-Huai ValleyGenetic distance among 8 soybean cultivars measured by SSR markers varied from 0.4376 to 0.8635 with the mean of 0.6877 and from 0.3124 to 1.0000 with the mean of 0.7679 based on the pedigree. SSR-based and pedigree-based cluster analysis revealed that genetic relationships for 8 soybean parents were basically consistent, and 8 soybean parents were grouped into 2 groups, one including 6 cultivars from middle and south of Huang-Huai Valley, the other consisting of one from Shanxi and one from America.The correlation between the genetic distance and mid-parent heterosis was moderate, especially, the correlative coefficient was 0.5209 which was significant at 0.01 level between mid-parent heterosis and the genetic distance measured by SSR between parents. Therefore, certain genetic distance is required for a cross with high heterosis and high yield, but genetic distance is not an only determinant factor for high heterosis and yield.4 Genetic analysis in terms of major-minor locus group constitutions of yield of hybrid soybean in Huang-Huai ValleyThere were 6 major locus groups plus minor locus groups detected in the genetic system of the 8 soybean parents and their hybrids. Genetic variation of the major locus groups and the minor locus groups explained 75.98% and 10.81% of the phenotypic yield variation, respectively, which indicated that major locus groups were the major source of genetic variation, with their additive effects (aJ) of 140.10,259.65,1.95,151.35,-32.70 and 45.00 (kg hm-2) and dominance effects (dJ) of 177.15,314.25,105.75,75.90,242.85 and 171.00 (kg hm-2), respectively, while the minor locus groups were a supplement source among the soybean hybrid. The genetic constitutions of the soybean hybrids were composed of heterozygous dominance effects of major locus groups, homozygous additive effects of major locus groups, heterozygous dominance effects of minor locus groups and homozygous additive effects of minor locus groups, with their relative importance in a descending order. The dissection of the relative importance of the genetic effects of major-minor locus groups helps to explain the genetic characteristics of the hybrid among the parents and provides the genetic basis for further mining the genetic potential of the soybean parental materials in the improvement of hybrid.5 Analysis of SSR loci and alleles associated with hybrid yield in soybean in Huang-Huai Valley38 SSR loci located on 17 linkage groups were identified to associate with hybrid yield in the diallel crosses with more loci on linkage groups D1a, M, etc, and 8 of the 38 loci were located within a region of±5 cM apart from known QTL identified from family-based linkage (FBL) mapping in the literature. Each of the loci explained 11.95%～30.20% of the phenotypic variance of hybrid yield. The allele pairs of the hybrids were composed of 4 parts, i.e. positive dominant heterozygous loci, positive additive homozygous loci, negative additive homozygous loci and dominant heterozygous loci, with their relative importance in a descending order. Among the 38 loci associated with hybrid yield, nine elite loci such as Satt449, Satt233 and Satt631 and nine elite alleles such as Satt449-A311, Satt233-A217 and Satt631-A152 were identified. Meanwhile, nine heterozygous allele pairs such as Satt449-A291/311, Satt233-A202/207 and Satt631-A152/180 were detected. These results will provide with some relevant information for understanding the genetic basis of heterosis and lay a foundation for hybrid soybean breeding by design.6 Quantitative genetic dissection of yield of elite crosses in soybean in Huang-Huai ValleyAmong genetic composition of hybrid yield in the 4 elite soybean crosses, heterozygous loci played a leading role, and the dominance effects were relatively larger; homozygous loci located in a secondary position, and the additive effects were relatively smaller. Different elite crosses had different elite heterozygous loci. A superior hybrid should take advantages of both heterozygous and homozygous loci, pyramiding more heterozygous loci to maximize the genotypic value of elite loci.