Holospora and Holospora -like bacteria in other ciliates. a Holospora sp. in the macronucleus of Frontonia salmastra . b Holospora -like bacteria in the macronucleus of Prorodon teres . c Holospora -like bacteria in the macronucleus of Trithigmastoma cucullulis . Bar 35 m m 

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... (the macronucleus or the micronucleus) does not guarantee the establishment of the symbiotic system, since bacteria may be rapidly lysed in the karyoplasm and/or may not produce reproductive forms (Skoblo et al. 1990 ; Fokin and Skovorodkin 1991a, b ; Skoblo et al. 1996 ; Fokin et al. 2003b). Rapid lysis of infectious forms of H. undulata and H. acuminata in the micronucleus and sometimes in the macronucleus, noted in some resistant paramecium clones, is a good example of interaction in a noncomplementary system (Skovorodkin and Fokin 1990 ; Ossipov et al. 1993 ; Fokin and Skovorodkin 1997 ; Fig. 12 ). The mechanisms of this phenomenon have not been studied. The lysis may be brought about by activation of the host enzymes: the rapidity of the process (3–5 h for H. undulata ) indicates this option. However, if this is the case, it is not clear how the host protects the structures of its own karyoplasm. In experiments involving genetic transformation of the lysis-resistant clone and the creation of heterokaryons bearing different combinations of nuclei from strains resistant or sensitive to infection, it has been shown that the character of lysis is determined by the genome of the macronucleus, not the micronucleus, where the process occurs (Rautian et al. 1996 ; Fokin and Skovorodkin 1997) . Another type of host reaction to infection was revealed when it was attempted to infect with H. obtusa the macronucleus of P. multimicronucleatum , a nonspecific host of this bacterium (Fujishima and Fujita 1985 ; Fokin et al. 2005) . In general, P. multimicronucleatum can be infected with infectious forms within 2 h, but it mainly loses the microorganisms from the macronucleus after 9–16 h. They are released from the host nucleus into the cytoplasm and then outside. This process in the infected cells can be stopped by low temperature or nocodazole treat- ment. This experiment indicates that some microtubules in the nuclear matrix probably play a role in the ciliate defense against intranuclear bacteria (Fokin et al. 2005) . It is not uncommon that the ciliate cell contains simultaneous infections by several endocytobionts at the same time, in different compartments or, less frequently, in the same one (Preer 1969 ; Görtz 1987 ; Fokin et al. 2000) . Examples of such complex endocytobioses can be found in special reviews (Görtz 1986 ; Fokin 1993) . These systems are usually unstable under laboratory conditions, apparently owing to relationships between the symbionts or between the symbionts and the host. Different bacteria may be antagonistic, especially when they are occupying the same compartment (Fig. 13 ). For instance, H. caryophila in the macronucleus of P. caudatum supersedes H. obtusa in the case of common infection; N. macronucleata supersedes H. obtusa (Fokin et al. 1987 ; Fokin 1988) ; in the case of common experimental infection of the micronucleus by H. undulata and H. recta , only the cells with either the former or the latter symbionts are soon observed in the infected paramecia culture, with the common infection disappear- ing (Fokin 1991) . Most probably, the antagonism expresses itself in modification of conditions in the cell compartment by one symbiont, so that they become unsuitable for other bacterial species. It may be also expressed in a direct inter- specific interaction, by production of metabolites or competition for some metabolites of the host (Görtz 1987) . Holospora infection of one of the paramecia nuclei specifically changes the state of the other one. For instance, at the vegetative stage of H. obtusa development in the macronucleus, the generative nucleus may be effectively infected by H. undulata , but as soon as numerous infectious forms appear in the macronucleus, the micronucleus become unsusceptible to H. undulata infection (Fokin and Skovorodkin 1991a, b) . A similar resistance of the macronucleus is observed when H. undulata in the micronucleus passes from the reproductive to the infectious stage. Later the double infection, as a rule, disappears (Fokin 1991, 1993) . On the other hand, there are cases when only the cells already infected by certain bacteria may be infected by additional endocytobionts (Preer 1969 ; Gibson 1973 ; Barhey and Gibson 1984) . As already mentioned, ten Holospora spp. have been found so far in paramecia. Very recently two more examples of bacteria definitely belonging to the genus were discovered in the Frontonia genus (Fokin et al. 2006 ; Ferrantini et al. 2007 ; Fig. 14a ). Although Paramecium and Frontonia are the sister genera, we can postulate now that Holospora are not exclusively endonucleobionts of Paramecium spp. as was always indicated before (Görtz and Schmidt 2005) . Thus, in the future it can be expected that some more holosporas in different groups of ciliates will be