Molecular analysis of introgression lines from Cucumis hystrix Chakr. to C. sativus L
Introduction
Cucumber (Cucumis sativus L., 2n = 14, CC) belongs to the genus Cucumis of the family Cucurbitaceae (Kirkbride, 1993). It is mostly grown in temperate to sub-tropical regions and is among top ten vegetables in world production (Tatlioglu, 1993). However, cucumber is a crop with narrow genetic base (Kupper and Staub, 1988) and also susceptible to many diseases and prone to insect damage, such as downy mildew, fusarium wilt, powdery mildew and root-knot nematode. Therefore, it is need of time to broaden the gene pool of cucumber by introgressing resistance genes from diverse sources to deal with the challenges facing in cucumber production.
Wide hybridization has been an important tool to introduce agriculturally important genes from one species to another and even creates new species or novel allopolyploids. Cucumis hystrix Chakr. (2n = 24, HH) is a wild Cucumis species having valuable traits, e.g. resistance to the root-knot nematode (Chen and Kirkbride, 2000), tolerance to low irradiance and temperature (Qian et al., 2002, Zhuang et al., 2002) and resistance to downy mildew and fusarium wilt (Chen et al., 2004). Hence, transfer of desirable genes in particular for disease resistance from C. hystrix into cultivated cucumber will be valuable for the continued improvements of cucumber cultivars.
Interspecific hybridization was first successfully carried out between C. hystrix and cultivated cucumber (Chen et al., 1997). Progress has been made towards achieving gene introgression following the production of fertile interspecific hybrids between the newly synthesized allotetraploid species C. × hytivus Chen and Kirkbride (2n = 4x = 38, HHCC) and C. sativus (Chen and Kirkbride, 2000, Chen et al., 2003). The resulting hybrid plants were selected and selfed for several generations, producing a series of introgression lines with various morphological characteristics. These introgression lines contain a low percentage of exotic DNA and favorable alleles can thus be easily and rapidly isolated and transferred into the elite varieties (Ishimaru, 2003, Kashiwagi and Ishimaru, 2004).
Recent advances in cytogenetic techniques and molecular markers provide opportunities for better understanding and manipulating the introgression throughout the breeding practice. The successful use of molecular markers to study alien gene introgression has been reported in a large number of crop species (Jena et al., 1992, Fulton et al., 1997, Lashermes et al., 2000). SSR and AFLP markers have gained popularity as powerful DNA markers. In the study reported here, two introgression lines from the interspecific hybrids were characterized using SSR and AFLP markers. An insight molecular analysis would be particularly useful for understanding the nature and the level of alien gene introgression.
Section snippets
Plant materials
The C. × hytivus (HHCC) was successfully synthesized via chromosome doubling of an interspecific hybrid between a Chinese cucumber “beijingjietou” and C. hystrix. The hybrids with a chromosome number of 2n = 14 were derived from the backcross between the C. × hytivus and “beijingjietou”. These hybrids were subsequently self-pollinated for five to six generations during 2000–2006. Two introgression lines (IL56 and IL73) were identified with heritable and novel morphological characteristics including
Morphology
The two introgression lines (IL56 and IL73) showed some difference from their cultivated parent “beijingjietou” as well as from the wild parent C. hystrix in morphological characteristics, such as fruit length, number of branches and size of seeds and leaf areas (Fig. 1). The introgression lines have small fruit and multiple branches, which is the characteristic of C. hystrix plants indicating the introgression from C. hystrix to C. sativus. All plants were fertile as judged by pollen
Discussion
This study presents the first use of molecular markers to measure introgression in interspecific hybrids from the cross between C. hystrix and C. sativus. The comparative study of the morphological characteristics and molecular analysis allow us to presume that the introgression of new genes and traits from C. hystrix to C. sativus, resulting in plants with a normal chromosome number (2n = 14) possessing resistances to downy mildew and fusarium wilt.
Simple sequence-repeat polymorphism is produced
Conclusions
Introgression of desirable traits into cultivated varieties from wide species could be a unique approach for crop improvement. This strategy is greatly facilitated by precise monitoring of alien gene introgression at molecular level. The present study demonstrated that the efficiency of SSR and AFLP markers for polymorphism detection and analysis of introgression in cucumber, thereby offering an insight into the effect of alien introgression and the possibility for genetic improvement in
Acknowledgements
This research was partially supported by the Key Program (30830079); the General Program 30671419 and 30700541 from the National Natural Science Foundation of China; the ‘863’, Programs (2006AA10Z1A8, 2006AA100108); the ‘111’, Project (B08025); the ‘973’ Program and the National Supporting Programs (2008AA10Z150, 2006BAD13B06, 2006BAD01A7-5-11) from the Ministry of Science and Technology of China; PSRF of Pickle Packer International Co.; and the Ph.D. Funding (20050307009) from the Ministry of
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