Phylogenetic tree of 18S RNA from the small ribosomal subunit of the most-studied ciliates with respect to their macronuclear structure. The tree was generated with PhyML with 100 bootstraps. The dinoflagellate Symbiodinium is used as an outgroup to root the tree. N. ovalis has a comparable macronuclear structure to Spirotrichea, its sister group in the tree (the lineage containing Euplotes through Sterkiella). MAC stands for macronucleus, MIC for micronucleus.