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the macronuclear anlagen developing from these 
division products of the synkaryon can range from 
one up to 15. This is complicated by the fact that 
variable numbers of these division products may 
degenerate without development (Raikov, 1972). 
There is really insufficient breadth of analysis to 
draw any firm conclusions of patterns, if indeed 
there are any, in relation to the subclasses of oligo-
hymenophoreans . 
 Autogamy results from the fusion in one cell 
of the haploid meiotic products of the maturation 
divison of the micronucleus (Corliss, 1952; Diller, 
1936). The progeny are thus homozygous, and 
this has been advantageous for the genetic explo-
ration of Paramecium in that mutations can be 
brought to full expression by inducing autogamy . 
Tetrahymena thermophila cannot be induced to 
 autogamy . However, geneticists can now achieve 
homozygosity in this species by matings with so-
called star strains , for example, strain A * . During 
this process, called genomic exclusion , the star 
strain loses its micronucleus during meiosis . A 
migratory gametic nucleus is transferred to this 
star strain partner, after which both partners, now 
isogenic, become diploid by endoreduplication 
(Allen, 1967; Bruns, 1986). 
 Conjugation and/or autogamy are now consid-
ered crucial to the continued existence of strains 
of ciliates. Sonneborn (1954) originally showed 
their importance, demonstrating that periodic bouts 
of autogamy in a so-called “Methuselah” strain of 
Paramecium biaurelia extended its clonal life. 
Later the same phenomenon was demonstrated for 
Tetrahymena species (Corliss, 1965). Without these 
sexual processes, senescence sets in at from 200–
350 cell divisions in members of the Paramecium 
“aurelia” complex and up to 1,500 cell divisions in 
Tetrahymena (Takagi, 1988, 1999). A single known 
exception is the amicronucleate Tetrahymena pyri-
formis , which has remained in culture for over 60 
years: while it is “genetically dead”, it is so-far 
physiologically immortal (Nanney, 1974). A variety 
of features indicates cells have entered senescence , 
among others: unequal distribution of macronu-
clear DNA at cytokinesis , a decreased viability of 
progeny after conjugation, a decreased ability to 
form food vacuoles, and a decreased fission rate 
(Smith-Sonneborn, 1981; Takagi, 1988). Clonal 
life span is undoubtedly under genetic control as 
mutants with variations in the clonal life cycle 
have been discovered (Komori, Sato, Harumoto, & 
Takagi, 2005; Takagi, Suzuki, & Shimada, 1987). 
Environmental factors can influence longevity , 
including UV and other forms of ionizing radiation 
(Smith-Sonneborn, 1981). 
 Conjugation is rarely observed in natural pop-
ulations of oligohymenophoreans (Lucchesi 
& Santangelo, 2004). However, populations of 
Paramecium and Tetrahymena can be dominated by 
immature individuals, suggesting that sex may be 
quite frequent in nature (Doerder, Gates, Eberhardt, 
& Arslanyolu, 1995; Kosaka, 1991b), although a 
population dominated by senile individuals has also 
been discovered (Kosaka, 1994). 
 Sonneborn (1957) also related breeding systems 
of Paramecium to characteristics of the life 
 history of the species. He proposed an inbreeding-
 outbreeding continuum : extreme inbreeders would 
have two mating types, a short period of imma-
turity, high fission rates, and local distributions, 
while extreme outbreeders would have the oppo-
site set of characters (reviewed by Landis, 1986; 
Nyberg, 1988). At that time, some Paramecium
“aurelia” species represented the extreme inbreed-
ers while Paramecium “bursaria” species repre-
sented extreme outbreeders . Tetrahymena species 
with their multiple mating types would be con-
sidered relative outbreeders . While this has been 
an attractive thesis, Nyberg (1988) concluded that 
there is contradictory data to refute it. Paramecium 
“bursaria” species, supposed extreme outbreed-
ers , appear to be restricted in their geographic 
distributions while some Paramecium “aurelia”
species, typical inbreeders , are globally distrib-
uted. Furthermore, Nyberg (1981b) demonstrated 
that continental geographic distances did not 
reduce the fertility of several Tetrahymena species 
while Przybos (1995) has demonstrated that North 
American and European isolates of “inbreeding” 
Paramecium triaurelia are not genetically isolated. 
