129 Abstract Ciliates in this class were thought to represent the pinnacle of ciliate evolution, along with the spirotrichs. However, small subunit rRNA gene sequences and the presence of postciliodes- mata in the somatic cortex strongly relate members of this class to the Class KARYORELICTEA. The heterotrichs are typically majestic ciliates of large cell size and with a conspicuous adoral zone of polykinetids or membranelles (AZM) that extend out over the peristomial surface. The ciliates in this class are not subdivided, and so there is one order – Order Heterotrichida. Heterotrichs are found in a diversity of habitats, from the marine benthos and hydrothermal vents to the plankton of high altitude oligotrophic lakes. They feed on a diversity of prey, ranging from bacteria up to small metazoa, like rotifers, and sometimes are conspicuous by carry- ing symbiotic zoochlorellae. Their body is highly contractile, elongated by postciliodesmal micro- tubules and shortened by contractile myonemes. The oral structures have a paroral and multiple paramembranelles. Stomatogenesis is parakinetal. Macronuclei can be nodular, and are divided by extramacronuclear microtubules. Conjugation has not been studied in any breadth in the class with the gamone-receptor system of Blepharisma being the only model. The heterotrich Spirostomum has been developed as a bioassay model for heavy metal. Keywords Ampliploid, microbiotest Heterotrichs are common and large ciliates, some Spirostomum species achieving body lengths of up to 4,000 µm. They include some of the best-known and most common ciliates in the phylum. Stentor , a typical representative, has long attracted attention from protozoologists and cell biologists because of its size, ubiquity, ease of general laboratory culture, contractility, and regenerative powers (Fig. 6.1). Typically heterotrichs are free-swimming and holot- richously ciliated, although members of the Family Folliculinidae secrete attached loricas in which they live. The group is at least 100–200 million years old as demonstrated by the fossil ized lorica of Priscofolliculina (Deflandre & Deunff, 1957). Heterotrichs were so-named because of the marked difference between their holotrichous somatic ciliation and the conspicuous, typically spiralling adoral zone of membranelles or oral polykinetids . They were conceived as the pivotal group in the evolution of the “higher” or polyhy- menophorean ciliates (Corliss, 1961, 1979). Doubt about this vision began to emerge in the late 1970s. Ultrastructural data indicated dramatic differences in their somatic kinetids compared to other polyhy- menophoreans ; and similarities in the heterotrich somatic cortex to that of the karyorelicteans sug- gested a closer relationship between the presuma- bly most derived and the presumably most ancestral groups (Gerassimova & Seravin, 1976; Lynn, 1981, 1991). The nature of membrane particle arrays in different ciliate groups also suggested a stronger relationship between heterotrichs and karyorelict- eans (Bardele, 1981). Finally, sequences of nuclear ribosomal RNA genes from heterotrichs and karyorelicteans supported their sister group status in an early diverging lineage (Baroin-Tourancheau, Delgado, Perasso, & Adoutte, 1992; Greenwood, Schlegel, Sogin, & Lynn, 1991b; Hirt et al., 1995). Chapter 6 Subphylum 1. POSTCILIODESMATOPHORA: Class 2. HETEROTRICHEA – Once Close to the Top 130 6. Subphylum 1. POSTCILIODESMATOPHORA: Class 2. HETEROTRICHEA – Once Close to the Top Fig. 6.1. Representative genera of the Class HETEROTRICHEA . Blepharisma with a somewhat linear arrangement of the adoral zone of polykinetids along the left margin of the oral region. In contrast, the oral polykinetids of Stentor and Climacostomum spiral out of the oral cavity in a counter-clockwise direction, bounding a peristomial field that is covered by kineties. The folliculinid Eufolliculina exemplifies this unique family of heterotrichs in being anchored in a lorica and in having its oral region drawn out into two extensive peristomial “wings” Thus, we are now certain that the postciliodesmata , shared by both karyorelicteans and heterotrichs , demonstrate their shared common ancestry. The ciliates now assigned to this class are united by four major features. First, they have highly ampliploid and dividing macronuclei. Macronuclear karyokinesis is accomplished primarily by extramacronuclear microtubules (Diener, Burchill, & Burton, 1983; Jenkins, 1973; Lynn & Small, 1997) and probably evolved inde- pendently of macronuclear karyokinesis in the Subphylum Intramacronucleata (Lynn, 1996a; Orias, 1991a; and see Chapter 4 ). Second, the microtubular components of their postciliodes- mata are more simply organized than those of the karyorelicteans : they appear as ribbons oriented perpendicular to the cell surface, only separated by a single microtubule. Third, the oral polyki- netids on the left side of the oral region are char- acterized as paramembranelles (de Puytorac & Grain, 1976), which form a conspicuous adoral zone often extending out onto the anterior cell surface. Fourth, the differentiation of these oral polykinetids during stomatogenesis occurs from the center of the oral primordium towards the anterior and posterior, a unique pattern within the phylum (Aescht & Foissner, 1998). 6.1 Taxonomic Structure There has been considerable change in the composition of this taxon since Corliss (1961, 1979). Corliss (1979) recognized six suborders within the Order Heterotrichida : (1) Suborder Heterotrichina ; (2) Suborder Clevelandellina ; (3) Suborder Armophorina ; (4) Suborder Coliphorina ; (5) Suborder Plagiotomina ; and (6) Suborder Licnophorina . Results from the study of ultrastructure and molecular sequences now suggest the following. The somatic kinetids of clevelandellids (Affa’a, unpublished, 2007; de Puytorac & Grain, 1969), armophorids (Schrenk & Bardele, 1991), plagioto- mids (Albaret & Grain, 1973), and licnophorids (Da Silva Neto, 1994a) do not exhibit postciliodesmata and also have different patterns of fibrillar associ- ates (Lynn, 1981, 1991). Furthermore, nuclear small subunit rRNA (SSrRNA) gene sequences separate the clevelandellids (van Hoek, van Alen, Sprakel, Hackstein, & Vogels, 1998, 2000b) and armo- phorids (Embley, Finlay, Thomas, & Dyal, 1992; van Hoek et al., 1998, 2000b) to a new class, the Class ARMOPHOREA (see Chapter 8 ), while plagiotomids (Affa’a, Hickey, Strüder-Kypke, & Lynn, 2004) and licnophorids (Lynn & Strüder-Kypke, 2002) are transferred to the Class SPIROTRICHEA (see Chapter 7 for details). The Suborder Coliphorina only included the Family Folliculinidae . However, the somatic kinetids of Eufolliculina are extremely similar to other heterotrichs in their fibrillar associates and the character of the postciliodesmata (Mulisch, Barthlott, & Hausmann, 1981), while SSrRNA sequences indicate this genus falls within the heter- otrich radiation and is the sister taxon to Maristentor (Miao, Simpson, Fu, & Lobban, 2005). Thus, this group should not be separated at such high rank, and we do not now recognize this suborder. Within the Suborder Heterotrichina , Corliss (1979) included the following: Family Bursariidae , Family Chattonidiidae , Family Climacostomidae , Family Condylostomatidae , Family Metopidae , Family Peritromidae , Family Phacodiniidae , Family Reichenowellidae , Family Spirostomidae , and Family Stentoridae . The somatic kinetids of Phacodinium (Didier & Dragesco, 1979; Da Silva Neto, 1993a), Transitella , a reichenowellid (Foissner, Adam, & Foissner, 1982; Iftode, Fryd-Versavel, Wicklow, & Tuffrau, 1983), metopids (Schrenk & Bardele, 1991), and bursariids (Gerassimova, Sergejeva, & Seravin,