INSIGHTS INTO THE EVOLUTION OF NUCLEAR DUALISM IN THE CILIATES REVEALED BY PHYLOGENETIC ANALYSIS OF RIBOSOMAL-RNA SEQUENCES

The small subunit rRNA gene sequences of the karyorelictean ciliates, Loxodes striatus and Protocruzia sp., and the heterotrichian ciliates, Climacostomum virens and Eufolliculina uhligi, were used to test the evolution of nuclear dualism in the Phylum Ciliophora. Phylogenies der ived using a least squares distance method, neighbour joining, and max imum parsimony demonstrate that the karyorelictean ciliates sensu Smal l and Lynn, 1985 do not form a monophyletic group. However, Loxodes an d the heterotrich ciliates form the first branch in the ciliate lineag e, and Protocruzia branches, in distance methods, basal to the spirotr ich lineage. It is proposed that Protocruzia be removed from the Class Karyorelictea, and placed in closer taxonomic association with the sp irotrich lineage. The distribution of nuclear division types along the phylogenetic tree is consistent with the notion that macronuclei inca pable of division represent a derived rather than a primitive or ''kar yorelictid'' character trait.