Extrachromosomal elements of extrachromosomal elements of Paramecium and their extrachromosomal elements.

Expression of killer traits in Paramecium is due to a complex interaction between the lower eukaryote host and two or three elements that can be viewed either as extrachromosomal elements or as endosymbionts. In all cases, the determinants of the killer trait are carried by obligate bacterial endosymbionts belonging to the genus Caedibacter. However, the actual genetic determinants for expression of these traits are not an integral part of the symbiont genome. They are located on extrachromosomal genetic elements (plasmids or bacteriophages) which essentially are molecular endosymbionts of Caedibacter. In the case of the plasmids, they are associated with yet another set of extrachromosomal genetic elements, which are transposons. These transposons have been observed to move into new sites in the plasmids and even to disrupt expression of R body production and the killer trait. Thus, the transposons can be considered either as extrachromosomal elements of extrachromosomal elements (plasmids) of extrachromosomal elements (C. taeniospiralis) of paramecia, or as molecular parasites of molecular endosymbionts (plasmids) of bacterial endosymbionts of paramecia.