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

Location: Floral and Nursery Plants Research

2021 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
Our studies of ash (the genus Fraxinus) in the eastern United States have made it clear that to understand the origin of eastern North American genotypes in the largest section of the genus (sect. Melioides), it is necessary to compare the eastern ashes with species from the southwestern part of the country. Although travel restrictions prevented remote fieldwork, colleagues from various parts of the southwestern United States supplied fresh tissue samples from a diverse sampling of plants (ca 70 individual trees, representing all recognized species of sect. Melioides in the area). This extended our Fraxinus sampling across the continent, and revealed unexpected cytological variability in southern Califonia and Nevada. Further work in this area is needed to understand the genetics of ash, and to assist in effective conservation of ash germplasm in case Emerald Ash Borer or ash decline enter the western U.S. In Quercus (the oaks), continued analysis of DNA HybSeq data is giving us a more detailed view of patterns of genetic variation, and the relative contribution of local selection and interspecific hybridization to genetic variability of native oak species. Collaboration is being arranged with a group of University and Non-governmental organization (NGO) programs doing oak genomics, which will allow our studies to be continued with larger sample sizes, and on whole-genome sequences rather than a limited number of genes. Analysis of genetic, morphological, and cytological data from native Celtis species, with collaborators from Agriculture and Agri-Food Canada, is clarifying patterns of genetic variability in the native apomictic complex. These plants are shrubs and small trees with tolerance to harsh conditions, but they have been underutilized because of diseases affecting many Celtis genotypes and a poor understanding of the available genetic resources. Our studies will help to characterize the diversity of germplasm available in these plants. A publication validating thirty cultivar registrations for genera of woody plants that do not have their own registrar was produced in collaboration with the Gardens Unit. This includes new cultivar registrations that have been made from December 2016-January 2021, and several older registrations that were never completed through formal publication. This work will ensure that horticulturists are aware of newly-released cultivars, that names in use are well founded and their application is clearly defined, and that information on their origins and characteristics are available to horticulturists. The Herbarium began a mass digitization project to image and database all 800,000 specimens in the collection. Using high-throughput conveyor technology and automated software, the contractors can generate about 4,000 images per day. To date, nearly 300,000 specimens have been imaged. This project also includes a new collections/digital asset management system (database), an online data transcription platform, and a website for virtual access to the specimens. The latter two items are still under development, but the database has launched and is now the herbarium’s primary data repository. In addition to digitization, staff have also focused on preservation efforts, including the establishment of a dedicated archival space and prioritizing the maintenance of priceless botanical objects that require special care. The herbarium grew by more than 1,250 sheets through acquisitions and newly-mounted material. In addition, 115 new type specimens were discovered within the collection. We continued to analyze data on genome size and morphology in escaped populations of Hedera (ivy), a genus important in horticulture that has become invasive in the United States. Analysis of cytological and morphological data from samples of invasive populations in the United States has revealed complex variation in tetraploid populations. Hybridization among the tetraploid groups may be contributing to the development of invasive genotypes in the western United States.


Review Publications
Svoboda, H., Mayfield, M.H. 2020. Lectotypification of the Allium perdulce (Amaryllidaceae). Phytotaxa. 434(1):128-130.