The classes of kappa in Paramecium aurelia.

Kappas (bacterial symbionts containing R bodies) have been studied in 16 strains of Paramecium aurelia, syngens 2 and 4. All produce toxins capable of killing sensitive paramecia. The first major class, the 51 group (consisting of the kappas of strains 51, 116, and 298), has R bodies which, when the pH is lowered below 6-5, unwind reversibly from the inside to form a tight tube; the outside end of the R-body ribbon forms an acute angle; no sheath surrounds the R body. The phage-like structures of the 51 group are helical; no capsomere-like structures are present; isolated R bodies do not have any killing activity. The second major class, the 7 group (strains 7, 576, Bl 166-1, 249, 1041, 310, 1039), has R bodies which unwind irreversibly from the outside to form a loose twisted ribbon whose outside tip is blunt, irregular or finger-like; a single membraneous sheath covers the R body. Phage-like structures in the 7 group are spherical; capsomere-like structures are present; isolated R bodies show killing activity. The third major class, the 562 group (strains 562, 517, 511), is similar to the 7 group except that there is no sheath, no capsomeres, and free R bodies are almost or completely inactive. The phage-like structures are spherical, and unlike those of the 7 group, do not stick to the R bodies. In addition to these 3 major classes there are 3 strains which show important differences from the major classes and also differ among themselves. 1038 is like 7 in all respects except that the phage-like structures are helical and isolated R bodies are not very active. 51ml, presumably a mutant of 51, has spherical phage-like structures and 7 type-R bodies which are inactive when isolated. 570 is the only known mate-killer with R bodies; it has spherical phage-like structures. It is noted that strain 1039 °f th 7 gP has very few phage-like structures, virtually all of which are empty. Since free 1039 R bodies are highly active, it is likely that intact phage-like structures are not essential for the toxic action on sensitives. The spherical and helical phage-like structures are probably very closely related because 51, which has helical structures, apparently gave rise to 51ml, which has spherical structures. Likewise the kappa symbionts of 1038 with helical phage-like structures are virtually identical to the kappas of the 7 group with spherical phage-like structures. The presence of phage-like structures in all strains with R bodies suggests that R bodies may be a product of phage activity. Strain 570, a mate-killer whose symbionts contain R bodies, provides a link between kappas and mus, the mate-killer symbionts. The symbionts of 570 are the only ones containing R bodies which are non-toxic when encountered by sensitives in the medium in which the paramecia are cultured.