Submitted to: Euphytica
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/4/2003
Publication Date: 10/20/2003
Citation: Chen, J., Luo, X., Staub, J.E., Qian, C., Zhuang, F., Ren, G. 2003. An allotriploid derived from a amphidiploid x diploid mating in cucumis i: production, micropropagation and verification. Euphytica. Interpretive Summary: There are many types of commercial cucumber varieties. However, the genetic (inherited) differences between these varieties is low (i.e., they are very similar to each other). When varieties are similar to each other they are susceptible to attack by disease and insects. Such attacks can significantly reduce yield and quality, and thus economically challenge the U.S. grower. There are opportunities to increase the genetic difference between varieties by crossing them to wild, exotic cucumber types that have genes for resistances now not in commercial varieties. Such transfer of genes through common crossing has allowed for gene transfer between closely related but different individuals (traditional plant breeding). Genes for disease resistance not found in commercial varieties and in wild closely related intercrossing (mating) individuals that can be found in related but cross incompatible (not able to cross with each other) individuals not of the same species. Such is the case in cucumber. A plant exists only in one province in China that has been genetically manipulated (by new biotechnologies) to cross with cucumber. The offspring of this manipulation have differing fertilities (degrees of being able to reproduce) ranging from infertile (sterile) to partially fertile (some ability to reproduce). Experiments were undertaken to identify the most fertile offspring. Through conventional microscope observation, partially fertile individuals were identified and their form (plant characteristics) were described. This is the first time that such individuals have been identified and characterized. Since they possess genetic characteristics that are not common to cucumber, they will, when crossed with cucumber, increase its genetic diversity. This will in turn allow plant breeders to transfer economically important genes not now present in cucumber, to increase the competitiveness of the U.S. grower and provide the consumer with a product at a low price.
Technical Abstract: A fully fertile interspecific hybrid (Cucumis hytivus Chen and Kirkbride, 2n = 4x = 38) between Cucumis hystrix Chakr. (2n = 2x = 24) and C. sativus L. (2n =2x = 14) was previously produced by means of F1 (2n = 19) embryo rescue and subsequent chromosome doubling. This amphidiploid, a new synthetic species, may serve as a genetic bridge in Cucumis, and thus is a source for broadening the genetic base of C. sativus. The identification and characterization of fertile progeny possessing lower ploidy levels would facilitate bridging among Cucumis species. Putative allotriploids (2n = 26) were recovered from C. hytivus x C. sativus matings by means of embryo culture, and experiments were designed to confirm their genetic constitution, describe their morphology, and establish an efficient protocol for their micropropagation. Apical and axillary buds of these putative allotriploid plants were used as explants to establish a micropropagation system for subsequent verification and characterization of ploidy. Of the array of microprogation media tested, the most effective for the induction of adventitious buds (desginated Stage II) was a Murashige and Skoog (MS) growth media containing 13.3µM BA + 1.1µM NAA or containing 10 µM BA only. The mean number of adventitious buds per explant in the two media was 6.8 and 6.5, respectively. Shoots resulting from adventitious buds produced roots (Stage III) in relative abundance (39 of 42, 92.8%) on half-strength MS medium containing 1.0 µM IBA. The survivorship of rooted plantlets after acclimatization as assessed by relative production of leaves in plantlets (designated Stage IV) was 91.4% (148 of 162). The chromosome number in putative allotriploid plants as determined in mitotic root tip figures in all plants was 2n = 26, the number expected for allotriploids derived from such a mating. An examination of pollen viability in five samples of each plant by cytochemical staining revealed stainability to be < 10%. Compared to their parents, the allotriploid genotypes possess a higher degree of parthenocarpy (20% higher) as measured by setting fruit in pollen-free conditions. While allotriploid fruit are black-spined and similar to the maternal parent C. hytivus, the dark green leaves typical of allotriploid plants mirrors that of the paternal C. sativus parent.