Evidence for a Viral Macronuclear Endosymbiont in Paramecium caudatum

ABSTRACT. Some strains of P. caudatum contain macronuclear inclusion bodies that are morphologically distinct from bacteria. They vary in number as well as in size in each macronucleus. The inclusion bodies are basically divided into peripheral and inner areas. The peripheral area consists of fibrillar proteins of 22–24 nm in thickness, which are specifically stained with fast green in 45% acetic acid. On the other hand, chromatin-like granules are within the inner area of large inclusion bodies. The granules within the inner area changed their distribution depending upon the physiological state of their host cells. Transplantation experiments and crossbreeding analyses revealed that genetic factors responsible for the multiplication of the inclusion bodies can 'infect' other macronuclei (or cells) via the cytoplasm. These results suggest that the inclusion bodies are a non-bacterial macronuclear endosymbiont, possibly produced by a virus or a virus-like element.