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a typical conjugation , although modi-
fied by a bizarre mutual exchange of portions of 
macronuclei. Other apostomes engage in what has 
been called syndesmogamy or zygopalintomy : two 
trophonts encyst together, undergo synchronized 
 palintomic divisions, and ultimately conjugate as 
 tomites (Chatton & Lwoff, 1935a; Minkiewicz, 
1912; Raikov, 1972). Conjugating endosymbionts 
may often be smaller than the average trophont, 
suggesting that they might undergo preconjugation 
divisions (Kazubski, 1963; Raikov, 1972). 
Paramecium and Tetrahymena served as the 
early model organisms for exploration of the 
genetics of the ciliates, and these efforts have 
been complemented and amplified by research on 
spirotrich genetics ( see Chapter 7 ). There is a very 
large literature on these two genera, representing 
the Subclasses Peniculia and Hymenostomatia , 
which we have used very selectively to touch on 
the characteristics of these sexual processes for 
the Class OLIGOHYMENOPHOREA . Late in the 
19th century, Maupas (1889) discovered the clonal 
life cycle of ciliates in his investigations of nuclear 
phenomena in Paramecium . During this clonal 
life cycle , cells of a clone pass through stages 
characterized as immature , adolescent , mature , 
and senescent , dependent upon the number of 
fissions since the last conjugation and upon their 
ability to engage in conjugation at the moment 
(Hiwatashi, 1981; Sonneborn, 1957; Takagi, 1999). 
As with other ciliates, oligohymenophoreans can 
be induced to conjugate after moderate starva-
tion and if they are sexually mature (Bruns, 1986; 
Fujishima, 1988). Relatively little research, other 
than that of a descriptive nature, has been focused 
on members of the other oligohymenophorean 
subclasses. Finley (1936) was able to predictably 
induce conjugation in Vorticella species, but only 
after they had encysted and excysted . Conjugation 
can also be induced in Paramecium by chemical 
means (Cronkite, 1974) and by mixing a single 
mating type with detached cilia from mature cells 
of the complementary mating type (Miyake, 1964). 
In Paramecium , the natural mating-type substances 
aiding agglutination are very likely proteins, and in 
 complementary mating types they may even have a 
precursor-derivative relationship (Xu et al., 2000). 
 Sonneborn (1937) discovered complementary 
mating types of Paramecium “aurelia” species 
while Elliott and Nanney (1952) reported a similar 
phenomenon in Tetrahymena . These discoveries 
opened the way to exploration of the genetics of 
ciliates (Nanney, 1980; Sonneborn, 1947). The 
 binary mating-type system in Paramecium with its 
two complementary mating types is probably the 
ancestral state in the oligohymenophoreans , and 
from it the multiple mating-type systems described 
for Paramecium bursaria and Tetrahymena 
thermophila have likely evolved (Bleyman, 1996; 
Miyake, 1996). Exhaustive sampling of species 
in these two genera has not expanded the number 
of mating types beyond a maximum of 8 and 7 
respectively, in dramatic contrast to the poten-
tially hundreds of mating types recorded for some 
 stichotrichs (Doerder et al., 1996; see Chapter 7 ). 
Jankowski (1972b) has demonstrated that 
Paramecium putrinum also has a multiple mat-
ing-type system . Mating type in binary systems 
has been further classified into three categories. 
In the A or caryonidal mating-type system , each 
developing macronucleus following conjugation 
is independently determined to express either 
 mating type. In the B or clonal mating-type 
system , the parental macronucleus epigenetically 
determines the expression of the mating type by 
the new macronucleus. In the C or genotypic 
mating-type system , the mating type expressed is 
under genotypic control (Bleyman, 1996; Simon 
& Orias, 1987; Sonneborn, 1977). A variety of 
environmental factors can influence the expression 
of the ultimate mating type in Tetrahymena with 
temperature and nutrition being important vari-
ables (Arslanyolu & Doerder, 2000; Doerder et al., 
1996). One of the most interesting environmental 
influences is the circadian rhythm of mating-type 
expression in Paramecium multimicronucleatum
(Barnett, 1966). While the complete analyses of 
the genomes of Paramecium and Tetrahymena
may eventually resolve the matter, the mating-type 
alleles of Tetrahymena exhibit serial dominance at 
a single locus, explained by a model for somatic 
DNA rearrangements during macronuclear devel-
opment (Orias, 1963, 1981). Those of Paramecium 
bursaria are explained by combinations of alleles 
at two or three different loci (Siegel, 1963). 
