Objective  To study the number and distribution characteristics of 45S and 5S rDNA sequences in the genome of 20 species of Cucurbitaceae plants, and provide a basis for studying the karyotype, genetic breeding and evolutionary classification of Cucurbitaceae plants.

Method  Using improved fluorescence in situ hybridization (FISH) method, the 45S and 5S rDNA probe sequences were modified by fluorescence at the 5′ ends, and the physical locations of 45S and 5S rDNA in the metaphase chromosomes of 20 species of Cucurbitaceae plants were determined. The chromosomes were observed under the Nikon 80i fluorescence microscope, and images were collected by cold CCD and then were analyzed.

Result  Numbers, locations and characteristics of 45S rDNA and 5S rDNA fluorescence sites on the metaphase chromosomes of 12 plants including Gymnopetalum chinense, Herpetospermum pedunculosum, Lagenaria siceraria, Momordica cochinchinensis, Momordica dioica, Cucurbita pepo, Trichosanthes anguina, Trichosanthes dunniana, Trichosanthes ovigera, Zehneria maysorensis, Coccinia grandis and Sechium edulewere were defined. We detected 3, 7, 2, 4, 2, 5, 3, 3, 5, 1, 2 and 2 pairs of 45S rDNA from these 12 plants respectively, and detected 2, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1 and 1 pairs of 5S rDNA respectively. The 45S rDNA and 5S rDNA of 20 plants were found to be located on short arms of chromosomes, terminal ends of short arms or centromeres.

Conclusion  FISH is an effective tool for building fine karyotypes of Cucurbitaceae plants. It can help to analyze the random body, differentiate chromosome and identify homologous chromosomes, and therefore provides strong evidence for karyotype analysis.