Localization of NORs in the nucleolus-DNA body complex of the oocyte from the nemertine Amphiporus lactifloreus.

Abstract

The nucleus of the premeiotic oocyte from the hoplonemertine Amphiporus lactifloreus contains a characteristic DNA-rich body that rapidly generates a peculiar nucleolar apparatus previously described as the nucleolus-DNA body complex. During early vitellogenesis, the nucleolar complex is centrally located in the nucleus and consists of many spherical and ribbon-shaped components-the nucleolar spherulae and lamellae-which produce hundreds of small peripheral nucleoli and several granular bodies as vitellogenesis progresses. Distributions of the nucleolar organizer regions (NORs) in various nucleolar components were studied by silver staining, and results were compared with previous findings obtained by cytochemical and autoradiographic techniques.The core of each spherula in the nucleolus-DNA body complex is strongly Ag-positive, and so, later on, is a minor part of each peripheral nucleolus. In contrast, the cortices of the nucleolar spherulae, in which slightly decondensed chromatin is distributed uniformly, and the nucleolar lamellae produced through self-peeling of the spherulae, do not react with silver. Thus, the NOR proteins identified by silver deposits correspond to numerous loci at which the most decondensed, transcriptionally active chromatin is made visible by the osmium-ammine reaction. These results are discussed in relation to the functional evolution of the initial DNA body of the oocyte from A. lactifloreus, the transcriptional abilities of its by-products, and the general process of the amplification of ribosomal genes.