Abstract: 【Objective】 Forty-one tree peony samples consisting of 21 from Xinan group, 18 from Zhongyuan group, 1 from Jiangnan group and 1 wild species were used to detect their genetic diversity and verify their phylogenetic relationship and discuss their genetic backgrounds for the new cultivars breeding. 【Method】Total genomic DNA was extracted from the fresh leaves of tree peony by a modified CTAB method. A total of 27 primers were selected from 60 ISSR universal primers designed by UBC on the basis of the establishment of an optimal ISSR-PCR reaction system. They were used in the PCR amplification to compare the genetic difference among different cultivars. Comparative analysis on DNA fragments amplified by ISSR-PCR technology was made by the relative genetic software. Genetic parameter such as percentage of polymorphic loci (PPB) was calculated by using POPGENE 32. Cluster analysis (UPGMA) and dendrogram based on Nei’s genetic distance were made to construct the relationship between cultivars and cultivars groups by using the Numerical Taxonomy Multivariate Analysis System (NTSYS-pc) ver. 2.1 statistical package.【Result】A total of 317 bands were obtained by amplifying 41 cultivars from 27 primers, among which 304 bands were polymorphic and percentage of polymorphic bands (PPB) attained to 95.41%. In average, 11.74 bands were produced by each primer. The genetic similarity coefficients among all the tested 41 samples ranged from 0.483 to 0.811, which ‘Daguanfen 4’ and ‘Lijiangzi 5’ had the highest similarity coefficient with 0.811, otherwise ‘Daguanfen 4’and ‘Caihui’ had the lowest coefficient with 0.483. The 41 cultivars were divided into four branches based on UPGMA cluster at the coefficiency of 0.625. The first branch was composed of 19 samples, which included 5 Zhongyuan peony cultivars and 14 Xinan cultivars, the genetic similarity coefficient of ‘Lijiangzi 5’ and ‘Daguanfen 4’ had the closest phylogenetic relationship, ‘Lijiangzi 5’ and ‘Shouanhong’ had the farthest phylogenetic relationship with a genetic similarity coefficient of 0.609; The second branch included 10 samples in which there were 5 Zhongyuan cultivars and 9 Xinan cultivars. The closest phylogenetic relationship existed between ‘Zijinhe 2’ and ‘Sichuanfenzi’ with a genetic similarity coefficient of 0.770. However, the farthest phylogenetic relationship was between ‘Zijinhe 2’ and ‘Luowuxianrui’,with the minimum genetic similarity coefficient at 0.625; The third branch contained 11 samples: one Jiangnan cultivar, 7 Zhongyuan cultivars and 3 Xinan cultivars. The closest phylogenetic relationship was between ‘Linghuazhanlu’ and ‘Zhushalei’ with the genetic similarity coefficient of 0.779. The farthest genetic relationship was between ‘Zhushalei’ and ‘Fendangbai’. The fourth branch involved only one wild species of Paeonia lutea. The result showed that most of the Zhongyuan cultivars with similar flower color had the closer relationship. ‘Yanzhilou’ belonged to Tianpeng cultivars with red flower had the much more closer genetic relationship with the Zhongyuan cultivars bloomed in dark red flower, while other Tianpeng cultivars with dark red flower had the closer genetic relationship with Zhongyuan cultivars bloomed in purplish red, pink, purple and red. Yunnan cultivar with purple flower had a certain genetic relationship with Zhongyuan cultivars bloomed in purple and purplish red flower. But other pink flower cultivars had some certain genetic relationships with Zhongyuan cultivars in purplish red, light red, light purplish red and light purple flower color. Tianpeng cultivars firstly clustered with Zhongyuan cultivars and then with Yunnan cultivars. While Yunnan cultivars except ‘Shishanhuanguan’ and ‘Xianyuban’ gathered together firstly and then they clustered with Zhongyuan cultivars, which Yunnan cultivars from different places with similar and same flower-color had the closer genetic relationship. 【Conclusion】The origin of Xinan cultivar group is complicated. Tianpeng cultivars have a closer genetic relationship with Zhongyuan group than Yunnan cultivars. However, Yunnan cultivars could not share the common ancestors with Tianpeng cultivars. Yunnan cultivars might be differentiated by several ancestors under different conditions. More evidences were supposed to make certain their origins. Based on the cluster results, it could be inferred that P. lutea is almost involved in the origin of Yunnan cultivars.

Key words: tree peony from Xinan group, origination, ISSR marker, phylogenetic relationship