Cyto-Molecular Genetics of the Interspecific Hybridization in Cucumber

Abstract

Cucumber, Cucumis sativus L. (CC, 2n = 2x = 14), is a valuable vegetable crop widely cultivated and consumed around the world. However, due to the narrow genetic base and lack of resistant genes, cucumber breeding has been hindered, especially in resistance breeding. Cultivated cucumber suffered from a range of devastating diseases, like downy mildew, powdery mildew, root-knot nematode, etc. Therefore, transferring specific traits from the wild relatives through interspecific hybridization has been highlighted for its importance by the breeders for a long time. Among more than 50 wild relatives, C. hystrix (HH, 2n = 2x = 24) is the only wild Cucumis species grouped into the same subgenus together with C. sativus, while all others are classified into Melo subgenus. Also, bearing multiple disease-resistant characteristics, C. hystrix is the only known wild species cross-compatible with C. sativus in this genus. The one and only successful synthetic allotetraploid C. ×hytivus J. F. Chen & J. H. Kirkbr (HHCC, 2n = 4x = 38) was obtained via an interspecific hybridization between C. hystrix and C. sativus. It has been reported that both genetic and epigenetic reprogramming in this C. ×hytivus, which might be the reason for the novel phenotypic variation, such as delay maturation. Hybridization and allopolyploidization frequently bring a ‘genomic shock’ that causes rapid genetic and epigenetic changes, due to the merger of two or more divergent genomes, which leads to many problems, like the meiosis abnormality, extensive abnormal chromosome pairing, imbalanced chromosome segregation, and karyotype variations. Still, according to the clear genetic background and small genome size with whole-genome released recently, the Cucumis allotetraploid could serve as an excellent system for studying immediate consequences following allopolyploidization. Cyto-molecular genetics and genomic information of this hybrid and its allotetraploid could provide a novel insight into the establishment of allopolyploids with different chromosome bases, as well as provide effective ways to create new species and materials, which can be employed for cucumber and melon improvement.