| Cultivars are the products of artificial selection,which contain rich information such as pedigree,parent selection a nd character characteristics compared with general germplasm resources,which make up of an important part of germplasm resource.Totally,2371 soybean cultivars had been released in China from1923 to 2015.It is of great significance to guide the improve ment of soybean cultivars to study the genetic basis,genetic composition and genetic mechanism of important agronomic characters.The major results are as follows:1 Genetic basis of soybean cultivars in China from 1923 to 2015Of all the 2371 soybean cultivars released in China from 1923 to 2015,1286,648,136,240,26 and 35 were bred in Northern China single cropping spring planting varietal ecoregion(I),Huang-huai-hai double cropping spring and summer planting varietal eco-region(II),Middle and Lower Changjiang valleys double cropping spring and summer planting varietal eco-region(III),Central-south multiple cropping spring summer and fall planting varietal eco-region(IV),Southwest plateau spring and summer planting varietal eco-region(V)and South China tropics multiple cropping all season planting eco-region(VI),respectively.A total of 2219 direct parents were used in the breeding process of 1975 hybrid cultivars,of which 82.69% were cultivars and breeding lines,while landraces and foreign varieties accounted for relatively small.In production,cultivars with excellent comprehensive characters(such as Hefeng 25,Jilin 20,Suinong 14 etc from eco-region I,Yudou 8 hao,Xudou 1 hao,Zhonghuang 13 etc from eco-region II,Aijiaozao,Zhongdou 32 etc from eco-region III,Guizao 1 hao,Xiangchundou 10 hao etc from eco-region IV)are often used as direct or indirect crossing parents to speed up the breeding process,which are the backbone parents of various eco-region.In this paper,40 cultivars released from 1996-2015 which were frequently used as direct parents frequently were summarized.Compared with cultivars released from 1986-1995,10,8 and 7 important direct parents were added in north,huang-huai-hai and south,respectively.The important direct parents have important guiding significance for the selection of breeding parents.2371 cultivars can be traced back to 1327 nuclear and 615 cytoplasmic ancestor parents,respectively.The number of ancestors has doubled in the past ten years,while the geographical sources are more extensive.As a whole,the genetic basis of cultivars has been broadened in late ten years.Totally,1327 ancestors came from six eco-regions of China and foreign countries,among which,I,II,III and foreign ancestral parents accounted for most part of the nuclear and cytoplasmic genetic contributions.Based on the analysis of the genetic contribution sources of cultivars in different ecological regions in the past 10 years,it was found that the nuclear(cytoplasmic)genetic contribution of ancestral parents in six ecoregions to the cultivars in this region was still dominated by local ecological germplasm,but its proportion decreased,the proportion of nuclear genetic contribution provided by foreign and hetero ecological germplasm increased.The results showed that the introduction of foreign species and the exchange of germplasm in different eco-regions were strengthened in late ten years.84.03% of the ancestors derived only 1-5 released cultivars,and a small number of important ancestral parents derived a large number of cultivars after many rounds of breeding,forming a family of ancestors.There was an imbalance in the genetic contribution of a single ancestor parent,and the genetic contribution was concen trated to the important ancestor parent.In this study,according to the cycles of derived cultivar,the ancestors were divided i nto two types: multi-generation ancestral parents and terminal ancestral parents.According to the four indexes of nucleus,cytoplasmic genetic contribution,the number of derived cultivars and the highest cycles of derived cultivars,58 multigeneration ancestral parents were nominated as core ancestral parents,which provided 51.51%and 58.88% nuclear and cytoplasmic genetic contribution rates for the cultivars from 1923 to2015.The cumulative number of derivative cultivars accounts for 71.59% of the total,which is very representative.Jinyuan,Baimei,silihuang,Duludou,etc are the important core ancestors in the northern reg ion.Binhai dabaihua,Tongshan tianedan,jimoyoudou,Yidu Pingdinghuang,huaxian dalvdou and so on are the important core ancestors in Huang-huai-hai region.51-83,Fengxian suidaohuang,Shanghai Liuyuebai,Aijiaozao etc are the important core ancestors in the southern region.The excellent genes of 58 core ancestral parents were transferred to their derived cultivars,which constituted the core structure of genetic basis of soybean cultivars in China;and 658 of 1151 terminal ancestral parents were new ance stral parents in late ten years,which realized the expansion of the genetic basis of soybean cultivars in China.2 Genetic basis of recent representative soybean cultivarsRecent representative soybean cultivars population(RRSC)was composed of 279,155,37,71,9,9 cultivars deriving from eco-region I,II,III,IV,V,VI.The 560 cultivars can be traced back to 604 nuclear and 180 cytoplasmic ancestor parents,and their nuclear and cytoplasmic genetic contributions mainly come from eco-region I,II and foreign ancestral parents.The local nuclear and cytoplasmic germplasm respectively provided lesser nuclear and cytoplasmic genetic contribution from I to VI eco-region.Compared with the nuclear and