Nevertheless, our ideas may be refined in the 
future as more molecular data accumulate. Stoeck, 
Przybos, and Schmidt (1998) have shown, using 
RAPD fingerprinting, that European populations 
of Paramecium sexaurelia , an extreme inbreeder , 
are more genetically isolated than populations 
of Paramecium triaurelia , a moderate inbreeder , 
consistent with Sonneborn’s predictions. Stoeck et 
al. (2000a) have also used this approach to char-
acterize P. novaurelia as a moderate inbreeder and 
P. pentaurelia as a weak inbreeder . 
 Sonneborn (1957) provided evidence that the 
 “genetic species” of the Paramecium “aurelia”
 complex were identical to the sibling species of the 
 fruit fly Drosophila . Nevertheless, because of the 
relatively onerous task of operationally identifying 
a species of Paramecium “aurelia” , Sonneborn 
(1957) was reluctant to name them as taxonomic 
species and instead chose to place them in syn-
gens ( syn , Gr = same, gens , Gr = kind). A similar 
situation was soon discovered for the Tetrahymena 
“pyriformis” species complex (Elliott, 1973b; 
Gruchy, 1955; Nanney, 1980). While analyses 
of cortical patterns suggested that some species 
of tetrahymenine hymenostomes might be sepa-
rated morphologically (Cho, 1971; Nanney, 1966, 
1968), multivariate morphometric analyses finally 
demonstrated that four species of the P. “aurelia”
complex could be separated but others could not 
(Gates & Berger, 1976b; Powelson et al., 1975). 
The discoveries of isozyme variation among spe-
cies of Paramecium by Tait (1970) and Allen, 
Byrne, and Cronkite (1971) and Tetrahymena
(Allen & Weremiuk, 1971; Borden et al., 1973a, 
1973b) were to provide an easy operational method 
to distinguish “genetic species”. These results lead 
Sonneborn (1975) and Nanney and McCoy (1976) 
to establish nominate species for the syngens of 
Paramecium “aurelia” and Tetrahymena “pyri-
formis” . DNA fingerprinting is now being used to 
distinguish species of Paramecium (Skotarczak et 
al., 2004; Stoeck et al., 1998), and to demonstrate 
that other morphological species of Paramecium , 
such as Paramecium duboscqui , are probably also 
 species complexes (Fokin et al., 1999). 
 While it had been difficult to morphologi-
cally resolve free-living species of Paramecium
and Tetrahymena , morphological variability even 
among populations of symbiotic species has been 
well established. For example, mobiline peritrichs 
on breeding carp showed statistically significant 
seasonable variability (Kazubski & Migala, 1968); 
 scuticociliate endosymbionts of sea urchins (Lynn 
& Berger, 1972, 1973) and bivalves (Berger & 
Hatzidimitriou, 1978) showed statistically signifi-
cant variation on a number of traits between host 
populations; and apostome symbionts showed sig-
nificant variation among host crustaceans (Landers, 
Zimlich, & Coate, 1999). These variations are 
likely due to a combination of factors, including 
invasion of the host by one to a few founders and 
adaptive responses to differing host environments 
(Berger & Hatzidimitriou, 1978). This dramatic 
morphological variation is contrasted with genetic 
uniformity in some symbionts from around the 
world: isolates of Orchitophrya stellarum have 
apparently identical nuclear genotypes in different 
 starfish hosts from around the world (Goggin & 
Murphy, 2000). Whether this holds for cytoplasmic 
genes, such as those from mitochondria, awaits 
future research. 
 15.7 Other Features