Subphylum 2. INTRAMACRONUCLEATA: Class 9. OLIGOHYMENOPHOREA — Once a Pivotal Group, Now a Terminal Radiation

Abstract

This class includes the two model ciliates— Tetrahymena and Paramecium—laboratory genetic models of protists and the first two ciliates to have completed genome projects. Along with these representatives of the Subclasses Hymenostomatia and Peniculia, there is a diverse variety of species distributed in four other subclasses: Scuticociliatia, Peritrichia, Apostomatia, and Astomatia. While the archetype of the class has a paroral and three left oral polykinetids, there is really no strong morphological or ultrastructural synapomorphy for the class. Molecular phylogenetics generally recovers an oligohymenophorean clade with moderate bootstrap support. The somatic kinetid is similar to that of the last three classes: a generally robust, anteriorly-directed kinetodesmal fibril, a divergent postciliary ribbon, and a radial transverse ribbon, except in the peniculines where it is tangential. Oral structures depart from the archetype in the apostomes with their rosette and complex cytopha-ryngeal apparatus and are absent in the astomes. Oligohymenophoreans are as broadly distributed across habitats as any class, but they include obligate symbionts: the apostomes typically with crustaceans and the astomes with annelids. Ichthyophthirius, the parasite of freshwater fishes, is an hymenostome that can plague aquaculture operations. Division morphogenesis is characterized as buccokinetal and parakinetal. There is a rich literature on the genetics of these ciliates, particularly Paramecium and Tetrahymena, which can be easily induced to conjugate in the laboratory, enabling a deeper understanding of their molecular biology.