Genetic diversity of pear (Pyrus L.) cultivar resources in Guizhou province analyzed by ISSR markers

Abstract:【Objective】Pear is part of the genera of the Rosaceae, Amygdaloideae, Maleae and Malinae plants. China is one of the origin places of pear. Pear had been cultivated for more than 3 000 years in our country. Although pear cultivar resources is the most abundant of resources in Guizhou province, because of land development, backward technology, the relatively concentrated mature period, diseases and serious pests, and environmental degradation, etc, a serious resource drain is occurring. There has been a gradual reduction in the genetic diversity of the pear germplasm resources with shrinking of the single variety and a few good varieties and also there are less export-oriented varieties. In addition, pear cultivars have become confused. In the process of long-term natural hybridization, not were only various types of cultivars formed, but the genetic relationship also became more complicated among the pear breeds. Furthermore, introduction and cultivation became mutual in different regions, causing the emergence of homonyms or synonyms, resulting in the chaos of species classification. It is difficult to reflect the genetic relationship among cultivars by the morphological characteristics of the leaf and fruit. Hence, researching on the genetic diversity of pear variety resources is very important in Guizhou. This study was carried out, in order to determine the genetic diversity and relationship of the local Pyrus L. species which provided a scientific basis for the development of species classification and cultivar identification of pear resources in Guizhou.【Methods】Inter-simple sequence repeat (ISSR) assessment of genetic diversity in 48 local Pyrus L. species was carried out.【Results】The genetic parameters of 48 varieties were analyzed by usingPopgene32 software. 12 primer pairs produced 126 loci of which 114 were polymorphic. The percentage ofpolymorphic loci was 90.48%. It showed that Pyrus L. had abundant diversity. The average value of the effective number of alleles, Nei's gene diversity, and Shannon's information index, were 1.487 4, 0.285 2and 0.430 7, respectively. According to the result of the PCR, the genetic similarity analysis for 48 culti-vars was calculated by using software of NTSYS pc-2.1e. The result of the genetic similarity analysisshowed that the Jaccard coefficient ranged from 0.484-0.952. The results showed that the Jaccard coeffi-cient of‘Baili'and‘Xiaobaili'were the highest, with a value of 0.952. These two cultivars belonged tothe P. bretschneideri. For‘Cili' (Weining) and‘Housui' (Japan) , the Jaccard coefficient is the lowest.The Jaccard coefficients are 0.484. We could draw the conclusion that the genetic differences betweenthese two varieties were large. A DNA molecular dendrogram was established for 48 cultivars which wasbased a on UPGMA cluster analysis. When the similarity coefficient value was 0.67, the 48 pear cultivarswere divided into 3 groups. The first group (I) included‘Weining Xueli'‘Anshun Xueli'‘Eli'‘Cili'‘Hululi'. In addition to‘Eli', the other four varieties were P. pyrifolia.‘Weining Xueli'and‘AnshunXueli'both had a similar coefficient of 0.817 and they were together with‘Eli'. The second group (II) on-ly contained one variety. The varieties were cross bred between‘Pingguoli'and‘Mishirazu'and weresuccessfully cultivated by the Institute of Pomology of the Chinese Academy of Agricultural Sciences in1965. 42 cultivars of the third group (III) were further divided into 3 subgroups. The first subgroup (IIIA) included 25 varieties, the second subgroup (IIIB) included 1 variety, the‘Housui'pear and the third sub-group (IIIC) included 16 varieties. 25 varieties of the first subgroup (IIIA) had a similar coefficient valueof 0.744, which can be divided into 6 subtypes. The third subgroup (IIIC) with a 0.714 level of value canbe divided into 2 subtypes: Type A includes‘Dabaizhaotongli'‘Fengtangli'‘Dahuangli'‘Jinggaili'‘Ji-azhaotongli'‘Zaoli'‘Huangpixiang'‘Jigali'‘Bingzili'‘Dangshansuli'in addition to‘Bingzili'and‘Dangshansuli'with the varieties being collected from Weining, Bijie; Type B includes 6 varieties, except for‘Haizili'and‘Xingyi Xueli', the varieties all belong to the P. bretschneideri.【Conclusion】Theclustering graph shows that there was a high genetic similarity among these varieties, such as‘Xiaobaili'‘Baili'‘Weining Xueli'and‘Anshun Xueli'. There were 25 pear varieties in the first subgroups (IIIA) , with apple pear being clustered individually; the results of this study were the same as Teng Yuanwen'sresearch. So apple pear was an independent group and independent of P. bretschneideri and P. pyrifolia.However, Qu baihong's research considered apple pear as belonging to the P. bretschneideri. This problem requires further research in order to identify more adequate classification evidence. This studyshowed that P. bretschneideri, ‘Yali'‘Jinhuali', ‘Xiangli'and‘Xianglanli'were clustered. There was ahigh proximity between the varieties. The genetic similarity coefficient was higher between‘Haizili'and‘Baili'in the system. Other varieties of pear were farther apart. Huang Lisen et al. observed this phenom-enon from the pollen morphology of the genus. TENG Yuanwen and Qu baihong et al used molecular marker methods and confirmed this similar phenomenon. The molecular level studies showed that the existenceof interspecific hybridization phenomenon was widely observed among P. bretschneideri and P. pyrifolia varieties. The interspecific hybridization between pear varieties is an important approach to the evolutionand development of pear varieties. The process of evolution of pear cultivars is very complicated. The ge-netic diversity of 48 pear cultivars was large, as was fundamentally matched with the traditional classifica-tion base on morphological classification results. However, the clustering results of the individual specieswere different from the actual situation.