Species and genus specificity of the intergenic spacer (IGS) in the ribosomal RNA genes of Cucurbitaceae.

The use of intergenic spacer (IGS) fragments of plant ribosomal DNA (rDNA) for the differentiation between genera and species is tested by cross-hybridization experiments with different IGS probes of two Cucurbitaceae, Cucurbita pepo (zucchini) and Cucumis sativus (cucumber). Hybridization with cloned fragments of different parts of the IGS of ribosomal DNA exhibit a different degree of conservation within and between the Cucurbitaceae genera. In general, Cucurbita species seem to be closer related to each other than the Cucumis species. A repetitive element of the external transcribed spacer (ETS) shows a more genus-specific pattern, reacting only with the respective genera; the region preceding the ETS is conserved between the Cucurbita species but also cross-hybridizes weakly with the Cucumis species. A GC-rich element of the Cucumis sativus IGS ("Cfo-cluster") is present in small amounts in Cucumis melo (melon) and even less represented in other genera of the Cucurbitaceae.