Studies On Genetic Diversity And Phylogenetic Relationship Of Cucumber(Cucumis Sativus L.)Germplasm By Morphological And Molecular Techniques

and molecular techniquesLi XixiangDirected by Professor Zhu DeweiInstitute of Vegetables and Flowersof Chinese Academy of Agriculture ScienceAbstractCucumber is an important worldwide vegetable. The genetic background ofcucumber cultivars in production becomes narrower and narrower because ofnatural genetic erosion and variety hybridization. It is urgently needed to evaluatecucumber genetic diversity and its distribution, understand the genetic relationshipof germplasm and identify some new genes both for effective cucumber germplasmcollection and management and for cucumber breeding.In this study, genetic diversity in 70 cucumber accessions from differentcountries or regions was assessed using morphological, RAPD and AFLP makers,and the genetic relationship among the accessions was revealed by cluster analysis atvarious levels. In morphological assessment, the average diversity index among theaccessions was 1.521 based on the data of 34 morphological characters. The meandiversity indices on qualitative and quantitative characters of Chinese germplasmwere 1.050 and 1.783 respectively. They were higher than those (0.753 and 1.647) ofexotic accessions. Meanwhile, it was found that the highest genetic diversity indicesof Chinese and exotic accessions were respectively displayed on differentmorphological characters. The results showed that the morphological diversityamong Chinese germplasm was abundant and the germplasm from other countriesor regions had their own special characters. 70 accessions were clustered into fivegroups based on morphological data. Wild germplasm was far from other accessions.Chinese and European-American cultivated varieties were classified into differentgroups. It was apparent that there was some association between morPhologicalsimilarity and geographical distribution.A total of 253 scorable bands were detected with 29 informative RAPDprimers among 66 accessions. Of them, 195 bands (77.08%) were polymorphic. Amean of 8.7 bands and 6.7 polymorphic bands per primer were generated. Theaverage heterozygosity of RAPD loci in 66 accessions was 0.388. The heterozygosityof Chinese germplasm was a little bit higher than that of the exotic accessions. Theaccessions from south China were more diverse than that from north China. Thegenetic diversity of south China ecotypes was higher than that of north Chinaecotypes. This demonstrated that Chinese germp1asm had broader genetic base thanthose from America and Netherlands. BesideS, it was found that the specific anddeletion bands of certain gemplasm could only be displayed through certain primers.This showed that primer selection was important for overall understanding ofgenetic diversity. 66 accessions could be put into eight groups based on AFLP databy cluster analysis. The clustering result was similar to the result frommorphological clustering in some aspects. For example, accessions fromXishuangbanna could be distinguished from other germplasm; and most germplasmfrom America and Netherlands went into the same group. However, there was notcertain association between the molecular genetic relationship revea1ed by RAPDmarkers and morphological similarity and geographical distribution especiallyamong the Chinese cultivated accessions.Eight pairs of AFLP informative primers generated 425 scorable bands among70 accessions, 286 (66%) of which were polymorphic. A mean of 53 bands/35.8polymorphic bands per primer pair was detected. The average expectedheterozygosity of AFLP loci among all accessions was 0.376. The heterozygosityamong Chinese accessions was higher than that among exotic accessions. Thisfurther proved that Chinese accessions were more diverse at DNA 1evel. Threegroups were clustered based AFLP markers, namely Xishuangbanna cucumbergroup, C. sativus var. Hardwickii group and cultivars group. Xishuangbannacucumber was the farthest from cultivars according to genetic distance. India...