Submitted to: Plant Breeding
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/20/2003
Publication Date: 9/20/2003
Citation: Zhuang, F.Y., Chen, J.F., Staub, J.E., Qian, C.T. 2003. Assessment of genetic relationships in cucumis species by SSR and RAPD marker analysis. Plant Breeding. 123:167-172. Interpretive Summary: Recently an exotic relative of cucumber, Cucumis hystrix, was discovered in a southern province in China. This species has the same number of chromosomes (24, structures in the plant's cell that hold genetic information) as melon, but when mated with melon does not produce offspring. However, it does mate successfully with cucumber (14 chromosomes) to produce sterile offspring. This is a very unusual occurrence in plants. The chromosomes of the sterile offspring of the cucumber and C. hystrix mating were doubled to produce fertile offspring (38 chromosomes) which themselves mate freely with cucumber. Since cucumber and the wild relative can mate and produce offspring that are fertile after chromosome doubling, this provides a means of transferring genes (specific entities on chromosomes) which govern such things as resistance to certain diseases which are absent in cucumber. Thus, one can use genetic manipulation (mating by controlled pollinations by humans) to improve the cucumber for traits such as disease resistance. However, it is a problem to identify the genetic nature (genes) of offspring to more clearly understand the relationships of the offspring, the parents, the original C. hystrix from which they were derived, and other closely related species. There exists biotechnology that allows for a way to identify differences between individuals called molecular markers (DNA that marks specific points on the chromosome). We designed experiments to use molecular markers to identify the differences between C. hystrix, its derived offspring from matings with cucumber and other closely related species. The objective was to determine the genetic relationships at the DNA level. Data indicated that molecular markers were useful for detecting such differences. The results of this experimentation describes relationships which will allow plant geneticists and plant breeders to be more effective and efficient in transferring genes for resistance from wild species (C. hystrix) to cucumber. This will allow for improved varieties which will make the U.S. grower more competitive.
Technical Abstract: The first successful production of a sterile interspecific hybrid obtained from a cross between C. hystrix Chakr. (2n = 2x = 24) and C. sativus var. sativus L. (2n = 2x = 14), and its subsequent fertility restoration through chromosome doubling provide an effective means for investigating genetic relationships among Cucumis species. In this study, RAPD and SSR markers were used to investigate relationships among C. s. var. sativus L, C. s. var. hardwickii (R.) Alef., C. hystrix, C. hytivus Chen and Kirkbride (the amphidiploid species from chromosome doubling of the C. sativus x C. hystrix interspecific hybrid, 2n = 38), C. melo L., (2n = 2x = 24), and C. metuliferus Meyer & Naudin (2n = 2x = 24). One hundred and nine SSR bands and 398 RAPD primed sites were used to calculate Jaccard's distance coefficients for cluster analysis using a UPGMA algorithm. The genetic relationships identified using SSR and RAPD markers were highly concordant, such that the correlation between SSR and RAPD genetic distance (GD) estimates was r = 0.94. SSR and RAPD analysis of 22 accessions allowed for their grouping into two distinct groups designated as CS and CM. While group CS consisted of 11 C. sativus genotypes, and the C. hytivus and C. hystrix accessions, group CM included six C. melo genotypes and C. metuliferus. The GD values between C. hystrix and C. sativus ascribed by SSR and RAPD matrices were 0.59 and 0.57, respectively. These GDs were smaller than those detected between C. hystrix and C. melo (0.87 and 0.70 derived from SSR and RAPD markers, respectively).