Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2006
Publication Date: 6/10/2006
Citation: Zhuang, F., Chen, J., Staub, J.E., Qian, C. 2006. Taxonomic relationships of a rare cucumis species (C. hystrix chakr.) and its interspecific hybrid with cucumber. HortScience. 41:571-574. Interpretive Summary: Taxonomy (the classification of plants) is an important science that defines the genetic and evolutionary relationships among plants. This science is critical to plant breeding since it allows the plant breeder to better understand the relationship between cultivated and non-cultivated plants (exotic plants that have important horticultural traits not present in cultivated plants) that are often used to improve crop species. That is, certain exotic plants cross readily with cultivated crop species while others do not. Plant breeders regular consider the expense of determining whether to use exotic plants based on their knowledge of plant taxonomy. In cucumber there exists a newly discovered exotic plant species called C. hystrix that may be important to plant improvement programs because it has resistance to diseases that are not present in cultivated cucumber. It is important to determine the genetic relationship between this species and cucumber in order for plant breeders to make important financial comments with their limited resources. Therefore, studies were carried out to determine the genetic relationship between C. hystrix and cultivated cucumber using biotechnology (tools that allow for the dissection of genetic components of difference between plants). Biotechnological tools (genetic markers) were used to determine the genetic relationship between these two types and a closely related species melon. It was determined that, although C. hystrix is genetically closer to cucumber than to melon, crossing C. hystrix with cultivated cucumber must consider its unique genetic constitution. The results will assist plant breeders in transferring the potentially useful traits of C. hystrix into cultivated cucumber such that plants derived from such crossing will improve cucumber. The grower will benefit from such improvements since the plants that are grown will be more disease resistant than existing commercial cultivars. This will allow for increased competitiveness in a global market place where cultivars that exhibit disease resistances not found in commercial types will increase the grower’s profitability.
Technical Abstract: In the current Cucumis taxonomic classification, Cucumis hystrix Chakr. is placed in subgen. Cucumis based on its morphological similarities to cucumber (C. sativus L., 2n = 14). However, the chromosome number of C. hystrix was identified as 2n = 24, the same number as in subgen. Melo. C. hystrix is therefore considered the first wild Cucumis species of Asiatic origin possessing 12 basic chromosomes. Thus research in its biosystematics would challenge the basic chromosome number and geographic location theories that govern the current taxonomic system. The successful production of the amphidiploid species (Cucumis ×hytivus Chen & Krikbride, 2n = 38) obtained from the cross between C. hystrix and C. sativus and subsequent chromosome doubling provides us with an effective way of investigating the relationship between Cucumis species with two different basic chromosome numbers. In the present study, RAPD markers were used to study the taxonomic placement of C. hystrix and its interspecific hybrid with cucumber. From 220 arbitrary primers screened, thirty-one were used, and among 402 polymorphic fragments generated, 96.3% were polymorphic. The genetic distances were computed with Jaccard’s coefficient, and the polygenic tree was established using the UPGMA method. Thirty-one accessions were grouped into two main groups (CS and CM). Under the coefficient threshold of 0.23, these two groups can be further divided into five clusters with C. hystrix, C. ×hytivus, and C. sativus as separate clusters in the CS group. A modified taxonomic system is proposed with the genus Cucumis containing subgen. Cucumis with three Series and subgen. Melo with six Series.