Crypthecodinium and Tetrahymena: an exercise in comparative evolution.

Nucleotide sequences have been determined for the highly variable D2 region of the large rRNA molecule for over 60 strains of dinoflagellates. These strains were selected from a worldwide collection that represents all the known sibling species (compatibility groups, Mendelian species) in the sibling swarm referred to as Crypthecodinium cohnii. A phylogenetic tree has been constructed from an analysis of the variations in a length of about 180 bases, using PHYLOGEN string analysis programs. The Crypthecodinium tree is compared with the previously published but here augmented tree constructed upon the same rRNA region for the sibling species of a worldwide collection of ciliated protozoa related to the genus Tetrahymena. The first reported sequence of Lambornella clarki, the parasite of tree-hole mosquitoes, is included. The dinoflagellate species complex is much more homogeneous with respect to ribosomal variation. The mean number of differences among sequences from different Crypthecodinium species is about 7, in comparison with 22 differences among the ciliate species examined. Moreover, all the diversity in the dinoflagellates can be explained by base substitutions, whereas insertions and deletions are common in the ciliates. The dinoflagellates are also much more uniform with respect to nutritional and genetic economies. The two complexes differ also in the relationship between molecular variations and breeding compatibility. All tetrahymenine sibling species thus far examined are monomorphic in the D2 region, but several dinoflagellate species are polymorphic. Several different dinoflagellate species, moreover, have identical D2 regions. This kind of ribosomal identity of incompatible strains is found in these ciliates only in one tight cluster of species--Group C. The tetrahymenine swarm is apparently much older than the Crypthecodinium swarm, and the dinoflagellate species produce incompatible progeny species much more readily than do the ciliates, perhaps by the acquisition of mutations that potentiate incompatibility in sympatric populations.