Diversity Analysis And Construction Of Primary Core Collection In Tibetan Turnip

Turnip,as a traditional medicine,food and fodder plant for Tibetan people,has become one of the most important characteristic economic crops in Tibetan plateau,which has been cultivated for more than 3000 years in local regions.At the same time,the genetic diversity of Tibetan turnip is rich because of the special environmental conditions in Tibetan region.However,lots of farmers in Tibet have begun to use turnip F₁ hybrid seeds on a certain scale in recent years,which will result in a sharp reduction of the landrace diversity of turnip in Tibet.As a result,the genetic diversity of germplasm resources is constantly destroyed or even lost.And the causes include of these aspects as follows:（1）the beginning of the national project of ecological protection in the Qinghai-Tibet Plateau;（2）the expansion of the planting area of turnip;（3）the acceleration of urbanization and the change of modern agricultural production mode.In addition,the previous studies showed that turnip was the only one subspecies which possessed the most resistant gene resources for clubroot in the genome AA plant among the Brassica crops.Therefore,new species of resistance to clubroot can be selected by hybridize and rotation cross.Besides,because the infection of Brassica is a comprehensive process of interaction between pathogen and host plant,it is also the most effective measure to prevent and control the disease.In this study,morphological investigation and SSR molecular markers have been employed to analyze the genetic diversity of turnip germplasm resources collected from the Qinghai-Tibetan Plateau.Meanwhile,we explored the genetic relationship and genetic structure among turnip germplasm in different geographical regions,and to construct primary core collection of turnip.These results provided the materials and basis for the improvement and preservation of turnip varieties in the future.At the same time,the resistance of clubroot disease has been identified.These results have been obtained as follows:（1）Genetic diversity analysis of the turnip germplasm resources using morphological markers.A total of 58 aboveground and underground morphological traits including 33 quantitative and 25 quality traits were measured in this study.Results showed that the similarity of leaf shape,leaf wax powder,vein brightness,vein glossiness,cotyledon color,root-acromion shape,lateral bud eruption calendar and bolting were over 97%in all individuals,the coefficient of variation was low and the genetic diversity was weak.However,the distribution of lateral roots,petiole color,cross-section shape of petiole,wavy margin,product maturity,hypocotyl color,root-acromion scar and underground root skin color were abundant.Among the quantitative characters,the genetic variation was also abundant among the traits of petiole length,single plant weight,root weight,fleshy root diameter,fleshy root diameter and upper root diameter.Through correlation analysis results,it was found that the weight per plant was positively correlated with cotyledon length,plant height,plant width,leaf number,leaf length,leaf width,petiole width,fleshy root transverse diameter,pistil width,fleshy root position and leaf tip shape,while root weight was positively correlated with plant width,plant width,leaf number,fleshy root transverse diameter,fleshy root longitudinal diameter,root shoot longitudinal diameter,leaf surface,fleshy root position,leaf split and product maturity.Thus,early screening of high-yielding varieties can be carried out.These results of the cluster analysis divided171 turnip species into one large and one small two large groups,and most of the varieties in Tibet were grouped into one group.It is considered that there is a certain relationship between the classification of groups and its geographical source,but it is not completely consistent with each other.（2）Genetic diversity analysis,kinship identification and population structure analysis of 171 turnip germplasm resources were successfully performed using SSR molecular marker technique.In this study,31 pairs of SSR primers with polymorphism were obtained by preliminary screening.Using them to amplify 171turnip materials,181 bands were amplified,of which 163 were polymorphic bands,18were common bands,and the polymorphism rate was up to 90.1%.The software NTSYS 2.10e was used to analyze the 181 marks produced by 31 pairs of primers to obtain the genetic similarity coefficient between turnip materials.The genetic variation between materials is very large,the genetic distance of the whole 171materials is between 0.12 and 1.09,and the genetic similarity coefficient is between0.73 and 0.95.Using the NTSYS 2.10e and PowerMarker V3.25 software based on the genetic similarity coefficient matrix,it was found that 171 germplasm were divided into two groups when the gs was 0.73 by the UPGMA cluster analysis method.The results of the amplification were depopulation structure analysis using structure software,and the germplasm was also divided into 2 main taxa.Using PAST software for principal component analysis,principal component 1 and principal component 2could explain 9%and 5.3%of the molecular genetic variation,respectively.The 171turnip germplasm resources have been divided into two parts by the PCA map.Results were consistent with the population structure analysis and the cluster map results.（3）The primary core collection has been constructed in Tibtan turnip.Based on the total sampling size of 15%,the primary core collection of turnip,which containing26 materials,was constructed.Results indicated the genetic redundancy was well removed,which represented the genetic distribution and trait correlation of the original germplasm.In this study,primary core collection were evaluated by comparing their mean difference percentage,variance difference percentage,polarity coincidence rate and coefficient of variation change values under different overall sampling sizes of 20%,15%and 10%.It was found that when the overall sampling ratio was 15%,the MD value was smaller and the VD,CR,and VR were larger.These results indicated that the sampling ratio was better representative.Principal component analysis and correlation analysis were used to identify the constructed primary core collection using phenotypic data.Comparison of the principal component analysis of the primary core collection and the original germplasm,it was found that the characteristic value,contribution rate and cumulative contribution rate of the primary core germplasm were all larger than those of the original population.Each of the first eight principal components,the eigenvalues are all larger than 2,the eigenvalues of the first principal component are 10.10 and 5.32,the contribution rate is 21%and 14%,and the cumulative contribution rate of the first eight principal components is 75%and 57%,respectively.Results showed that the construction of primary core collection can exclude genetic redundancy and make the cumulative contribution rate significantly higher.（4）The clubroot resistance has been identified,and 55 resistant varieties and 87susceptible varieties were obtained.In this study,171 turnip accessions were identified for clubroot resistance by diseased soil method.And the growth status and incidence of turnip clubroot were observed and recorded,and the condition of turnip root swelling disease was graded.It was found that of the 24 high resistant landraces,10 were from Tibetan region,5 from Japan,4 from other provinces except Tibet in China,and all 4 ECD series from Europe showed high resistance to clubroot.Heading Chinese cabbage from South Korea also showed high resistance,but Heading Chinese cabbage varieties from China were always susceptible.These accessions with high resistance provided the new inbreds and research clues for the study of clubroot in Brassica crops.