Cucurbit Genetics Cooperative Report 19:34-35 (article 13) 1996
Regeneration of Interspecific Hybrids of Cucumis Sativus L. x C. hystrix Chakr. by Direct Embryo Culture
J.F. Chen and J.E. Staub, Y. Tashiro
Vegetable Crops Research, USDA/ARS, Department of Horticulture, University of Wisconsin-Madison, WI 53706 U.S.A., Plant Cell Engineering, Faculty of Agriculture, Saga University, Saga 840, JAPAN
Introduction. Cucumis hystrix is a wild species of Cucumis subgen. Cucumis, which originated in Asia (Kirkbride, 1993). It has a taste and flavor typical of cucumber (2n =14) (Chen et al. 1994), but its diploid chromosome number is 24 (unpublished data). Previous work employing isozyme analysis hypothesized, a triangular phylogenetic relationship among C. hystrix, C. savitus and C. melo (Chen et al.,1995). However, the genetic distance between C. hystrix and C. Sativus was smaller (0.50) than that calculated between C. hystrix and C. melo (0.71).
Materials and Methods. Interspecific crosses were made by conventional crossings of two types of Chinese cucumbers (maternal parents with C. hystrix (paternal parent) in Japan between October 8 to 12, 1995, in the field. The fruits were harvested 50 days after pollination and then stored for 25 days at room temperature to improve maturity. The embryos were recovered and immediately cultured on MS hormone free solid medium with 3% sucrose, 0.8% agar, pH 6.0 at 25 C. Plantlets with four true leaves were then transferred to containers of vermiculite covered with a plastic bag for 5 days, followed by a 5-day period of gradual exposure in a greenhouse. When the plants had grown to about 30 cm in height, they were transferred to pots containing soil and fertilized weekly with a commercial nutrient formulation.
Results. All pollinations resulted in mature fruit. The embryo halted its development at the "rabbit-ear" stage. of 235 dissected seeds, 158 embryos ("heart" to rabbit-ear stages) were obtained. Embryos began to grow within 3 days and turned green in 5 days on MS medium. Roots developed in 8 days and embryo growth and development increased. A total of 59 normal plants were obtained. The regeneration rate was 37.3%. Hybrid plants grew vigorously and were relatively uniform for such traits as the diameter and internode length of stem, shape and size of leaf, and shape and size of flower when compared to the parents. However, some characteristics, such as multiple branching, densely brown hairs (especially on flowers), orange yellow corolla, and ovate fruit were obviously inherited from the paternal parent. The flower position in the stem was similar to that of the maternal parent. Some characteristics of flower structure, such as three separately elongated degenerated stigma on staminate flower and elongated stigma with brown hair on pistillate flower were not apparent in either parent.
The somatic chromosome number of these hybrid plants was 2n=19. Malate degydrogenase banding patterns observed using starch gels and glutamate oxaloacetate transaminase banding patterns present on polyacrylamide gels also confirmed the hybrid nature of these plants. Preliminary observation showed that the meiosis in PMCs was irregular. The staminate flower was highly sterile. No pollen grains and only a few tetrads were observed at anthesis. Backcross to the female parent resulted in 60% fruit set, but embryos were not present in the seeds which were produced. The check (without pollination) developed fruit parthenocarpic ally.
Discussion. The importance of wild Cucumis species for cucumber and melon breeding has long been recognized because Cucumis crops are susceptible to a number of devastating fungal, bacterial, viral and insect diseases (Kirkbride, 19193). In several instance, attempts have been made to produce interspecific hybrids between cucumber, melon and other species. However, thus far these attempts have been without repeatable success (Dane, 1991). Although the interspecific hybrid we obtained is not horticulturally acceptable (it can not produce offsprings naturally), this development should be viewed as an important first step in the process of interspecific hybridization in Cucumis species.
Previous investigations on the taxonomy and evolution of Cucumis have been based on the theories suggesting a basic chromosome number of 2n=24 (Africa group). Researchers have found no justification keeping melon and cucumber in the same genus because of their different chromosome number, centers of origin, morphological characters, and strong cross incompatibilities (Pangalo, 1950; Ramachandran, 1986; Sujatha and Seshadri, 1989). It has been difficult to establish the taxonomic and evolutionary relationship between these two subgenera. The cucumber was found to be the most distantly related group among Cucumis species, i.e. having a large D value with all other species (Perl-Travis et al., 1985). C. hystrix, which is cross compatible with cucumber, might have a close taxonomic and evolutionary relationship with cucumber, and therefore melon. As such, it might bemused as a bridge species in the genus Cucumis and therefore enrich our knowledge of this genus. We believe some answers about the evolution of Cucumis species might be obtained through further study of the relationship between C. hystrix and other species in Cucumis.
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