EXTRACHROMOSOMAL RIBOSOMAL-RNA GENES IN TETRAHYMENA - STRUCTURE AND EVOLUTION

The macronuclear rRNA genes from a number of strains within several species of Tetrahymena (T. capricornis, T. australis, T. cosmopolitanis, T. pyriformis, T. thermophila, T. americanus, T. borealis, T. pigmentosa, T. canadensis, T. tropicalis and T. hyperangularis) were characterized. Restriction enzyme analysis revealed that individual strains all contained entirely homogeneous populations of extrachromosomal palindromic ribosomal[r]DNA, varying in molecular size from 12 .times. 106 to 14 .times. 106 in different strains. Considering that the evolutionary distance among some of the species may be 106 years, the rDNA from all the species exhibited a strikingly high similarity in the localization of their restriction sites. Nevertheless, differences both inside and outside the gene regions were clearly detectable, showing that the rDNA sequences have diverged in all species. Genetic polymorphism as to rDNA structure exists in Tetrahymena, but seems to be rare. In 2 of the 5 spp. examined (T. borealis and T. pigmentosa), interbreeding strains differing in rDNA structure were found. While the differences detected in the T. borealis rDNA were confined to a small size difference located at the non-coding ends of the molecule, several differences were detected in the rDNA from the T. pigmentosa strains. One of the differences was due to the presence of an intervening sequence within the structural gene for 26 S rRNA in some of the strains. An intervening sequence of similar size located at the same position with the 26 S gene region was found by R-loop mapping in all strains of the species T. thermophila. Restriction enzyme analysis indicates that the rDNA from 2 other species contains a similar intervening sequence, and the size and localization of the intervening sequence may be evolutionarily stable. The 2 intervening sequences examined so far are not identical, as revealed by restriction enzyme mapping.