Nondenatured and denatured total RNA from the tapeworm Diphyllobothrium dendriticum (Cestoda) was analysed by agarose gel electrophoresis. It was found that the large subunit ribosomal RNA (lrRNA) is 28.0S and the small subunit ribosomal RNA (srRNA) is 19.5S. Following denaturation the 28.0S rRNA was disrupted into a 19.5S subfragment and a 20.7S subfragment due to the presence of a centrally located hidden break. By hybridization of Northern blot membranes with oligonucleotide probes specific for the 5′- and 3′-ends of the lrRNA respectively, we have shown that the 19.5S sub-fragment is from the 5′-end (the α-subfragment) and the 20.7S subfragment from the 3′-end (the β-subfragment) of the 28.0S rRNA of D. dendriticum.

The ribosomal DNA (rDNA) unit of the liver fluke Opisthorchis viverrini has been cloned and characterized. The results demonstrated that the total length of this unit is approximately 13 kb, containing 4·2 kb of large subunit (LSU) rDNA, 2·0 kb of small subunit (SSU) rDNA, 1·0 kb of transcribed spacer DNA and 5·8 kb of non-transcribed + external transcribed spacer DNA. Examination of the non-transcribed spacer region between different rDNA units showed variation in the restriction sites rather than in the length. Judging from hybridization of the rDNA plasmid to the restriction endonuclease digest of genomic DNA, rDNA units represent 6·1% of the total genomic DNA. At the RNA level, the LSU rRNA of O. viverrini and Fasciola gigantica contained a hidden break. The molecule consisted of two fragments co-migrating with a SSU rRNA, when electrophoresis was carried out under denaturing conditions. Ribosomal RNA sequence comparison has been previously used to determine phylogenetic classification of parasitic organisms. The sequence of 381 nucleotides at the 5′ terminus of the LSU rRNA gene was determined and compared with those from species previously reported by other investigators. Phylogenetic classification of O. viverrini, as determined by rRNA gene sequence comparison, is comparable with the conventional taxonomic classification.