ARS | Publication request: Taxonomic Relationships of a Rare Cucumis Species (C. Hystrix CHAKR.) and Its Interspecific Hybrid with Cucumber

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 15, 2004
Publication Date: November 20, 2004
Citation: Zhuang, F., Chen, J., Staub, J.E., Qian, C. 2004. Taxonomic relationships of a rare cucumis species (C. hystrix Chakr.) and its interspecific hybrid with cucumber. HortScience. 41:571-574.

Interpretive Summary: Plants are classified into genus and species designations which are the lowest levels of plant classification. Cucumber and melon on in the genus Cucumis and in their respective species categories as sativus and melo, respectively. It has been more than two and half centuries since Linnaeus the great taxonomist (plant name classifier) defined the genus Cucumis. There exists another plant species called by genus and species Cucumis hystrix which has its origin in China. All plants have chromosomes that are composed of genes that are made up of DNA. Cucumis hystrix is more closely related to cucumber than melon although it has the same number of chromosomes are melon. Since it important for plant improvement programs to determine what the relationships are among closely related species a study was designed to use DNA makers (biotechnological tools that can distinguish between the DNA of species) to determine the differences between cucumber, melon and Cucumis hystrix. Data indicate that in fact, C. hystrix is more closely related to cucumber than melon based on DNA analysis. This information will allow plant geneticists and plant breeders to use C. hystrix in conventional crossing with cucumber to introduce genes not present in cucumber such as resistance to nematodes and fungi. This will allow the U.S. grower to eventually grow cucumber more economically making them more competitive.

Technical Abstract: Currently, Cucumis taxonomic classification of the wild species, Cucumis hystrix Chakr. (2n = 24) found solely in Southern China places it taxonomically in the subgen. Cucumis based on its morphological similarities to cucumber (C. sativus L., 2n = 14). Given that the chromosome number of C. hystrix is identical to species of subgen. Melo that are of African origin, its Asiatic origin suggests a unique taxonomic status. Thus, data defining the taxonomic relationships among African and Asian Cucumis species might be used to investigate long-held theories of Cucumis species origin that have been based on chromosome number and geographic location. The creation of an amphidiploid Cucumis species (C. hytivus Chen & Krikbride, 2n = 38) obtained from an initial cross between C. hystrix and C. sativus (2n = 19) with subsequent chromosome doubling provides an opportunity of investigating the genetic relationships among Cucumis species with different chromosome numbers (2n = 14 vs. 24). A set of RAPD markers was, therefore, employed to study genetic affinities among 31 Cucumis African and Asian accessions. Thirty-one of 220 arbitrary RAPD primers used to screen these accessions provided 417 bands for analysis of which 97.7% were polymorphic and reproducible. After genetic distance calculation (Jaccard's coefficient) and cluster analysis, accessions were grouped into two main clades designated as CS [C. sativus (2n = 14)] and CM [C. melo (2n = 24)]. At the genetic distance threshold of 0.37, these clades were further divided into six clusters such that the C. hystrix, C. hytivus, and C. sativus accessions examined were partitioned into separate clusters in CS group. Data recapitulates the previously existing taxonomic position and structure of subgen. Melo containing six Series (Humifructuosi, Melo, Hirsuti, Metuliferus, Angurioidei, and Myriocarp). Based on these results and those of previous studies, a testable hypothesis is proposed which provides for the creation of a provisional Cucumis species complex to include a subgenus Cucumis containing three Series; Hystrix, Hytivus, and Sativus. This hypothesis requires rigorous testing (e.g., chloroplast marker analysis), and would be the initial step in the precise taxonomic placement of C. hystrix, C. hytivus, and C. sativus.