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Title: Chloroplast sequence data differentiate Maleae, and specifically Pyrus, species in the USDA-ARS National Plant Germplasm System

Author
item Volk, Gayle
item Henk, Adam
item Richards, Christopher
item Bassil, Nahla
item Postman, Joseph

Submitted to: Genetic Resources and Crop Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/4/2018
Publication Date: 10/1/2018
Citation: Volk, G.M., Henk, A.D., Richards, C.M., Bassil, N.V., Postman, J.D. 2018. Chloroplast sequence data differentiate Maleae, and specifically Pyrus, species in the USDA-ARS National Plant Germplasm System. Genetic Resources and Crop Evolution. 66(1):5-15. https://doi.org/10.1007/s10722-018-0691-9.
DOI: https://doi.org/10.1007/s10722-018-0691-9

Interpretive Summary: The USDA-ARS National Plant Germplasm System (NPGS) maintains an extensive Maleae collection, including Malus (apple), Pyrus (pear), and 23 additional taxa with production, ornamental, and plant breeding value. The collections are maintained primarily in the field in Geneva, New York (Malus), Corvallis, Oregon (Pyrus, Sorbus, and Cydonia), and Washington, D.C. (ornamentals). In this work, we sequenced 4 regions of the chloroplast from 728 accessions to determine the phylogenetic relationships amongst Maleae species within the NPGS. We focused in particular on the pear accessions and found that the genetic diversity of these chloroplast sequences was more appropriately represented as a network rather than branching tree structure. These data resolved genetic differences between Asian (Pashia) and European (Pyrus) species. Knowledge of the genetic relationships among Maleae taxa will help identify plant materials that could be used as rootstocks or for use in breeding programs. In addition, the tabular representation of Maleae genera in the NPGS facilitates the process of identifying species that would be valuable additions to NPGS collections to ensure the conservation of important Rosaceae genetic resources.

Technical Abstract: The USDA-ARS National Plant Germplasm System (NPGS) maintains a vast collection of Maleae, including 49 Malus taxa, 36 Pyrus taxa, and 24 additional genera with ornamental and plant breeding value. These plant genetic resources are primarily maintained clonally as trees or shrubs in field conditions, and seeds are also conserved for some species. We used NPGS Maleae taxa to assess the genetic diversity across the tribe Maleae and placed Pyrus taxa within this broader context using analytical methods that displayed the genetic relationships as a network, rather than as a traditional dendogram. Sequence variation from four conserved chloroplast regions unraveled the complex and often reticulate genetic relationships among and within 109 economically important Maleae taxa. In a broad sense, chloroplast haplotypes differentiated Pyrus species within Sections Pyrus and Pashia. The genetic relationships amongst Pyrus species were found to be complex, likely resulting from multiple hybridization and expansion/contraction events during the speciation process. Knowledge of the genetic relationships among Maleae genera and/or species may aid in the selection of materials for use as rootstocks and or breeding (hybridization) programs. Future collection efforts to augment the NPGS accessions within the tribe Maleae will improve the coverage and representation and assure conservation of important Rosaceae genetic resources in the NPGS.