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(Beier et al., 2002). It is the so-called R body, 
constructed by these obligate endosymbionts, that 
kills susceptible host cells (Preer et al., 1974; 
Quackenbush, 1988). The existence of “killer” 
Paramecium in nature is probably maintained 
by natural selection as “killers” are rarely found 
with “sensitives” in natural collections (Landis, 
1981, 1986). Another obligately symbiotic bacte-
rial species group is an assemblage belonging to 
15.2 Life History and Ecology 295
the genus Holospora (Görtz, 1988b, 1996). The 
nuclei of several Paramecium species are infected: 
the macronucleus by some Holospora species 
(e.g., Fokin, Brigger, Brenner, & Görtz, 1996; 
Fujishima, Sawabe, & Iwatsuki, 1990) and the 
 micronucleus by others (e.g., Görtz & Dieckmann, 
1980; Ossipov, Borchsenius, & Podlipaev, 1980). 
These latter species can essentially genetically 
castrate the Paramecium by destroying the micro-
nucleus (Görtz, 1988b). On the other hand, con-
jugation may be a way that some Paramecium
can rid themselves of the macronuclear endosym-
bionts , although symbiont strategies can avoid 
this by migrating to the anlage or by causing the 
fusion of infected pycnotic fragments with the 
anlage (Fokin, 1998). Some clones of Paramecium
are resistant to infection by Holospora , and this 
resistance can evolve (Lohse, Gutierrez, & Kaltz, 
2006). Microsurgical transfers of nuclei suggest 
that the lytic abilities of the host are mediated 
by macronuclear activity (Fokin & Skovorodkin, 
1997). A variety of other bacterial endosymbi-
onts have been described in Paramecium species 
(Fokin, Sabaneyeva, Borkhsenius, Schweikert, & 
Görtz, 2000; Görtz, 1996), in the scuticociliates 
Uronema (Soldo, Brickson, & Vazquez, 1992; 
Soldo, Godoy, & Brickson, 1974), Schizocaryum
(Lynn & Frombach, 1987), Cyclidium (Esteban et 
al., 1993b), and Conchophthirus (Fokin, Giamberini,
Molloy, & de Vaate, 2003), and in the hymenos-
tome Ophryoglena (Fokin et al., 2003). Holospora
may enhance the success of entry of these other 
symbionts into the nuclei of Paramecium (Fokin, 
Skovorodkin, Schweikert, & Görtz, 2004). While 
the host-symbiont relationship of most of these 
symbionts is unknown, scuticociliates from anaero-
bic habitats are known to harbor methanogens , 
which make use of hydrogen generated by the host 
(Esteban & Finlay, 1994; Esteban et al., 1993b). 
 From a human perspective, possibly the most insid–
ious examples of “endosymbiotic” bacteria carried 
by ciliates are species of the “pneumonia-causing”
genus Legionella , which have been found infect-
ing and confirmed to proliferate in Tetrahymena
species (Barbaree, Fields, Feeley, Gorman, & 
Martin, 1986; Steele & McLennan, 1996). This 
led to Tetrahymena being called “ Trojan Horses 
of the microbial world” (Barker & Brown, 1994). 
 Bacteria are occasionally observed also as epibi-
onts on the cell surface of oligohymenophoreans 
(e.g., Bauer-Nebelsick, Bardele, & Ott, 1996; 
Beams & Kessel, 1973; Esteban & Finlay, 1994; 
Lynn & Frombach, 1987). 
 The diversity of eukaryotic endosymbionts of 
 oligohymenophoreans pales in comparison to the 
 prokaryotes . Gillies and Hanson (1963) described 
a Leptomonas species that infected the macronu-
cleus of Paramecium species. Suctorians have been 
reported as “parasites” of peniculines and peri-
trichs (Jankowski, 1963; Padnos & Nigrelli, 1947; 
Pérez Reyes & López-Ochoterena, 1963), and 
 rhynchodids can “parasitize” peritrichs (Chatton & 
Lwoff, 1939b). The astome Spirobuetschliella was 
reported to be parasitized by the microsporidian 
Gurleya (Hovasse, 1950). 
