Figure 1. Schematic diagram of plant ribosomal DNA organization and the probes used in this study. Abbreviations on the figure denote the following: 18S: 18S rDNA; ITS: internal transcribed spacer; 25S: 25S rRNA coding region; IGS: intergenic spacer. The position of the probes used for sequential probing is shown below the repeat unit. (a) The 8.95 kbp Eco RI fragment of wheat rDNA cloned in pTA71 (Gerlach and Bedbrook, 1979) as heterologous probe, (b) The internal transcribed spacers ITS1, 5.8S and ITS2 rDNA (ITS), (c) An EcoRI- BamHI fragment (1000 bp) corresponding to the 3' end of the 25S fragment of wheat rDNA (May and Appels, 1987), (d) The intergenic spacerIGS region.