Evolutionary conservation of a common pattern of activity of nucleolus organizers during spermatogenesis in vertebrates.

The patterns of activity of the nucleolus organizer regions (NORs) in the spermatogeneses of ten species of all non-mammalian classes of vertebrates and one species of the cephalochordates were investigated with the silver (Ag)-staining technique. The Ag-stainability of the NORs is a measure of the transcriptional activity of the ribosomal RNA genes. In all species, there is a very similar pattern of NOR-activity in the various stages of spermatogenesis. The qualitative analysis of the Ag-stainability of the NORs was in very good agreement with the results obtained for mammals: Ag-stained NORs are detectable during the entire meiotic prophase up to the pachytene stage, completely absent in the meiotic metaphases I and II, and again demonstrable in early spermatid nuclei. The results confirm the occurrence of postmeiotic reactivation of the RNA genes. The preferential inhibition of rRNA synthesis by low doses of actinomycin D induced a rapid decline of the Ag-stainability of the postmeiotically reactivated NORs. The significance of the evolutionary conservation of the postmeiotic NOR-reactivation is discussed.