 The vast majority of research on eukaryotic 
symbionts has focused on the endosymbiotic 
Chlorella species of Paramecium bursaria (Görtz, 
1996; Reisser, 1986). These Chlorella symbionts 
enhance the growth rate , maximum population 
density , and survival of their host Paramecium
(Karakashian, 1975). Several strains and species 
of Chlorella , which typically release several times 
more sugar by cell dry weight than non-infective 
isolates, have been isolated from different strains 
of P. bursaria world-wide (Reisser, Vietze, & 
Widowski, 1988; Weis, 1979). Karakashian and 
Rudzinska (1981) demonstrated that vacuoles 
containing infective Chlorella inhibited lysosomal 
fusion, and speculated that this was due to altera-
tion of the vacuolar membrane, a prediction con-
firmed by Meier, Lefort-Tran, Pouphile, Reisser, 
and Reisser (1984). While “infection-capable” 
Chlorella species may influence food vacuolar 
membrane properties, these species are also dis-
tinguished by the presence of glucosamine in 
the cell wall (Takeda, Sekiguchi, Nunokawa, & 
Usuki, 1998). Nevertheless, Chlorella cells can be 
digested by the host ciliate, and this is particularly 
enhanced in the dark (Gu, Chen, Ni, & Zhang, 
2002). Perhaps darkness increases the mortality of 
the Chlorella , which cannot then “control” their 
vacuolar environment. Chlorella -type symbionts 
have also been observed in another peniculine 
Frontonia (Finlay & Maberly, 2000) and in the 
 peritrichs Vorticella (Graham & Graham, 1980) 
and Ophrydium (Woelfl & Geller, 2002). The 
abundances of these ciliates, coupled with the 
photosynthetic activity of their symbionts, can at 
times make them significant contributors to the 
 primary production of some waters (Sand-Jensen 
et al., 1997; Woelfl & Geller, 2002). 
 Oligohymenophoreans demonstrate behavioral 
responses to a variety of environmental parameters. 
The repertoire of these responses, in turn, can 
help to explain their ecology. Symbiont-bearing 
P. bursaria show positive photokinesis and pho-
toaccumulation (Cronkite & Van den Brink, 1981; 
Nakaoka, Kinugawa, & Kurotani, 1987). This 
response occurs with different Chlorella species 
and requires algal photosynthesis . At least 50 indi-
vidual Chlorella cells must be present in an indi-
vidual Paramecium to induce this behavior (Niess, 
Reisser, & Wiessner, 1982). Hymenostomes , such 
as the ophryoglenid Ophryoglena , and scuticocili-
ates , such as Porpostoma , have cup-like organelles 
in the oral region. These ciliates also demonstrate 
complex life cycles with theront , trophont , pro-
tomont , tomont , and tomite stages (Fig. 15.1). 
 Photobehavior is related to the life-cycle stage: 
the tomont or dividing stage typically exhibits 
negative phototaxis while the dispersing theront 
stage exhibits little preference (Kuhlmann, 1993; 
Kuhlmann, Bräucker, & Schepers, 1997). Theronts 
of Ophryoglena do exhibit chemotaxis (Kuhlmann, 
1993), a behavior that has been thoroughly investi-
gated in species of Paramecium and Tetrahymena . 
Species in these latter genera are attracted to 
inorganic compounds and organic compounds, 
including amino acids (Almagor, Ron, & Bar-Tana, 
1981; Levandowsky et al., 1984; Hellung-Larsen, 
Leick, & Tommerup, 1986; Van Houten, 1975, 
1982). Tetrahymena is particularly sensitive, at 
3 × 10 −8 M, to some proteins, such as plate-
let-derived growth factor (Hellung-Larsen et al., 
1986), and this may explain the facultative his-
tophagy exhibited by a number of species in this 
genus ( see above ). Finally, Paramecium shows 
behavioral hypothermia as these organisms seek 
lower temperatures apparently to survive hypoxic 
conditions, a behavior also exhibited by a variety 
of animal species (Malvin & Wood, 1992). 
 As noted above, oligohymenophoreans can 
exhibit complex polymorphic life cycles . Stages 
in these life cycles exhibit different behaviors, 
undoubtedly of adaptive significance (Fenchel, 
1990). A prime stress in the life cycle of any pro-
tist is the disappearance of food. There are two 
typical responses to starvation or the absence of 
appropriate food. The first is transformation to 
the theront or “hunter” phenotype