Submitted to: Theoretical and Applied Genetics
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/1/2002
Publication Date: 12/10/2002
Citation: Chen, J., Staub, J.E., Tao, C., Jiang, J., Luo, X., Zhuang, F. 2002. Reproduction and cytogenetic characterization of interspecific hybrids from cucumis hystrix chakr.xc. sativus l.. Theoretical and Applied Genetics. Interpretive Summary: Intraspecific hybridization (crossing or mating between species that are closely related) has been widely employed in crop species to produce improved plant varieties for commercialization. Recently, however, the genetic diversity (genetic differences between plant species that are the result of gene differences) in many crops has become increasingly narrow (the genes between species have become more alike) following continued use of varieties with similar lineages (similar or identical parents) in plant breeding programs. Such narrowing of present day varieties makes such varieties more susceptible to death by insects and diseases. This phenomenon is especially true in crops such as cucumber where commercial varieties have very similar pedigrees. The incorporation of genes from wild cucumber species into commercial cucumber varieties would broaden the genetic diversity of cucumber. This has been a goal of plant geneticists and breeders for over 100 years. Such intraspecific crossing or mating in cucumber has not been successful. Recently, a wild species of cucumber has been discovered in China that can successfully mate with commercial cucumber (i.e., produce seed). This species has genetic resistance to nematodes (worms) that is not found in commercial cucumber. If the genes for nematode resistance could be transferred to commercial cucumber through simple matings between these species, then commercial cucumber would be more productive in growing areas infested with nematodes. The first mating experiments been Hystrix and commercial cucumber have been conducted in our laboratory through cooperative exchange of seeds of this species. These matings have resulted in progeny that are very different than commercial cucumber, but possess many of the economically important traits of the wild species including nematode resistance. These progeny will be important to future breeding efforts of U.S. private and public breeders to improve the disease and insect resistance of cucumber. The use of these progeny will directly effect the consuming public by providing them with disease resistant, high yielding varieties with good fruit quality.
Technical Abstract: Interspecific hybrids between Cucumis hystrix Chakr. (2n = 2x = 24) and C. sativus L. (2n =2x = 14) were reproduced by means of F1 (2n = 19) embryo rescue and subsequent chromosome doubling. The hybridity was confirmed by genomic in situ hybridization (GISH) and chromosome analysis. The amphidiploid (2n = 38) was self-pollinated and backcrossed to cucumber resulting in lines with improved crossability to C. sativus. Considerable variability in morphology, and isozyme and DNA marker loci was observed among and between BC1 and BC1S1 offspring. Examination of shape, stainability, and germination rate of pollen grains and yield as a function of mature fruit set per 10 pollinated flowers indicated a tendency for increased fertility in BC1S1 progeny when compared to F1 and amphidiploid offspring. Cytogenetic characterization of F1 and amphidiploids progeny was performed. Generally normal meioses, pollen viability and frequent fertilization resulted in fertility restoration in amphidiploid progeny. Generally, the intervening time from metaphase I (M I) to telophase II (T II) was considerably shorter than that from prophase I to M I in pollen mother cells (PMCs) of both hybrids when compared to cucumber. In PMCs of the F1 hybrid progeny, 19 univalents were observed at diakinesis and M I. Chromosome anomalies such as fry-pan trivalent, chromosome lagging and spindle mis-orientation were observed at M I. In contrast, examination of amphidiploid progeny revealed that more than 90% of the figures at M I consisted of the predicted 19 bivalents with less than 5% of the figures containing multivalents [trivalents (2.3%) + quadrivalents (0.3%)], suggesting the presence of preferential pairing and parental genome distinctiveness. The chiasma observed between homoeologous chromosomes further demonstrated the introgression of the C. hystrix genome into that of C. sativus.