Genetic Diversity and Distribution within Cultivated Gene Pools of Chionanthus retusus (Oleaceae) in the United States

The chinese fringe tree (Chionanthus retusus Lindl. & Paxton) has a broad distribution in China with disjunct populations found in Korea, Japan, and Taiwan.  Although introduced to the United States on several occasions from different sources and provenances, the extent to which these introductions were established in cultivation, and their contribution to the current cultivated C. retusus-gene pool is unknown. Genomic simple sequence repeat (SSR) markers were used to screen a broad sampling of C. retusus from commercial nurseries and botanical collections to assess genetic diversity and structure. Seventy-one SSR loci tested were highly polymorphic across 63 accessions of C. retusus. The mean number of alleles per loci and mean polymorphic information content was 8.7 and 0.61, respectively.  The mean observed heterozygosity (0.35) was low relative to mean expected heterozygosity (0.64).  Nineteen loci (26.7%) deviated significantly from Hardy Weinberg equilibrium and the mean frequency of null alleles was 0.33. Low observed heterozygosity and disequilibrium are indicative of the narrow genetic base for the initial introductions of C. retusus and subsequent inbreeding and clonal propagation. Distance and cluster analyses are congruent, and the resulting population clusters (K=7) are supported by collection and provenance data. Commercially available C. retusus consists primarily of three populations. The largest and most diverse population originates from northeastern China, and includes historical introductions and their progeny augmented with new Chinese-sourced commercial seed.  The second population is based on the original Japanese introduction in 1901, exhibits low diversity, and is essentially derived from the cultivar Arnold’s Promise.  The third population originated from a small initial Taiwanese introduction and is widely cultivated in hardiness zone 7 and south.  The southern ecotype previously identified in the literature is associated only with material from Taiwan, and is represented primarily by the cultivar China Snow. The newly introduced cultivar, Tokyo Tower, is genetically similar to a fourth population from Tsushima Island, Japan, which had not previously contributed to the cultivated gene pool.  Additional populations from China and Korea are only represented in botanical collections.  Therefore, American plant breeders have additional, though limited, genetic diversity available for the development of novel C. retusus cultivars.