Diversity Analysis And Racial Classification Of Maize Landraces In China

Maize is one of the most important cereals and forage crops in China,and plays a key role in Chinese agriculture.During the long term adaptation to the complicated eco-environments in the nearly 500 years since its first importing into China,a vast number of landraces have formed,in which abundant genetic diversity has accumulated.Being important basic germplasm resources,these landraces have heavily improved the development of maize production in the early stage in China,and will still be of great importance to the current and future improvement of maize cultivars.So it is of full necessary to carry out a systematic and fine evaluation on the phenotypie and DNA level before the thorough use of these landraees germpalsm.On the other hand,from 1950s to the end of 1990s,a total of 16779 accessions of maize was gathered and preserved in China National Genebank(CNG),including 13521 landraces accessions.Though great accomplishments have gained in the maize germplasm collection,systematic evaluation and related analysis of these germplasm, especially of the maize landraees,are extremely feeble.What's more,there are still no normal reports on the racial classification of maize landraees preserved in CNG.Based on the geographical distribution and phenotypie analysis of all the landrace accessions conserved in CNG,a core collection of all the maize landrace accessions was established.We fingerprinted the core aeeessions with 30 phenotypic traits and 55 simple sequenee repeat(SSR)markers,and the goals were to:(1)study the phenotypic diversity of Chinese maize landraces thoroughly,and compare the differences of phenotypic diversity among 9 maize regions;(2)evaluate the genetic diversity of maize landraces preserved in CNG on DNA level,and analyze the distribution of genetic diversity among different maize regions;(3)reveal the population structure of Chinese maize landraces and the genetic diversity of each subpopulations;(4)examine the races of maize germplasm in China;and(5)establish a mini core set of the core collections,so as to construct a feasible and easily attainable platform of maize landraces for the efficient management,thoroughly utility and succeeding related researches.A total of 760 landrace accessions were used for the phenotypic analysis and racial classification while 846 landrace accessions were used for the evaluation of genetic diversity.The main results were as follows:1.Based on the analysis of 27 quantitative traits and 3 qualitative traits,the whole set of 760 core maize landrace accessions preserved in CNG showed a relatively high level of phenotypic diversity.Among all of the 30 traits,100-kernel weight and plant height represented the highest level of phenotypic diversity,with the value of Shannon-Weaver diversity indices of 2.10 and 2.09 separately,while the traits of cob color and tassel-pollenshedding interval showed the lowest phenotypic diversity,with Shannon-Weaver diversity indices only of 0.64 and 0.84 respectively.At the same time, there existed distinct discrepancy among the accessions from 9 maize regions in China, among which Southwest Region possessed the highest phenotypic diversity,while Northwest Region and Plateau Region possessed the lowest.The results of phenotypic diversity analysis suggested that maize landraces from Southwest Region contained broader phenotypic variation,existing relatively high protection value and utility potential.2.Fingerprinted with a set of 55 TP-M13-SSRs(simple sequence repeat with tailed primer M13)covered the entire maize genome,all of 846 maize core landrace accessions existed a high level of genetic/gene diversity.Totally,we observed 805 alleles,114 specific alleles and 597 rare alleles across 55 SSRs,with an average of 14.64,2.07 and 10.85 alleles per locus respectively,and the total genetic diversity was 0.65,conforming the broad genetic base of maize landraces in China.Comparisons among landraces with different geographical origins showed slightly difference of genetic diversity among the accessions from 9 maize regions,while distinct discrepancy existed in total alleles,specific alleles,and rare alleles among these regions,and Southwest Region held higher number of these three types of allelcs than the other 8 regions,implied the most abundant genetic diversity.Among the whole set of 55 simple scqucnce repeats,dinuclcotide SSRs could capture more alleles and genetic diversity than other repeat SSRs(P=0.0144 and p＜＜0.001 respectively),while gene-based SSRs could capture higher genetic diversity than genomic SSRs(P=0.0125),but almost the same alleles as the latter(P=0.3563).3.Model-based population structure analysis showed that 846 core accessions could be grouped into 5 subpopulations,which were Group1,Group2,Group3,Group4,and Groups in turn.Among these 5 subpopulations,Group2,Group3,and Group4 were formed by the accessions from Southwest Region,Huang Huaihai Region,and North Region respectively,and Groups was formed by the accessions from provinces of Guangdong and Hainan and part of Jiangxi province in South Region,while Groupl was formed by the accessions form the other provinces and regions in South Region. The result of population structure of 846 landrace accessions showed a strong geographical trend.4.Principle component analysis(PCoA)of 30 phenotypic traits determined 12 candidates for the racial classification of all the 760 landrace accessions,and the results of PCoA indicated that the maize landraces preserved in CNG included 9 races. Based on the geographical origin and performance of each race on the integrated trait of maturity constructed by four flowering related traits and leaves per plant,these 9 races were named by southwestern dents,waxy race,northern flints,southwestern white flints,yellow flints,southwestern yellow flints,derived race,northern dents,and popcom race.5.To efficiently assist in the use of maize landrace germplasm,we define mini core set of these accessions that capture the maximum number of SSR alleles using the Line Selection function in PowerMarker V3.25.A mini core set contained 285 accessions was established,of which 233 mini corc accessions were from 29 provinces,cities,or regions throughout China,and the other 52 mini core acccssions wcrc imported from America,Europe,and other countries and regions around the world,which indicated a roughly broad geographical distribution of all the 285 mini core accessions.All of these 285 mini core accessions were fingerprinted with the same set of 55 SSR markers that covered the entire maize genome,the results showed that the mini core set captured 91.05%alleles and 83.32%rare alleles of the original core accessions,and there existed no significant diffcrcnccs between the mini core set and the core set on both of alleles and rare alleles(P=0.4119 and 0.2126 respectively),as well as genetic diversity(P=0.6019).Phylogenetic analysis of all the 285 mini core accessions based on 55 SSRs indicated that the accessions from North Region,Huang Huaihai Region, Southwest Region,and South Regin could be separated into two different subclusters, which implied a rather broad genetic variation among these mini core accessions. The geographical distribution and genetic diversity of the whole set of 285 mini core accessions confirmed that the mini core set established in this research existed an extensive wide geographical origin,and a broad genetic variation,which suggested that this mini core set well represented the core accessions.All the results confirmed that in stead of the maize landrace accessions conserved in core set and CNG full scaly, the mini core set could be thoroughly used in the improvement of maize breeding in China,as well as mining of the favorable alleles contained in these germplasm and other related succeeding researches on maize landraces...