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

Location: Floral and Nursery Plants Research

2022 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
We studied native American Fraxinus (ash) species and clarified several taxonomic questions in the genus by examining morphological traits and chromosome numbers in extensive samples of wild ash populations across the United States. This work documents distribution of major gaps in genetic diversity in the genus, which will inform more effective conservation efforts. Restriction site-associated DNA sequencing (RAD-seq) data for 194 North American wild ash were obtained, allowing more detailed documentation of the taxonomy and genetic diversity of our native ash populations. We analyzed the geographical variation in leaf shape and structure of Quercus (oak) species, which was used to investigate statistical methods for designing efficient sampling schemes for the genus. The distribution, chromosome diversity, and genetic variation of Celtis (hackberry) from eastern North America was documented for species that reproduce clonally and sexually. We also gathered extensive data on chromosome variability and pollen stainability for several Celtis species. We surveyed invasive populations of Hedera (English ivy) for morphological and chromosome diversity, confirming that four species in the genus are invasive in North America, and clarifying their invasive ranges. Two tetraploid species appear to hybridize extensively in California, possibly creating new, more invasive genotypes. Several triploid plants were found, which could prove to be seed-sterile, thus incapable of dispersing far outside of cultivation. Accessions of these latter genotypes were brought into cultivation to test whether they can produce viable seeds. The mass digitization of the U.S. National Arboretum Herbarium concluded, culminating in nearly 2.8 million digital image files (four file versions for each specimen) and a complete catalogue of the herbarium’s taxonomic names. Collection information from each specimen’s label is currently being transcribed using a crowdsourcing platform, which is on track to finish all 700,000 accessions by the end of the fiscal year. This was a 13,500% increase in productivity compared to the digitization efforts previously employed and allowed the National Arboretum to become the first major federal natural history collection to be completely imaged. In addition to digitization, we made considerable advancements related to the management and growth of the herbarium: 1067 new incoming acquisitions; 1822 outgoing exchange and gift specimens; 87 new type specimens discovered; distribution of 308 specimen images for scientific investigation; four outgoing loans, including material used as evidence in a federal criminal trial; 785 newly mounted specimens via a pilot at-home volunteer project; and hosting seven visiting researchers. A large collection of preserved material from the U.S. Geological Survey was donated and incorporated into the National Arboretum Herbarium, which included more than 6500 accessions. We made significant progress on existing backlogs of unmounted and/or unlabeled material, often requiring consultation of GRIN-Global or original source (analog) documentation to create adequate labels for permanent preservation of the vouchers.

1. Modernization of the USDA’s preserved plant collection. Preserving plants and the data about them is an important means of documenting how people interact with the natural world and provides a biological snapshot of a given time and place. The U.S. National Arboretum in Washington, D.C., maintains an extensive repository of specimens, known as an herbarium, that is consulted by researchers worldwide for myriad uses and scientific discovery. However, the wealth of information housed in this herbarium has been relatively out of reach for many because of the necessity to visit the facility in person or request a loan. In 2021, ARS scientists at the National Arboretum worked with a contractor to use high-throughput conveyor technology and a crowdsourcing platform to digitize the entire 700,000-specimen collection to increase the accessibility and relevance of these invaluable botanical resources to a diverse audience. Once made available online, this digital collection will revolutionize how the USDA’s preserved plants are discovered, utilized, and appreciated by the public.

Review Publications
Whittemore, A.T., Fuller, R.S., Brown, B.H., Hahn, M., Gog, L., Weber, J.A., Hipp, A.L. 2021. Phylogeny, biogeography, and classification of the elms (Ulmus). Systematic Botany. 46(3):711-727.
Whittemore, A.T., Schori, M. 2022. A new nothospecies in Lagerstroemia (Lythraceae). Phytotaxa.