Subphylum 2. INTRAMACRONUCLEATA: Class 8. PLAGIOPYLEA — A True Riboclass of Uncommon Companions

Abstract

This class is truly a riboclass because it assembles three groups of ciliates that were never suspected of being phylogenetically related, and yet there is an extremely strong signal from the small subunit rRNA gene sequences that they are. The now “classic” plagiopyleans, the sonderiids and plagiopylids, are now united with the trimyem-ids and tentatively also the odontostomatids. These ciliates are all considered anaerobic to microaer-ophilic, and are often found in sapropelic habitats. Several species have conspicuous assemblages of hydrogenosomes and methanogens, which presumably enable these ciliates to survive in these anoxic habitats. There are really no unifying morphological features. The somatic kinetids are monokinetids in the sonderiids, plagiopylids, and trimyemids and highly unusual dikinetids in the odontostomatids. Oral structures in the plagiopylids and sonderiids are modifi ed extensions of somatic kineties; trimy-emids apparently have a kind of “circumoral” ciliature; and odontostomatids have several small oral polykinetids. Stomatogenesis is apparently holotelokinetal in all but the odontostomatids, and we are ignorant of how this latter group divides. There remains much to be learned about their life cycle, sexual processes, and nuclear features.