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of variation in the features of 
 conjugation among and even within the different 
Fig. 4.17. Conjugation involves fusion of the two cells of complementary mating type . This fusion can occur in 
different body regions depending upon the group of ciliates. ( a ) Loxodes – Class KARYORELICTEA . ( b) Euplotes
– Class SPIROTRICHEA . ( c) Stylonychia – Class SPIROTRICHEA . ( d ) Strombidium – Class SPIROTRICHEA . 
( e ) Metopus – Class ARMOPHOREA . ( f) Coleps – Class PROSTOMATEA . ( g) Actinobolina – Class 
 LITOSTOMATEA . ( h ) Litonotus – Class LITOSTOMATEA . ( i ) Chilodonella – Class PHYLLOPHARYNGEA .
( j ) Spirochona – Class PHYLLOPHARYNGEA . ( k) Paramecium – Class OLIGOHYMENOPHOREA . ( l) Vorticellid 
peritrich – Class OLIGOHYMENOPHOREA . (Redrawn from Kahl, 1930.)
classes of ciliates. Some of this variation is 
touched on in the section Nuclei, Sexuality, and 
Life Cycle in each chapter, but see reviews by 
Raikov (1972), Vivier (1984), and Miyake (1996). 
Briefly, conjugating cells are typically the same 
size, hence isomorphous conjugation , but cells can 
differ in size, hence anisomorphous conjugation . 
In anisomorphous conjugation, which occurs often 
in sessile forms, a migratory microconjugant dis-
perses and may totally fuse with a stationary mac-
roconjugant resulting in only one exconjugant cell 
(Fig. 4.17j, 4.17l). Mating type systems are either 
bipolar or multipolar. In bipolar systems, there are 
only two mating types: for example, the “odd” and 
“even” mating types of Paramecium (Sonneborn, 
1957). In multipolar systems, there are many 
more than two mating types: in the stichotrich 
Stylonychia , there may be over 50 mating types 
(Ammermann, 1982). A further variation occurs 
in the length of the period of immaturity ; when 
this period is short, the species is classified as a 
relative inbreeder and when it is long, the species 
is classified as a relative outbreeder (Bleyman, 
1996; Landis, 1986; Sonneborn, 1957; Stoeck et al., 
Fig. 4.18. The nuclear events of conjugation , modeled after Tetrahymena . Two ciliates of complementary mating type 
fuse (on the left) and their micronuclei undergo meiosis . One of the meiotic products survives and divides mitoti-
cally, giving rise to two gametic nuclei – one stationary and one migratory. Fertilization occurs after the migratory 
gametic nuclei cross the conjugation bridge . The synkaryon divides twice, in this case, and two products differenti-
ate as macronuclei and two differentiate as micronuclei. The old macronucleus becomes pycnotic and is resorbed. 
(Redrawn after Nanney, 1980.)
4.6 Nuclei, Sexuality and Life Cycle 117
118 4. Phylum CILIOPHORA – Conjugating, Ciliated Protists with Nuclear Dualism
2000a). Even though extreme “inbreeding”, identi-
fied as selfing or intraclonal conjugation , has been 
identified in some Tetrahymena species, it does 
not always lead to clonal death, although viability 
is typically much reduced (Simon & Meyer, 1992; 
Simon & Orias, 1987). 
 A discussion of the nuclei of ciliates would not 
be complete without brief mention of the recent 
successful genome projects on Tetrahymena (Eisen 
et al., 2006) and Paramecium (Aury et al., 2006) 
and the earlier discovery of genetic code deviations 
among ciliates. In reference to the latter phe-
nomenon, detailed investigation of protein-coding 
genes in ciliates demonstrated that the univer-
sal stop codons UAA and UAG coded glutamine 
in the oligohymenophoreans Tetrahymena and 
Paramecium , which used only UGA as the stop 
(Caron & Meyer, 1985; Horowitz & Gorovsky, 
1985; Preer, Preer, Rudman, & Barnett, 1985). 
