FINE-STRUCTURE OF THE MACRONUCLEUS IN THE RESTING CYSTS OF THE CILIATE TILLINA-MAGNA

In the ciliate Tillina magna, small fragments separate from the macronucleus during formation of resting cysts, before separation of the previously known large extrusion bodies. These fragments contain loose fibrogranular material but neither nucleolar fibrillar cores nor chromatin bodies. Extrusion of this kind is supposed to be related to macronuclear reorganization during encystment. In early resting cysts of young clones, the macronucleus contains many nucleolar granules, which indicates nuclear activity related to cyst formation. Later they become less numerous. In resting cysts of an old clone, maintained in the laboratory for many years, segregation of fibrillar and granular nucleolar components occurs. Subsequently, the latter disappears, which indicates the cessation of both the synthesis and processing of rRNA. Large aggregates of microtubules are also formed in the macronucleus of this clone. Soon after the experiment, the old clone has been lost due to inability to form viable resting cysts.