found. Indeed, several other bacteria, which have a life cycle similar to Holospora represented by two morphological forms, have been mentioned from different ciliates (Fig. 14 ). All these bacteria appear to be infectious, though experimental evidence on the matter exists only for one of them (Görtz and Maier 1991) . There are macronuclear symbionts of Metopus caudatus (Jankowski 1964) , Prorodon teres (Stein 1867 ; Fig. 14b ), Stentor multiformis (Görtz and Wiemann 1987), S. polymorphus (Balbiani 1892 ; Fokin 2004a) S. roeselii (Stein 1867) , Trithigmostoma cucullulus (Görtz and Maier 1991 ; Fig. 14c ), Vorticella sp. (Kirby 1941b) , and Zoothamnium pelagicum (Laval 1970). For the majority of the ciliates Holospora -like bacteria were just mentioned without serious further investigation or were even shown as figures without any information ( Trichodina pediculus ). According to the morphology of the infectious form, it is possible that not all of them are holosporas since sometimes the distribution of the periplasmic part and the tip part morphology deviated quite a bit from that of “classical” holosporas (Görtz and Wiemann 1987; Görtz and Maier 1991 ; Fokin et al. 2006) We know especially little about the actual situation regarding the presence and maintenance of bacterial infection in ciliates in natural bodies of water. The low percentage of bacterial infection in most ciliates revealed so far appears to indicate that most endocytobionts are parasites or commensals , and not mutualistic symbionts , otherwise, their broad distribution in the population as a result of positive selection would have been observed. This is apparently true for Holospora , which should be considered as parasites of paramecia. Living in the host cell and feeding at its expense, parasitic bacteria should negatively influence the growth of ciliate populations. The presence in some populations of genotypes that do not support infection and stabilization of bacteria in the cell may be considered as a kind of defensive mechanism limiting the opportunities for such endocytobiosis. For example, in some regions of Japan the share of such populations of P. caudatum may reach 40–60% (Fokin et al. 2003b). However, in places where holosporas are widespread (Europe), the bacteria may influence the population structure. An increased proportion of amicronuclear (genetically dead) cells and cells with the polymorphic micronucleus and cortical anomalies, inability of infected paramecia to enter the sexual process, are the usual “payment” for main- taining H. undulata as well as, probably, H. elegans and H. recta infections in the population (Ossipov 1981 ; Fokin 1985, 2004a ; Görtz 1986, 2008) . The results of laboratory investigations cannot always be extrapolated for natural ciliate populations and yet our knowledge is mostly based on such works. Experiments show that infectious endocytobionts may increase the presence of other bacteria in ciliate cells by means of coinfection and the diversity of endocytobionts should thus be higher in the populations that are, or were, infected by holosporas. Thus, holosporas can be regarded as native vectors and a factor of increasing endocytobiont diversity . The host specificity of Holospora spp., ten of which in nature infect six Paramecium species, invites the supposition about coevolution of these genera. Experimental cross-infections of eight Paramecium spp. by five species of Holospora revealed groups infected by similar sets of bacteria, pointing to relatedness of paramecia in these groups. The good agreement of this classification with phylogenetic groups of Paramecium revealed by 18S rDNA analysis of the same set of species (Fokin 2000 ; Fokin et al. 2004) testifies to the sensitivity of this “symbiotic” method. A recent model of the evolution of mixed transmission of parasitic genotypes has shown that transmission proceeds either predominantly vertically or horizontally (Lipsitch et al. 1996). In contrast to this prediction, the example of Holospora points to adaptive phenotypic plasticity as an alternative to genetic specialization in dealing with ecological variability. This parasite has a mixed, phenotypically plastic strategy, ranging from exclusive expression of vertical transmission to predominant expression of horizontal transmission , which possibly evolved as an adaptation to variable host growth conditions (Kaltz and Koella 2003 ; Fels and Kaltz 2006 ; Restif and Kaltz 2006) . An experimental study of coevolution in the microcosm of several P. caudatum clones infected with H. undulata made recently revealed an increase in host resistance , but not in parasite infectivity (Lohse et al. 2006) . Cross-infection experiments showed significant host clone-parasite isolate interactions, and the evolved host tended to be more resistant to its own (local) parasites than to H. undulata isolated from other host clones. The authors postulated de novo evolution of host resistance, associated with both direct and indirect costs, and thus illustrated how interactions with holosporas can lead to the genetic divergence of initially identical ciliate populations ...