The classes of endosymbiont of Paramecium aurelia.

Abstract

ABSTRACT The endosymbionts of Paramecium aurelia appear to consist of a number of different Gram-negative bacteria which have come to live within many strains of paramecia. It is not known whether in nature this relationship is mutually beneficial or not. The symbionts from one paramecium may kill other paramecia lacking that kind of symbiont. We identify the following classes of endosymbiotic organisms. First, kappa particles (found in P. aurelia, syngens 2 and 4) ordinarily contain highly characteristic refractile, or R, bodies, which are associated with the production of a toxin which kills sensitive paramecia. In certain mutants of kappa found in the laboratory both R bodies and ability to kill have been lost. Second, mu particles (in syngens 1, 2 and 8) produce the phenomenon of mate-killing. Third, lambda (syngens 4 and 8) and sigma particles (syngen 2) are very large, flagellated organisms which kill only paramecia of syngens 3, 5 and 9, and are enclosed in membrane-bound vacuoles. Fourth, gamma particles (syngen 8) are minute endosymbionts, surrounded by an additional membrane resembling endoplasmic reticulum. They have strong killing activity but no R bodies. Fifth, delta particles (syngens 1 and 6) possess a dense layer covering the outer membrane. At least one of the two known stocks is a killer. Sixth, nu particles are a heterogeneous group of particles (syngens 2 and 5) which do not kill or possess distinctive morphological characteristics. Seventh, alpha particles (syngen 2) are the only known nuclear symbionts of P. aurelia; they are found in the macronucleus. Alpha is also exceptional in being the only particle which is highly infectious, though certain of the other symbionts can also be taken up by paramecia lacking them, under special conditions.