The Symbiont Theory in Explanation of the Apparent Cytoplasmic Inheritance in Paramecium

Abstract

The number of bodies responsible for the killer effect in P. aurelia (about 250), as recently discovered by Preer, corresponds roughly to the number of green sym-bionts (Zoochlorellae) in P. bursaria, and on the basis of this correspondence it is suggested that the bodies in question (the kappa factor of Sonneborn) are colorless symbionts related to the green symbionts in P. bursaria. It is further suggested that mutations in the symbionts might sometimes have effects, as on mating type, similar to those of nuclear gene mutations in Paramecium, for which effects the term "cyto-copies" is proposed, and which would be analogous to phenocopies, or the imitations of gene mutations produced by the environment. An alternative possibility (suggested earlier by the writer) is that [kappa] is a viroid (a useful symbiont related to a virus), and this is still a possibility, but in view of the numerical correspondence above referred to, the present suggestion seems more likely. The 2 suggestions have an important point in common, namely, that the [kappa] factor is a symbiont rather than a plasmagene, and this is indicated by the relative independence of Paramecium and [kappa], as shown both by their independent division rate, and by the ability of the one (Paramecium) to live without the other (k) .