The nucleolar cycle in the myxomycete Physarum polycephalum. II. Mode of formation of the nucleolus

Abstract

The previous ultracytochemical investigation has revealed that the numerous nucleolar remnants observed during the mitotic stages in Physarum polycephalum possess the same characteristics as chromatin and originate from the fibrillar zones of the mature nucleolus. In the present study these nucleolar minichromosomes are shown to be incorporated as well-defined structural elements within the two daughter nuclei. At very early interphase these minichromosomes are first scattered throughout the nuclear cavity, but within a few minutes they aggregate to form several highly irregular masses corresponding to prenucleolar bodies. By the time these latter bodies fuse into typical horseshoelike nucleoli, pervading granular material starts accumulating in more noticeable amounts. Following labeling for 10 min with uridine radioautographic grains are already detected in very early interphase nuclei over dense regions undoubtedly corresponding to the minichromosomes. Incorporation of this RNA precursor increases as the minichromosomes fuse into prenucleolar bodies. By the time large nucleoplasmic zones have formed, radio-autographic grains are present in still larger quantity over the irregularly shaped nucleolus as well as over diffuse chromatin thoughout the nuclear cavity. Such early RNA synthesis is interpreted to indicate that the early stages of nucleolar formation do not merely involve fusion of the preexisting minichromosomes but also imply synthesis of new material.