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subse- quent to cell division that are necessary to com- plete differentiation. For example, in many sessile forms, like folliculinid heterotrichs , suctorians , or chonotrichs , the offspring are quite different from the parents. These so-called buds or swarmers must undergo considerable development once they themselves have found a suitable place to settle. These morphogenetic processes can be complex, and include, for example, the development of the characteristic oral arms in folliculinids , the development of attachment stalks in suctorians and chonotrichs , and the development of oral structures, such as tentacles , in suctorians . 4.6 Nuclei, Sexuality and Life Cycle As noted in the characterization of the phylum, ciliates are typified by having two nuclei \u2013 the macronucleus is typically \u201c polyploid \u201d or ampli- ploid , and the micronucleus is presumed to be diploid, but is likely polyploid in some taxa (Figs. 4.9A, 4.19A) (Aury et al., 2006; Génermont, 1984; Raikov, 1996). Prescott (1994) categorized macronuclei into two types: (1) those with gene- sized DNA molecules , roughly 0.4\u201320 kb in size, each with telomeres and typically including one gene; and (2) those with subchromosome-sized DNA molecules , roughly 100\u20132,000 kb pairs, also with telomeres . During development of the macro- nucleus from the micronucleus , the micronuclear genome size can be considerably reduced before amplification, especially in the gene-sized macro- nuclei, hence the term ampliploid was introduced, since the entire genome is not duplicated as it would be in a true polyploid (Raikov, 1982, 1996; Schwartz, 1978). Regardless of the type of macro- nucleus , chromosome-like elements are difficult to observe in macronuclei, and in contrast to the micronucleus , there also are no centromeres and so no means of attachment for spindle microtubules during karyokinesis . There is a huge range of variation in size and shape of macronuclei, ranging from 1.4 pg of DNA in Uronema to over 38,000 pg of DNA in Bursaria (Raikov, 1995). However, DNA amount can vary depending upon the stage in the cell cycle , the age of the cell, and the nutritional state of the cell (Berger, 2001; Raikov, 1995). While macronuclei are typically single, the tintinnid choreotrichs , for example, generally always have two nodules, and other spirotrichs can have dozens. Macronuclear nucleoli are also variable in size and number, but can only be unambiguously discriminated from larger chromatin aggregates when either ribosomal precursors or a nucleolar organizing center can be demonstrated (Figs. 4.9A, 4.19A). Thus, it is a mis- take to describe nucleoli unless at least one of these features has been definitively demonstrated. Raikov (1982, 1994a, 1996) has characterized in detail the range of variation in the macro- nuclei of the Class KARYORELICTEA , which have near diploid to paradiploid DNA amounts. Measurements of DNA amounts in the karyore- licteans indicate that Loxodes , for example, can have macronuclei with up to 6C DNA (Bobyleva, Kudriavtsev, & Raikov, 1980). Karyorelictean macronuclei do not divide, and their number is maintained by division of micronuclei: the karyorelictean micronucleus divides twice at each cell division, once to reproduce itself and once to provide a new macronucleus. After division, the micronucleus differentiates, a process that might include some sequence elimination followed by amplification (Kovaleva & Raikov, 1978). This differentiation process occurs in all other classes of ciliates when macronuclei differentiate following conjugation (see below). As noted above, ciliates spend most of their life cycle reproducing asexually by binary fission . Late in the 19th century, E. Maupas (1889) discovered that Paramecium had a clonal cycle superimposed on these eukaryotic cell cycles: cells could be clas- sified as immature , adolescent , mature , and senes- cent (Fig. 4.16) (Hiwatashi, 1981; Miyake, 1996; Sonneborn, 1957). These periods are operationally defined by the ability of cells to mate or undertake conjugation : in the immature period , cells are unable to conjugate; in the adolescent period , there is some unpredictability in the ability to conjugate; in the mature period , cells are completely sexually competent; and finally in the senescent period , the ability to conjugate becomes initially unpre- dictable and then is lost (Fig. 4.16). Conjugation will rejuvenate the clonal life cycle, \u201cturning the clock back\u201d, so to speak to the immature period. If cells are not able to find partners to conjugate, some species can undergo autogamy , a kind of self\u2212fertilization, to \u201crestart the clock.\u201d Conjugation is often stimulated in the laboratory setting by starvation (i.e., depriving the ciliates of food), and this is likely a stimulant in natural settings as well. Other stimulants to conjugation have been observed, for example, temperature and light (Rapport, Rapport, Berger, & Kupers, 1976; Vivier, 1984). There need to be cells of comple- mentary mating type present to ensure success. Prior to fusion of the cells, cell-to-cell communi- cation needs to take place, either by direct contact between cells or through indirect means. Direct contact occurs when individuals of Tetrahymena and Paramecium touch each other over a period of time prior to forming successful pairs (Watanabe, 1978, 1983; Wolfe & Grimes, 1979). Indirect \u201ccontact\u201d occurs when, for example, individuals of Blepharisma and Euplotes secrete soluble sub- stances called gamones , which prepare potential partners for mating (Miyake, 1981, 1996; Miyake & Beyer, 1974; Heckmann & Kuhlmann, 1986; Fig. 4.16. The clonal life cycle of a ciliate, modeled after Paramecium . After conjugation , the exconjugants separate and undergo growth and binary fissions transiting through an immaturity stage during which conjugation is not pos- sible. In maturity , the ciliates can conjugate with cells of complementary mating type . If cells in the clone are unable to conjugate they undergo a period of senescence with death temporarily delayed by autogamy or self-fertilization. (Redrawn after Hiwatashi, 1981.) 4.6 Nuclei, Sexuality and Life Cycle 115 116 4. Phylum CILIOPHORA \u2013 Conjugating, Ciliated Protists with Nuclear Dualism Luporini, Miceli, & Ortenzi, 1983; Luporini, Vallesi, Miceli, & Bradshaw, 1995; Vivier, 1984). Once stimulated, cells will fuse in a variety of ways: side-to-side, anterior-to-anterior, among oth- ers (Fig. 4.17). During this fusion process, the region of fusion becomes differentiated in preparation for the exchange of the gametic nuclei, which derive by meiosis from the micronuclei of each partner. This conjugation bridge or conjugation basket is often supported by microtubules and microfilaments, which are believed to be involved in the transfer of the migratory gametic nucleus from partner to partner (Geyer & Kloetzel, 1987a, 1987b; Lanners & Rudzinska, 1986; Orias, Hamilton, & Orias, 1983). The migratory gametic nucleus then fuses with the stationary gametic nucleus in karyogamy , forming the synkaryon or zygotic nucleus . The synkaryon may divide twice to form four products, two of which develop into macronuclei and two of which develop into micronuclei (Fig. 4.18), but there is much variation in postkaryogamic develop- ment (Raikov, 1972). During this postkaryogamic phase, the restoration of the original nuclear condi- tion occurs. This involves the programmed death of the parental macronucleus (Ejercito & Wolfe, 2003; Endoh & Kobayashi, 2006; Kobayashi & Endoh, 2003) and the simultaneous differentiation of the new macronucleus, with the elimination of sequences and the amplification of the genome (Jahn & Klobutcher, 2002; Prescott, 1994; Raikov, 1995). There is a great range