Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/15/2002
Publication Date: 11/15/2002
Citation: Staub, J.E., Dane, F., Reitsma, K., Fazio, G., Lopez-Sese, A. 2002. The formation of test arrays and a core collection in cucumber (cucumis sativus l.) using phenotypic and molecular marker data. Journal of the American Society for Horticultural Science. Interpretive Summary: Genetic variability (differences among genes) is essential for crop improvement, and therefore substantial resources have been directed towards collecting, preserving, and evaluating collections to access their genetic diversity. National genebanks, such as the U.S. National Plant Germplasm System (NPGS), maintain substantial collections of diverse genotypes of worldwide origin. These crop germplasm collections differ in size, scope, and reproductive biology, and as a consequence require different management strategies for their curation and genetic maintainence. Resources at genebanks (places where seeds are stored) are often not sufficient to adequately conserve, manage, and evaluate germplasm collections of ever-increasing size. The concept of "core collections" was introduced to improve managerial effectiveness of germplasm curation (the process of cataloging and maintaining seed for public use), and increase the availability of unique genetic resources for plant improvement. A core collection is a representative sample from an entire collection that allows for more efficient evaluation of genetic diversity, and for the identification of accessions of potential economic value. The NPGS cucumber collection is relatively small, consisting of about 1,350 seed lots (accessions) collected worldwide between 1925 to the present. A study was designed to construct a core collection for cucumber by examining the accessions visually and by studying the genetics of their proteins. A core collection of 147 accessions was made which will increase the efficiency of managing the national collection and providing the end-user (public and private scientists) with groups of accessions specific to their requests. This was not possible before this study was completed and will directly decrease the time it takes to develop new varieties.
Technical Abstract: Genetic relationships among 970 cucumber plant introductions (PIs) in the U.S. National Plant Germplasm System (NPGS) were assessed by observing variation at 15 isozyme loci. Allozyme frequency data for these PIs were compared to allozyme variation in heirloom and modern (H&M) cultivars released between 1846 and 1985 (H&M cultivars; 178 accessions), and experimental commercial (EC) germplasm (EC germplasm; 82 accessions) in use after 1985. Multivariate analysis defined four distinct groups of accessions (Groups A-D), where Group A consisted of PIs received by the NPGS before 1992, Group B contained PIs from India and China obtained by NPGS after 1992, Group C consisted of EC germplasm, and Group D contained H&M cultivars. Morphological, abiotic stress (water and heat stress tolerance) and disease resistance evaluation data from the Germplasm Resources Information Network (GRIN) for the PIs examined were used in conjunction with estimates of population variation and genetic distance estimates to construct test arrays and a core collection for cucumber. Disease resistance data included the evaluation of angular leafspot [Pseudomonas lachrymans (E. F. Smith) Holland], anthracnose [Colletotrichum lagenarium (Ross.) Ellis & Halst], downy mildew [Pseudoperonospora cubensis (Berk. & Curt) Rostow], rhizoctonia fruit rot (Rhizoctonia solani Kuhn), target leafspot [Corynespora cassiicola (Berk. & Curt) Wei] pathogenicity. The test arrays for resistance/tolerance to angular leafspot, anthracnose, downy mildew, rhizoctonia fruit rot, target leafspot, and water and heat stress consisted of 17, 16, 17, 16, 17, 16 and 16 accessions, respectively. The core collection consisted of accessions in these test arrays (115) and additional 32 accessions which helped circumscribe the genetic diversity of the NPGS collection. The core collection of 147 accessions (115 + 32) represents about 11% of the total collection's size (1352). Given estimates of genetic diversity and theoretical retention of diversity after sampling, this core collection could increase curatorial effectiveness and the efficiency of end-users as they attempt to identify potentially useful germplasm.