Parasites are believed to have a major impact on host evolution. On the other hand, it is necessary for a parasite to adapt its growth dynamics to the host species or even to its genotype because the host must not be killed before the parasite has developed infectious stages. Optimal virulence should result in high parasite fitness in a host, whereas maladapted virulence could drive a parasite or the host extinct ('Suicide King' hypothesis; e.g., Dybdahl and Stor-fer 2003). Co-evolution between parasites and hosts in some cases resulted in very specific (genotype-specific) interactions. Certain host genotypes can persist only together with certain parasite genotypes (towards which they are well adapted), but not with others (towards which they are not adapted).
Various Paramecium species in general are infected by different Caedibac-ter species (Pond et al. 1989), but certain Caedibacter species live in different host species. C. taeniospiralis or C. pseudomutans, both, occur in P. tetraurelia, and C. varicaedens or C. paraconjugatus inhabit P. biaurelia. Furthermore, C. caryophilus of different genotypes inhabit different host species and here live inside different compartments of the cell (Kusch et al. 2000). C. caryophilus inhabits the macronucleus of P. caudatum, and lives inside the cytoplasm of P. novaurelia, too. A 16S-rDNA sequence of 1695 base pairs length differed in a total of only one base pair in C. caryophilus of the two Paramecium species. This difference, though only small, may indicate that the bacteria have different adaptations. The infection of specific host cell compartments as a result of interspecific communication may depend on certain host genes, but not on basically different traits of the infecting endocy-tobiont species. Yet, so far we do not know about the extent of genetic differences of the total genomes of Caedibacter strains; they may be larger than the differences of their rDNA. Although this example hints to a quite specific adaptation of Caedibacter genotypes and host species, we do not know whether different genotypes of one Caedibacter species occur in different genotypes of one Paramecium species. Alternatively, the infection of different compartments of a cell in different host species could result from different infection mechanisms (e.g., cytoplasmic exchange during conjugation versus co-infection with nuclear specific Holospora species).
Recent work shows that Caedibacter bacteria are not limited to the genus Paramecium as hosts. Predators of paramecia, e.g., Didinium nasutum (Cilio-phora) or Amoeba proteus (Amoebozoa, Gymnamoebia) were frequently infected when the predators were fed on infected paramecia (Reisser et al. 1985; Kusch et al. 2002). Most of the newly infected predators lost their parasites during a few cycles of vegetative reproduction, but some did not. The genus Acanthamoeba comprises numerous species of free-living amoeba that are increasingly being recognized as hosts for obligate bacterial endocytobionts. Acanthamoebae serve as natural vehicles for the human pathogen Legionella pneumophila and for a variety of other clinically relevant bacteria (Barker and Brown 1994; Atlas 1999). In contrast to these transient interrelationships, which result in lysis of the infected amoebas, the presence of stably infecting, obligate bacterial endocytobionts has been reported for about 25% of all Acan-thamoeba isolates (Fritsche et al. 1993). Comparative analyses of 16S rDNA sequences revealed that the endocytobionts of Acanthamoeba strains are related to Caedibacter caryophilus, Holospora elegans and Holospora obtusa, all of which live in the nuclei of Paramecium caudatum (Horn et al. 1999). With overall 16S rRNA sequence similarities to their closest relative, C. caryophilus, of between 87 and 93%, these endocytobionts of amoebas represent three distinct new species that were proposed to be classified as Caedibacter acanthamoeba, Paracaedibacter acanthamoebae and Paracaedibacter sym-biosus. The Caedibacter-specific R body production was not observed in these endocytobionts from acanthamoebae. These findings suggest a specific association of Caedibacter-species with different host species of unrelated higher taxa and a relatively high diversity of hosts. The specializations and ecological adaptations that enable a certain Caedibacter species (or genotype) to colonize a special host, but not others, should be of great interest for future research.
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