Cap 4

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 – the 
 macronucleus is typically “ polyploid ” 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 0.4–20 kb in size, 
each with telomeres and typically including one 
gene; and (2) those with subchromosome-sized 
DNA molecules , roughly 100–2,000 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, “turning the 
clock back”, 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−fertilization, to “restart the clock.” 
 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 
“contact” 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 – 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