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Fig. 1. Cluster of Caedibacter caryophilus in the macronucleus of Paramecium cauda-tum. Some bacteria contain R bodies (arrows) with phage capsids (arrowheads). Electron micrograph, courtesy of Michael Schweikert, University of Stuttgart. Bar 0.5 |m

Fig. 2. R bodies are coiled protein ribbons that are refractile in phase contrast. a The unrolled ribbons have a length of up to 20 |m and a width of 0.4 to 0.8 |m. b By appropriated triggers, such as low pH, they are induced to unroll. Certain R bodies are associated with phage capsids (arrowheads)

All R body producing, obligate endosymbionts of paramecia were combined into the genus Caedibacter and classified based upon morphological, functional, and phenotypic properties (Preer and Preer 1982). Within the genus Caedibacter, six different species were identified in paramecia (C. caryophilus, C. macronucleorum, C. paraconjugatus, C. pseudomutans, C. taeniospiralis, C. varicaedens) (Preer et al. 1974; Preer and Peer 1982; Schmidt et al. 1987; Fokin and Görtz 1993, for review see Görtz and Schmidt 2004). In recent years, endocytobionts of small free-living amoeba were classified as Caedibacter based upon rDNA-similarities to C. caryophilus (Horn et al. 1999). However, none of these bacteria in amoeba was observed to express R bodies. R bodies of C. caryophilus, C. macronucleorum, C. paraconjugatus, C. pseudomutans, C. varicaedens are often associated with phage capsids, and the proteins of R bodies are encoded by phage genomes or plasmids (see for reviews Quackenbush 1988; Pond et al. 1989). The plasmids, in addition may contain transposons. Thus, killer paramecia are subject of multilevel infections. The presence of killer-symbionts was first recognized by Sonneborn as an example of extrachromosomal inheritance in Paramecium (Sonneborn 1938), and the complexity of this symbiosis is most impressively addressed in a title of a paper by Bob Quackenbush and his co-workers: Extrachromosomal elements of extrachromosomal elements of Paramecium and their extrachromosomal elements (Quackenbush et al. 1986a). In this title the genome of Caedibacter being an extrachromosomal element of Paramecium, the plas-mids (or phage genomes) being extrachromosomal elements and the transposons found in plasmids are addressed. In earlier times many authors discussed the significance of Caedibacter for its host. Most authors have emphasized the advantage paramecia are getting from their killer symbionts (see Landis 1988). Yet, neither is the nature of the association clarified, nor has the origin and relationship of plasmids and phages been discovered. Do we have to expect a gene transfer from Caedibacter to the host genome, as has been suggested repeatedly? Is this a mutualistic symbiosis or rather a good example of parasitism, and why do we still find uninfected paramecia in nature? There are some new answers to these questions that shall be addressed in this chapter.

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