Subsequently, genetic deviations were discovered 
in the spirotrich Euplotes (Harper & Jahn, 1989), 
the heterotrich Blepharisma (Liang & Heckmann, 
1993), and representatives of several other classes 
(Baroin-Tourancheau, Tsao, Klobutcher, Pearlman, 
& Adoutte, 1995; Kim, Yura, Go, & Harumoto, 
2004; Sánchez-Silva, Villalobo, Morin, & Torres, 
2003). Baroin-Tourancheau et al. (1995) concluded 
that evolution of these genetic code deviations must 
have occurred independently during the evolutionary 
diversification of the phylum. These variations are 
mechanistically explained by altered tRNAs (Caron, 
1990; Grimm, Brunen-Nieweler, Junker, Heckmann, & 
Beier, 1998; Hanyu, Kuchino, & Nishimura, 1986; 
Sánchez-Silva et al., 2003) and by changes in the 
specificity of eukaryotic release factor 1 (Caron, 
1990; Lozupone, Knight, & Landweber, 2001; 
Moreira, Kervestin, Jean-Jean, & Philippe, 2002). 
 4.7 Other Conspicuous Structures 
 Three other prominent kinds of structures are 
briefly mentioned below. More details on each of 
these can be found in the subsequent chapters relat-
ing to each class. 
 The osmoregulatory system of ciliates is cen-
tered on the contractile vacuole and its complex of 
vesicles and canals, which have long been known 
as responsive to ionic changes in the environment 
(Allen, 2000; Estève, 1984a; Kitching, 1967). As 
noted by Patterson (1980), the ciliate contractile 
vacuole complex is one of the most elaborately 
organized of those exhibited by protists. The 
cytoplasm surrounding the contractile vacuole is 
termed the spongioplasm , the region of cytoplasm 
responsible for the sequestration of water and ions, 
in part through the action of proton-translocating 
V-type ATPases (Allen; Stock, Gronlien, Allen, 
& Naitoh, 2002). The spongioplasm tubules may 
connect directly to the contractile vacuole or, as is 
often the case in larger ciliates, indirectly by col-
lecting canals that radiate out from the contractile 
vacuole – Types A and B of Patterson (1980). This 
organelle received its name because of the rapid 
expulsion of its contents, inferred to be caused by 
a contractile mechanism. However, it now appears 
that cytosolic pressure is sufficient to explain the 
expulsion dynamics (Naitoh et al., 1997). The fluid 
is expelled through one or more pores that are 
typically permanent features of the somatic cortex. 
The pores are supported by a thickened epiplasm, 
a special set of helically-disposed microtubules, 
and a set of radial microtubules that contact the 
 contractile vacuole itself (Fig. 4.19F) (McKanna, 
1973a; Patterson, 1980). In addition to its activ-
ity being related to the external environment, the 
 contractile vacuole is also influenced by ambient 
temperature and the size of the cell (Lynn, 1982; 
Nematbakhsh & Bergquist, 1993). 
 Mitochondria are also prominent organelles, typ-
ically several microns long and about 1 µm wide, 
distributed in the cortex of ciliates, underneath 
the cortical ridges and often in close association 
with the epiplasm (Figs. 4.9C, 4.10G, 4.19A) 
(Aufderheide, 1983). In all ciliates so far exam-
ined, the mitochondria have tubular cristae (Fokin, 
 Two variations in mitochondria merit brief discus-
sion. First, scuticociliates are typified by having per-
haps a single mitochondrion, at least extending from 
the anterior to the posterior of the cell beneath each 
cortical ridge. These adjacent long mitochondria may 
extend laterally to join with their neighbors, forming 
one giant mitochondrion underlying the entire cortex 
– a structure truly worthy of the term chondriome 
(Antipa, 1972; Beams & Kessel, 1973). Second, mito-
chondrial variation occurs amongst anaerobic ciliates 
from different classes (i.e., Classes ARMOPHOREA , 
which have mitochondria in which the tubules are 
Fig. 4.19. Ultrastructural features of conspicuous organelles of ciliates. A The macronucleus ( MA ) and its nucleolus
( N ) of the colpodean Colpoda steinii . Note the closely adjacent micronucleus ( MI) with its condensed chromosomes 
and several mitochondria ( M ). B–E . Extrusomes of ciliates. B A rod−shaped mucocyst of the oligohymenophorean
Colpidium campylum (from Lynn & Didier,