Differentiation of nuclear structure during the sexual cycle in Tetrahymena thermophila: I. Development and transcriptional activity of macronuclear anlagen

Abstract

During the postzygotic period of the sexual cycle (conjugation) in the ciliated protozoan Tetrahymena, transcriptionally active somatic macronuclei develop from descendants of the germinal micronuclei, which are mostly transcriptionally inactive. This nuclear differentiation process is accompanied by the amplification of ribosomal genes, genome polyploidization, and reorganization. In this context, the developmental sequence of macronuclear anlagen represents an interesting system for the study of finestructural aspects of nuclear differentiation in combination with an analysis of the onset and location of transcriptional activity. From our data, the differentiation of the fine structure of macronuclear anlagen can be divided into three distinct stages. During the first stage, the anlagen appear very electron lucent and exhibit a loose, fine fibrillar substructure (achromatic stage). 3H-Uridine incorporation was low, and the autoradiographic label was evenly distributed over the nuclear areas. During the second stage, specific nuclear regions showing arrays of small, condensed flocculent patches could be seen. In such areas, primary nucleolar structures are obviously formed; these structures initially only consist of fibrillar components and show very little or no transcriptional activity. During the last stage of differentiation, the macronuclear anlagen attain a fine structure similar to that exhibited by macronuclei in starved vegetative cells. The fibrillogranular substructure becomes more dense, and typical heterochromatic chromatin bodies appear. The multiple extrachromosomal nucleoli are exclusively located at the periphery of the nuclei and organized as either single units or nucleolar aggregates containing a cortex of granular components. The autoradiographic label was dense and clearly concentrated over the nucleoli. The combined fine-structural and autoradiographic data indicates that, during the first stage of macronuclear differentiation, only nonribosomal RNA transcription occurs and that there is a lag phase between the first appearance of nucleolar structures and the expression of the amplified ribosomal genes.