cytoplasmic genetic contribution provided by the ancestral parents of the cultivars in1923 to 2015,the utilization of heterotypic germplasm in I,II,V and VI regions was strengthened,while that in regions III and IV was weakened.In the present study,41 core ancestors of the RRSC population were nominated,which provided 44.45% and 54.46% of the nuclear and cytoplasmic genetic contributions to the population,respectively.The number of derived cultivars accounted for more than 62%of the total.41 core ancestral parents were completely included in the core ancestral parents nominated in 1923-2005 and 1923-2015,which formed the important core genetic basis of RRSC population.28,066 SNPLDB markers were constructed using 277,581 SNP markers.A total of84124 alleles were detected,and the average genetic diversity(π)was 0.127.The average number of alleles in the north,Huang-Huai-Hai,and the south ranges from 179.57 to 375.02,and the genetic diversit y ranges from 0.094 to0.143,the least in the north and the most in the south.There are 59,523 common allele in three region,accounting for 70.76% of the total,and there are 6749,4198 and 6063 unique allele in these three regions,respectively.The analysis of the population structure of RRSC showed that the cultivars in the same region tend to gather together,which indicating that the genetic basis of the RRSC population was related to the geographical distribution of cultivars.463 cultivars(82.68%)in RRSC population belong to 11 main ancestor families,including Jinyuan,Silihuang,Binhaidabaihua,Jimo youdou,51-83,and so on.It was found that the COP and GSC among ancestor families in the same eco-region were relatively high,while among ancestor families in different ecoregion were relatively low.The changing trend of the average value of COP and GSC among main ancestor families was relatively consistent.Based on pedigree and SNPLDB markers,the COP matrix and GSC matrix of RRSC population and each major ancestor family were constructed.Mantel test of the two matrices found that the correlation between the two matrices was significant,indicating that the relationship between cultivars was evaluated by COP and GSC,and the conclusion was relativel y consistent.3 Analysis of genetic constitution of cultivars released in late ten yearsThe genotype × environment was significant(P < 0.01)in main stem node number,internode length,first flower,reproductive period,R8 full maturity,seed weight,seed protein and seed oil of RRSC,while environment was significant(P < 0.01)in plant height which implies that the population performed quite inconsistently between the two environments.In this paper,the G×E model of RTM-GWAS was used to analyze the genetic composition of these traits.In RRSC population,42,83,82,23,68,41,65,64 and 40 QTLs were identified from the 560 cultivars to be correlated with 9 traits(plant height,main stem node number,internode length,first flower,reproductive period,R8 full maturity,seed weight,seed protein,seed oil);The sum of phenotypic interpretation rates of QTL major effects detected were between 47.18% to 68.17%.There are 388,475,413,141,493,349,492,484,419 alleles and corresponding QTL-allele matrix were constructed respectively.The percentage of common allele of 9 traits in northern,huang-huai-hai and southern subgroups were between 60.28% to 65.70%.The southern region may be the origin center of soybean,and the allelic variation of southern cultivars is less than that of their ancestors.Therefore,compared with the southern subgroups,the proportion of common alleles of internode length trait is lowest while of seed protein trait is highest;the proportion of emerged alleles of first flower trait is lowest while of internode length trait is highest,the proportion of excluded alleles of R8 full maturity trait is lowest while of first flower trait is highest.We found that the number of endemic allele of 9 traits in Huang-Huai-Hai subgroup was significantly lower than that in the northern and southern subgroups,which may be due to the mutual utilization of cultivars in Huang-Huai-Hai subgroup and other two subgroups.Based on the analysis of the differences in genetic composition of 9 traits in southern subpopulation,it was found that common,emerged and excluded alleles were the main factors that influenced the types and frequency of allele among subgroups,the recombination of common alleles is also one of the factors that lead to the variation of individual trait phenotype with genotype.There were emerged or excluded alleles in all the 9 traits among the three subpopulations,and there were more alleles in some QTL loci.19-33 important QTL loci were screened out respectively by the phenotypic explanation rate.These QTL will be the focus of functional verification in the future.The differences of genetic constitution among cultivars provides genetic basis for their hybrid combination among materials to produce different types of offspring.The optimal combination design of 9 traits was carried out,and it was found that the super parent combination appeared in the 5th and 95 th percentile.The comprehensive optimal combination design of multi-characters was put forward,and 10 elite crosses match well with the conditions: plant height is below 100 cm,main stem node number is more than 25,internode length is less than 4.5 cm/node,which realized the design and breeding of multicharacter comprehensive selection at the whole genome level and improved the efficiency of hybrid combination selection. |
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