Ultrastructure of the macronucleus in the resting cysts of the ciliate Bursaria truncatella

Abstract

Summary The ultrastructure of the macronucleus and the micronucleus in the resting cysts of the ciliate Bursaria truncatella was studied by electron microscopy. The data obtained were compared with those for B. truncatella mature vegetative cells. Macronuclear chromatin in the resting cysts was shown to be organized in compact electron-dense bodies twice to three times as large as those in the vegetative B. truncatella cells completed their growth and differentiation. In the cyst macronucleus we observed clearly separated zones with chromatin bodies of different morphology. Some of these zones contained large, closely associated chromatin bodies, while smaller, more sparsely distributed ones were observed in others. During encystment the chromatin bodies increase in size at first. Then they fuse into rather extended linear or circular chromatin strands of 150 to 300 nm in diameter, resembling chromonemata that are typical for chromosomes of higher eukaryotes. Nucleoli in the early cysts underwent segregation observed mainly in the cortical regions of the macronucleus. The distribution of RNA-containing structures in cysts and the extrusion of nucleolar material into the cytoplasm were studied using RNase A-colloidal gold complexes. The extrusion of nucleolar material appeared to be coupled with the segregation of the nucleoli. The development of chromatin bodies and nucleoli might reflect the functional state of the macronucleus of encysted B. truncatella , indicating that some low transcriptional activity may be left in the cyst macronucleus.