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Cucumis Taxonomy, Evolution, Interspecific Hybridization

USDA Vegetable Crops Research Unit
Cucumber and Melon Genetics

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Demonstrated Accomplishments of the Laboratory

3. Accomplishment: Cucumis Taxonomy, Evolution, and Interspecific Hybridization

Exotic germplasm used during breeding constitutes a major source of genes for the introgression of economically important genes during plant improvement. Several wild African (2n = > 22) and Chinese (2n = 24) Cucumis species possess disease resistance and water stress tolerance traits not available in commercial cucumber (C. sativus, 2n = 14) and melon (C. melo, 2n = 24). The laboratory has cataloged and reclassified formerly misclassified African Cucumis accessions, and has defined genetic relationships (#17, #24, #34, #35, #37, #58, and #66) in Cucumis species to make the first interspecific hybridization in Cucumis. A wild species, C. hystrix, originates from and is solely endemic to one southern Chinese province. Although not well studied, it has tolerance to nematodes, a pest of worldwide importance to both cucumber and melon production. The laboratory has: 1) acquired C. hytrix germplasm and accomplished the first successful interspecific hybridization in Cucumis (C. hystrix x C. sativus) (#109,#100,#112); 2) synthesized a new amphidiploid species (#147,#100,#142), and; 3) studied its genetic relationship with other Cucumis species (#123) after karyotype reevaluation (#116). Role: The senior author in studies of interspecific hybridization (Exhibit 1a) was a visiting scientist under the Dr. Staub's direction where he provided funding, and expertise in systematics, breeding, design and implementation of experiments to reconfirm previous preliminary experiments, and wrote a majority of the manuscript. In synthesis experiments (Exhibit 3b) the Dr. Staub was a team member and had major responsibility with the morphological measurements, with the determination of ploidy, and writing of the manuscript. In genetic relationship studies (Exhibit 3c), the senior author was a visiting scientist where the Dr. Staub's supervision included providing all funds for experimentation, expertise in cytology and cytogenetics, and writing a majority of the manuscript. Impact: The reclassification and biosytematic analysis of wild African species which are cross-incompatible with cucumber and melon has allowed for a more precise understanding of the potential utility of this germplasm for gene transfer by biotechnologists interested in introducing unique virus and insect resistance, and drought and salinity tolerance genes into cucumber and melon. In this regard, Dr. Staub has characterized the efficiency and effectiveness of RAPDs and isozymes for species identification in cucumber and melon accessions which led to recommendations to geneticists and biosystemicists for their use in species identification (i.e., Joseph Kirkbride, USDA, ARS). In addition, Dr. Staub, in cooperation with a visiting scientist, used this information to make a cross between C. hystrix and C. sativus producing a sterile F1 (2n = 19), which was then retrieved by embryo rescue, and subsequently chromosome doubled to produce a fertile amphidiploid (2n = 38). This amphidiploid or the derivatives there from offer the first possible genetic bridge between cucumber and melon where genes (e.g., disease and abiotic stress resistance) could be exchanged to improve either species and/or make a new synthetic species for commercial consideration. The classification of germplasm in the U.S. cucurbit collection supports NP #301 by increasing the knowledge of species in this collection. Furthermore, the creation of a new synthetic species (raw materials) utilizes germplasm that works toward ensuring American agricultural productivity by overcoming crossing barriers between cross-incompatible but related crop species (i.e., cucumber and melon) to introduce unique, economically important genes. See publications #109; #147; #123)

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