Interspecific Hybridization in Cucumis—Progress, Problems, And
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Interspecific Hybridization in Cucumis—Progress, Problems, and Perspectives Jin-Feng Chen1 and Jeffrey Adelberg Department of Horticulture, E-142 Poole Agriculture Center, Clemson University, Clemson, SC 29634 Interspecific hybridization is used to im- the genus Cucumis, an amphidiploid was re- The recent cross between cucumber and C. prove crops by transferring specific traits, ported from the cross of C. anguria L. and C. hystrix Chakr. (2n = 24) was the first repeat- such as pest and stress resistance, to crops dipsaceus E. ex S. (Yadava et al., 1986). able cross between a cultivated Cucumis spe- from their wild relatives (Bowley and Taylor, However, in the Cucurbitaceae only in cies and a wild relative (Chen et al., 1997b), 1987). When applicable, this approach is a Cucurbita has interspecific hybridization been and represented a breakthrough in interspe- very effective method of gene transfer. In successfully utilized for crop improvement cific hybridization in Cucumis. The success of nature, ≈30% to 35% of flowering plant spe- (Robinson and Decker-Walters, 1997). this cross was even more surprising because cies were created by interspecific hybridiza- Cucumis contains two species of economic the parental species have different chromo- tion, followed by chromosome doubling importance, melon (C. melo L., 2n = 24) and some numbers. The original F1 hybrid (2n = (Stebbins, 1971). Starting with interspecific cucumber (C. sativus L., 2n = 14). The impor- 19), obtained by embryo rescue following hybridization, allopolyploids, such as allotet- tance of wild Cucumis species has long been pollination of C. sativus by C. hystrix (Fig. raploids, can be developed by doubling the recognized because they possess resistance to 1A), has 7 chromosomes from C. sativus and chromosome number of the F1 hybrid. Suc- pathogens, such as powdery mildew [caused 12 from C. hystrix, and was both male- and cessful construction of an allopolyploid re- by Sphaerotheca fuliginea (Schlechtend.: Fr) female-sterile. To restore fertility, reciprocal sults in the creation of a new combination of Pollacci], downy mildew [caused by crosses were made and the chromosome num- genomes, or the production of a species that Pseudoperonospora cubensis (Berk. & M.A. bers of the progeny were successfully doubled did not exist previously. Curtis) Rostovzev], anthracnose [caused by (Fig. 1B) (Chen et al., 1998). Pollen grains However, great effort may be required to Colletotrichum orbiculare (Berk. & Mont) were produced by these progeny when C. hybridize cultivated and wild species. The Arx], and fusarium wilt (caused by Fusarium hystrix was used as the seed parent; the plants first man-made interspecific hybrid was syn- oxysporum Schlechtend.: Fr.) (Kirkbride, 1993; produce fertile flowers (Fig. 1C) and set fruit thesized in 1717 between carnation (Dianthus Leppick, 1966; Lower and Edwards, 1986). (Fig. 1D) with viable seeds (Fig. 2), indicating caryophyllus L.) and sweet william (Dianthus Genetic variation is relatively limited in cu- that fertility was restored. This restoration of barbatus L.) (Stalker, 1980). Since then, thou- cumber (Staub et al., 1987); thus, efforts to fertility marked the creation of a new synthetic sands of interspecific crosses have been at- create interspecific hybrids become more criti- species, which has close phylogenetic rela- tempted, but success has been rather limited. cal and meaningful. In 1859, Naudin first tried tionships with its parental species, but is dis- Chromosomal, genetic, cytoplasmic, or me- to cross melon with cucumber and other spe- tinctively different from each. It has the ge- chanical isolation barriers can handicap suc- cies (Naudin, 1859). Historically, various ap- nome HHCC and chromosome number 2n = cessful hybridization and utilization. It took proaches (traditional and biotechnological) for 4x = 38. This synthetic species might be useful plant breeders about 100 years to produce interspecific hybridization have been used in as a new Cucumis crop. In addition, as a C. triticale—a new crop species created from the Cucumis to overcome the fertilization barriers hystrix x C. sativus hybrid, it might be useful cross of wheat (Triticum aestivus L.) and rye between cucumber, melon, and wild species, as a bridging species for transfer of useful traits (Secale cereale L.) (Zillinsky, 1985). Signifi- but with only limited success. to cucumber. cant benefits and difficulties make interspe- cific hybridization an important objective for geneticists and plant breeders. AB Interspecific hybrids in the Cucurbitaceae have been produced in several genera, includ- ing Cucumis (Deakin et al., 1971), Citrullus (Valvilov, 1925), Luffa (Singh, 1991), and Cucurbita (Weeden and Robinson, 1986). In Received for publication 18 Mar. 1999. Accepted for publication 12 July 1999. Journal paper no. 4482 of South Carolina Agricultural Experiment Station, Clemson, S.C. The authors sincerely thank Dr. Yosuke Tashiro, Dr. Shiro Isshiki, and Dr. Sadami Miyazaki, Faculty of Agriculture, Saga Univ. of CD Japan, Saga, for the isozyme data presented in this paper, and Dr. Bill Rhodes and Dr. Vance Baird, Dept. of Horticulture of Clemson Univ., and Dr. Jack Staub, USDA/ARS, the Univ. of Wisconsin– Madison, for their review of this paper. This re- search was supported by the National Education Committee of the Peoples Republic of China, and by Research Grant Award No. US-2809-96R from the United States–Israel Binational Agricultural Re- search and Development Fund (BARD). The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked adver- tisement solely to indicate this fact. Fig. 1. (A) Embryo obtained from the interspecific hybrid between Cucumis sativus and C. hystrix. (B) The 1 To whom reprint requests should be addressed. F1 diploid, sterile, hybrid plant form embryo rescue (left) and its chromosome-doubled tetraploid, fertile Current address: Nanjing Agricultural Univ., Nanjing plant (right). (C) Female flowers of C. hystrix (left), C. sativus (right), and the F1 hybrid (middle).(D) 210095, P.R. China; e-mail: [email protected]. Fruits set on the amphidiploid. HORTSCIENCE, VOL. 35(1), FEBRUARY 2000 11 FEATURE Fig. 2. Seeds harvested from the amphidiploid (below) and its diploid progenitors (Cucumis hystrix, upper left; C. sativus, upper right). PROGRESS: SYSTEMATIC STUDIES AND INTERSPECIFIC CROSSES IN CUCUMIS The genus Cucumis includes two distinct groups or subgenera, different in their origins and basic chromosome numbers (Jeffrey, Fig. 3. The current Cucumis systematic system proposed by Kirkbride (1993). 1980). Melon, and most other species in this genus with the basic chromosome number n = Table 1. Grouping of Cucumis species by the studies on phylogenetic affinity. 12, are referred to as the African group. Methods No. groups No. species used Source Cucumis sativus var. sativus and C. sativus Crossability 4 14 Deakin et al., 1971 var. hardwickii (Royle) Alefeld, with the basic Morphology and 5 13 (2n = 24) Singh and Yadava, 1984a chromosome number n = 7, are referred to as chromosome pairing the Asian group. Under the current systematic Crossability, 3 8 Singh and Yadava, 1984b system (Fig. 3), 30 species are grouped into six chromosome pairing, series in the subgenus melo (Kirkbride, 1993), and pollen fertility instead of four groups (Jeffrey, 1980). ChlDNA variation 6 21 Perl-Treves and Galun, 1985 Angurioidei is the largest of the six series in the Isozyme variability 6 21 Perl-Treves et al., 1985 Isozyme pattern 6 24 Puchalski and Robinson, 1990 subgenus melo, and includes 19 species that are cross-compatible and can stimulate fruit set in members of the series melo. The species African Cucumis have n = 12, and that Asian of these results were not repeatable and did not C. sativus and C. hystrix are included in the Cucumis have n = 7, which has governed the result in fertile hybrids. Our current under- subgenus Cucumis (Kirkbride, 1993). understanding of systematics and standing of the cross relationship based on the Successful utilization of wild species to phylogenetics in Cucumis for decades. The previous experiments is presented in Fig. 7. improve a crop species largely depends on taxonomic position of C. hystrix is of special More work is needed for a precise placement species relationships. To understand the phy- interest because it bears a morphological re- of C. hystrix in the genus Cucumis and a better logenetic affinities among species, studies on semblance and biochemical affinity to C. understanding of its specific relationship. comparative morphology, crossability, chro- sativus while its chromosome number is the Knowledge of species relationships are the mosome pairing, isozyme variability, and DNA same as C. melo (Chen et al., 1995). Isozyme key to success. variation in Cucumis have been carried out variability suggested a phylogenetic relation- (Table 1). Although the number of groups ship between C. hystrix and both C. sativus varied with each study, the basic phylogenetic and C. melo (Chen et al., 1997a). For instance, MAJOR PROBLEMS IN trees developed from the different experiments C. hystrix has four bands in the pattern of INTERSPECIFIC HYBRIDIZATION were similar. For instance, most of the African malate dehydrogenase (MDH) (Fig. 6). The AND THEIR SOLUTIONS Cucumis species form a close group (Anguria), first band is shared by all three species, indi- which is distant from both melon (C. melo), cating the common