Dynamic use of the National Collection of Genetic Resources for Pecans and Hickories

Authors: Grauke, Larry

Submitted to: Journal of the American Pomological Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2019
Publication Date: N/A
Citation: N/A

Interpretive Summary: When the National Clonal Germplasm Repository for Pecans and Hickories was first envisioned in the late 1970's, it was based on collections of grafted trees that had been brought together for use as parents in the USDA Pecan Breeding Program. A team of plant, insect and disease specialists from across the United States and Mexico worked together in 1984 to plan how to make collections of seeds across the range of pecan and hickory species. Many collections were made and test systems were established in order to find diversity, not just to find a special tree to propagate. Graftwood and seed have been distributed from this collection to cooperators in the United States and internationally. Most of what has been distributed has been commercially important pecan cultivars (cultivated varieties) and seedstocks. Test systems have been been used to study the differences in this group of trees across their ranges. New methods have been developed that will help the next generation of breeders select better rootstocks and grafted cultivars for commercial orchards in widely different regions. The new methods will also help develop new strategies for selecting improved materials for other users, including plant breeders from other countries. The long-lived trees in the test systems of the NCGR-Carya will continue to be valuable for research, to educate students and to guide breeding. The dynamic use of genetic diversity in this collection has transformed the way we breed pecans.

Technical Abstract: Our unit was initially designated as a National Clonal Germplasm Repository based on an existing collection of grafted cultivars developed for use as parents by the USDA ARS Pecan Breeding Program. From its inception, the NCGR-Carya has pursued a dynamic collection strategy with three distinct targets: 1) collection of named cultivars from culturally distinctive growing regions to represent selections made over the 170+ year history of grafting pecans; 2) provenance collections of self-rooted seedlings to represent genetic diversity in the context of geographic origin; and 3) representative collections of Carya species to provide the broadest base of potentially compatible genetic diversity. The entire collection currently is maintained as living inventories at two worksites in Texas, where 5 ARS workers maintain trees on 242 hectares. Graftwood distributions have historically been requested from only a small subset of accessions, primarily the most recently released cultivars, while seed distributions have been from a small subset of primarily standard seedstock sources. Graftwood exchange is impacted by the presence of disease caused by the bacterial pathogen Xylella fastidiosa. The broad genetic diversity available in the collection is being used by multiple international teams for developing improved methods of molecular characterization based on genome sequencing. Inventory specific records of NCGR-Carya provide the "type" collections for genomic profiles of taxonomic species and are a particularly valuable laboratory for studying interspecific hybridization. Phenotypic records linked to pecan provenance inventories in replicated test configurations provide evidence of regional adaptation related to genetic diversity that varies between geographic subpopulations. Historically documented passport records of regional pecan cultivars linked to their phenotypic and genomic characterizations provide the foundation for identity verification procedures and tools for development of marker assisted selection. This accessible, diverse, living collection of trees is contributing to the development of the next generation of Carya scientists, as they work toward the refinement of improved genomic techniques. The next generation of breeders will use these tools to select cultivars with traits targeted for improved regional performance from broad foundations of genetic diversity available within the collection. Long-lived test systems on NCGR-Carya worksites will provide foundations for future development of improved techniques such as remote imaging. As this generation of breeders works cooperatively within these diverse collections, we hope to facilitate continued coordinated use.