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Research Project: Systematics, Nomenclature, and Genetic Diversity of Priority Genera of Woody Landscape Plants

Location: Floral and Nursery Plants Research

2019 Annual Report

Objective 1: Conduct systematic and genetic analyses of priority woody landscape plant genera (chiefly Ulmus, Quercus, and Fraxinus) to elucidate their systematic relationships, revise their taxonomic classifications, and assess the amount, apportionment, and nature of the genetic diversity they contain. Record and disseminate characterization data via GRIN-Global and other data sources. Subobjective 1A – Elucidate the taxonomy and cytology of Fraxinus (ash) in eastern North America. Subobjective 1B – Determine the distribution of alleles of expressed loci in oaks (Quercus). Subobjective 1C – Clarify the taxonomy and diversity of Ulmus (elms), emphasizing potentially disease-resistant germplasm in subg. Oreoptelea. Subobjective 1D – Clarify the taxonomy and reproductive biology of Celtis. Sub-objective 1E – Carry out taxonomic research that meets the needs of stakeholders, such as the production of regional floras and botanical manuals, in collaboration with other scientists and horticulturists. Objective 2: Efficiently and effectively maintain and expand the U.S. National Arboretum Herbarium, Seed Herbarium, and associated documentation as foundational biological resources for agricultural, botanical, and plant genetic resource research. Objective 3: Determine the identity and basic biological characteristics of selected invasive woody plants in the United States.

Work will involve study of plant material, both in wild habitats and cultivated in our botanical garden. We will carry out classical studies using the morphology of the plants, flow cytometry to estimate genome size and thus ploidy level, and study of various DNA markers, including SSR markers for simple parentage studies and NextGen sequencing studies to gather detailed information on evolutionary relationships and history of plant groups. Research will also involve a wide variety of analyses, including phenetic analyses for investigating overall patterns of variation, and searching for clusters and discontinuities in the distribution of plants through multivariate character space; phylogenetic analysis for investigating ancestral relations among groups of plants; and admixture analysis for detecting hybridization.

Progress Report
Significant progress was made on all three objectives, which fall under NP301. We continue to collect material of Fraxinus (ash) with collaborators, determining the ploidy using flow cytometry and collecting material for DNA extraction. Work concentrated on tetraploids of uncertain affinity from the southeastern United States. DNA sequencing using both RAD-seq and shotgun techniques has been run and both data sets are being analyzed to determine relationships among the species, and the nature of polyploidy in the group (whether autopolyploidy or allopolyploidy). In addition, we have collaborated in genetic work to identify alleles conferring tolerance to emerald ash borer conducted primarily by the U.S. Forest Service and Kew Gardens in the UK. For our studies of Quercus (oaks), we are using two next-gen sequencing approaches for different questions. Previous studies by our lab and others provided strong evidence that hybridization between related oak species affects different parts of the genome to different degrees. We used RAD-seq data to identify loci that show relatively little alteration in groups with a history of hybridization, and can serve as efficient markers to allow accurate identification of the species in molecular data sets. We are also analyzing HybSeq DNA sequences of selected oaks made using a set of baits that include loci functioning in drought resistance and spring budbreak. This will allow us to directly characterize oaks for genes that affect physiological characteristics that are important for nursery use. Both of these studies are being carried out in the lab of a collaborator. We also participated in a study of variation in morphological characters in bur oak, to test different methods for quantifying and coding so they can be used as genetic markers. In Ulmus (elms), we have completed a phylogeny for the genus using RAD-seq DNA sequencing. This will allow us to identify species with maximum potential for introducing compatible genes for disease resistance and other desirable traits. We are also collaborating on genomic work on American elm with the U.S. Forest Service. We have completed a survey of genome size in Carya (pecans and hickories), in collaboration with the ARS Pecan Germplasm Repository. This will help scientists at the pecan repository plan genetic work with their collection. Herbarium work continues to progress with a temporary herbarium curator, an essential position. Approximately 2000 records were added to the database, and several hundred specimens, including many collections documenting recent research and germplasm projects, were added to the collection. We continued to survey genome size and morphology in escaped populations of Hedera (ivy), a genus important in horticulture that has become invasive in the United States, from across North America, in order to determine the distribution and degree of invasiveness for different species. Additional triploid ivies were discovered and brought into cultivation, so we can test them to determine whether they are able to produce seed. This work allows us to determine the relative invasiveness of different cultivated ivies, and to look for less invasive alternatives for commercially available ivies that are highly invasive.


Review Publications
Gallinat, A.S., Primack, R.B., Willis, C.G., Nordt, B., Stevens, A., Fahay, R., Whittemore, A.T., Du, Y., Panchen, Z.A. 2018. Patterns and predictors of fleshy fruit phenology at five international botanical gardens. American Journal of Botany. 105(11):1824-1834.
Lura, S.B., Whittemore, A.T. 2018. International registration of cultivar names for unassigned woody genera March 2013 - November 2016. HortScience. 53(8):1232-1237.