Submitted to: Genetic Resources and Crop Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2016
Publication Date: 9/7/2016
Citation: Thammina, C., Olsen, R.T., Kramer, M.H., Pooler, M.R. 2016. Evaluation of genetic diversity and relatedness of boxwood accessions using genic simple Ssquence repeat markers. Journal of the American Society for Horticultural Science. DOI 10.1007/s10722-016-0436-6.
Interpretive Summary: Boxwood (Buxus L. spp., Buxaceae) are popular woody landscape shrubs or small trees grown for their diverse forms and broad-leaved evergreen foliage. They are used in the landscape as specimen plants, hedges, parterres, mass plantings, in containers, or as topiary. Each year, more than 13 million boxwood plants are sold in the United States, with an annual market value of over $100 million. Boxwood plants grown in temperate zones are increasingly threatened by a destructive new fungal blight disease called boxwood blight. To date, all tested cultivated Buxus taxa are affected by boxwood blight, although some taxa appear to be more susceptible to the fungus than others. The National Boxwood Collection at the U.S. National Arboretum (USNA) contains more than 700 Buxus accessions, making it one of the most complete collections in the world and a valuable genetic resource for developing blight-tolerant varieties. However, genetic diversity and relatedness of these accessions have not been determined. We used molecular markers to characterize the diversity and identity of the accessions in the National Boxwood Collection at the U.S. National Arboretum. This information will be used for breeding programs and collection management, and for identifying possible sources of disease tolerance in boxwood.
Technical Abstract: Boxwood (Buxus L. spp., Buxaceae) are popular woody landscape shrubs grown for their diverse forms and broad-leaved evergreen foliage. Boxwood plants grown in temperate zones are now threatened by a destructive new blight disease caused by the ascomycete fungus Calonectria pseudonaviculata Henricot (syn. Cylindrocladium pseudonaviculatum, Cylindrocladium buxicola). We used genic simple sequence repeat markers (genic-SSRs) to assess genetic diversity and relatedness of 275 accessions from the National Boxwood Collection at the U.S. National Arboretum. Flow cytometry was conducted to determine the relative ploidy level of each accession to confirm marker data with respect to allele numbers. Genic-SSR loci were highly variable among the accessions, detecting an average of 6.7 alleles per locus using 17 primer pairs. Data were analyzed using the R polysat package, with a distance matrix based on Jaccard’s similarity index, followed by UPGMA clustering. Seven major clusters were identified consisting of individual accessions from B. balearica, B. harlandii, B. microphylla, B. sempervirens, B. sinica, and their interspecific hybrids. The accessions generally clustered by cultivar, provenance, or species. Clustering within each group generally reflected breeding pedigrees and was supported by bootstrap replicates. This information will be used for breeding programs and collection management, and for identifying possible sources of disease tolerance in boxwood.