| Rhododendrons belong to the family Ericaceae, and are among the ten famous traditional flowers in China with a long cultivation history since the Tang Dynasty. The genus Rhododendron (Ericaceae) is famous world-wide for the diversity of floral colors and forms of its more than1000species. Among them, over800species are distributed throughout the Northern Hemisphere, ranging from tropical to polar climates and more than500species are native to China. The genus is divided into eight subgenera (i.e. Azaleastrum, Candidastrum, Hymenanthes, Mumeazalea, Pentanthera, Rhododendron, Therorhodion and Tsutsusi). The two subgenera Azaleastrum and Tsutsusi together with the subgenus Pentanthera comprise the ’azaleas’. There are over28000cultivars in the world, but only about300cultivars in China, most of which were exotic cultivars arrived from Japan and Europe. The present classification system is put forward by Huang and Qiang (1984) according to the phenotype and the origin:(1) East azaleas, formed by cultivars with small leaves and hose-in-hose flowers;(2) West azaleas, including plants with large flowers and semi-double to double petals;(3) Hairy azaleas, characterized by large simple flowers and finely pubescent stems and leaves; and (4) Summer azaleas, cultivars blooming in the early summer.Over the years, just a little work has been done on the cultivar diversity and classification. In this research, we collected130azaleas, analyzed the genetic relationships and diversity among them based on numerical taxonomy, AFLP and SSR markers, and managed to put up a new classification system based on the principles of "Dual Classification Method". The main results were as follows:1. Numerical taxonomy on azalea cultivars in ChinaSixty-six evergreen azalea cultivars were investigated in this study based on thirty morphological characters (14qualitative and16quantitative). Cluster analysis using unweighted pair group method with arithmetic average (UPGMA) and principal coordinate analysis (PCO) was performed then using NTSYS software. Mantel’s test showed a good fitness between the cluster analysis and Euclidean distance matrix. The first three axes in PCO explained49.94%of the total variation, and revealed that14characters, most of which are related to flowers, played an important part in numerical taxonomy. Both cluster analysis and PCO suggested that cultivars in the East and Summer groups were distinct from other groups in the four-group classification system, but the West and Hairy groups were closer when concerned primarily with phenetic relationships.2. Genetic relationships between azalea cultivars using AFLP markersAmplified fragment length polymorphism (AFLP) markers were employed to obtain a clear view of azalea cultivars in China. One hundred and thirty genotypes collected from gardens and nurseries, including cultivars classified in the groups East, West, Hairy, and Summer, unknown cultivars and close species, were analyzed using three primer combinations. A total of408polymorphic fragments were used in data analysis with an average of136fragments per primer pair. Genetic similarities were generated based on Dice coefficients, used to construct a neighbour joining tree, and bootstrapped for100replicates in Treecon vl.3b. Principal coordinate analysis was performed based on Dice distances using NTSYS-pc software. The first two axes explained a total variation of15.6%. The AFLP technique was useful for analyzing genetic diversity in evergreen azaleas. Cluster analysis revealed that cultivars in the West and Summer groups were quite distinct from other groups in the four-group classification system and the East and Hairy groups should be redefined.3. Genetic diversity analysis based on SSR markersSix SSR loci developed by Dendauw et al. and labeled with fluorescent dyes were used to evaluate the genetic diversity of130azaleas. A total of98alleles were detected with an average of16.33alleles per locus. The size of alleles ranged from 127bp to293bp. All the loci were polymorphic with polymorphism information content (PIC) ranged from0.73to0.92. The largest number of alleles was27detected from locus A02, from which the highest PIC and Shannon’s information index (I) were detected. UPGMA was used to analyze the genetic relationships based on Dice coefficients. Cluster analysis revealed that the four-group classification system was approximately reasonable, further more the cultivars in West or East group were more complicated than in Summer or Hairy group.4. Classification system of azalea cultivars using "Dual Classification Method"We did lots of work on cultivar collection, morphological character investigation, and genetic diversity analysis. Based on the principles of "Dual Classification Method" and these researches mentioned before, we managed to put up a new classification system of spring azalea cultivars, which took into consideration both cultivar origin and flower types. The three ranks were defined by branch habits, flower forms, and flower shapes separately. A total of fifty-eight spring azalea cultivars we investigated were classified into two groups, six subgroups, and fourteen forms according to the system. |
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