 In the oligohymenophoreans , the molecules 
signalling the readiness for mating are firmly 
bound to the cell surface of the ciliate, either on 
the ciliary membranes or the plasma membrane; 
Miyake (1996) calls this type, gamone-carrying . 
Mature Tetrahymena prepare for cell fusion in 
two stages, called initiation (Bruns & Brussard, 
1974) and costimulation (Finley & Bruns, 1979). 
In contrast, mature Paramecium agglutinate upon 
first contact after which the cells prepare for con-
jugation (Fujishima, 1988; Nanney, 1980). Other 
morphological differentiations occur in cells prior 
to fusion. In Tetrahymena , a region of the cor-
tex anterior to the oral region becomes smooth 
and flattened. This tip transformation prepares 
the cells for fusion (Wolfe & Grimes, 1979). In 
Paramecium , a broad region of cortex anterior 
and posterior to the oral region becomes decili-
ated prior to cell fusion (Watanabe, 1978). Cell 
membranes ultimately fuse to enable transfer of 
the pronuclei, at the anterior end in Tetrahymena
and at the paroral cone region in Paramecium
(Fujishima; Inaba, Imamoto, & Suganuma, 1966; 
Wolfe, 1985). In Tetrahymena , microtubules have 
been implicated in the movements of micronuclei 
prior to formation of the gametic nuclei (Nakajima, 
Ishida, & Mikami, 2002), and in the formation of 
the complex, microtubular-microfilamentous trans-
fer basket that envelopes the migratory pronucleus 
in each partner, enabling transfer (Orias, Hamilton, 
& Orias, 1983). 
 As with other ciliates, the micronuclei of 
 oligohymenophoreans undergo typically three 
maturation divisions with the exception that some 
 peritrich micronuclei may only undergo two matu-
ration divisions (Raikov, 1972). These patterns are 
confirmed in more recent reports for scuticocili-
ates (Coppellotti, 1990), hymenostomes (Martín-
Gonzalez, Serrano, & Fernández-Galiano, 1984), 
and peritrichs (Sola, Guinea, & Fernández-Galiano, 
1989a). Raikov (1972) noted that the “crescent 
stage” is probably typical of micronuclear meio-
sis in peniculines , scuticociliates , hymenostomes , 
and some peritrichs , but a “parachute stage” , 
found in nassophoreans , has been observed as 
examples, in one astome , one apostome , and in 
Paramecium putrinum (Jankowski, 1972b, Raikov, 
1972). The “crescent stage” micronucleus is prob-
ably elongated by microtubular growth (Suganuma 
& Yamamoto, 1992). The chromosomes appear to 
be arranged in parallel with their telomeres aggre-
gated near one end of the developing “crescent”, 
15.6 Nuclei, Sexuality and Life Cycle 323
which may facilitate the pairing of homologues 
(Loidl & Scherthan, 2004). There is either mutual 
exchange of gametic nuclei, or in the case of total 
conjugation in peritrichs , the fusion of the single 
“migratory” gametic nucleus of the microconju-
gant with the single “stationary” gametic nucleus 
of the macroconjugant . This makes the exconju-
gants isogenic. The zygotic nucleus or synkaryon 
then undergoes typically two or three divisions, 
although in some Frontonia species it may divide 
only once while in some Paramecium species it 
may divide four times (Raikov, 1972). This