Phylogenetic relationships of the Nassulida within the phylum Ciliophora inferred from the complete small subunit rRNA gene sequences of Furgasonia blochmanni, Obertrumia georgiana, and Pseudomicrothorax dubius.

Using comparisons of complete small subunit rRNA sequences from the ciliated protozoans Furgasonia blochmanni, Obertrumia georgiana, and Pseudomicrothorax dubius we inferred the phylogenetic position of the Nassulida (Class Nassophorea) within the Ciliophora. In distance matrix analyses the Nassulida share a common ancestry with the colpodean ciliate Colpoda inflata. Distance matrix and parsimony methods convincingly demonstrate that the Nassulida plus Colpodida are members of a complex ciliate assemblage that also includes the oligohymenophorans and phyllopharyngeans. These phylogenetic inferences are largely congruent with recent analyses of 23S-like rRNA gene sequences and morphogenetic features. Groups traditionally thought to represent ancestral lineages now appear as highly derived ciliates. In contrast, heterotrichs which were considered to represent a highly evolved group, diverge at the base of the ciliates.