A Model for de novo Nucleolar Assembly during Drosophila melanogaster Embryogenesis and the Roles of pre-rRNA and RNA Polymerase I

Abstract

The nucleolus was first discovered over 200 years ago, in 1781. Early research determined its critical role in ribosome biogenesis, and its dynamic nature throughout the cell cycle. However, the exact timing and mechanisms surrounding the initial construction of the nucleolus are still largely unknown. Though the nucleolus is often called “an organelle formed by the act of building a ribosome,” recent studies have shown that, in some species, the nucleolus can be constructed before rRNA synthesis and ribosome production have begun. This study therefore investigates de novo nucleolar formation during Drosophila melanogaster embryogenesis, focusing on the temporal relationship between the initiation of rDNA transcription by RNA Polymerase I (Pol I), the presence of ribosomal RNA precursors (pre-rRNA), and nucleologenesis. In situ hybridization, immunofluorescence, and transgenic fluorescent proteins were utilized in order to illustrate a time course of nucleologenesis. Mutants lacking rDNA were used to explore which factors are necessary and sufficient for nucleolar formation. The results suggest a model of Drosophila nucleologenesis in which pre-rRNA is not maternally donated, is first produced zygotically at nuclear cycle 11, re-localizes to nucleolar organizing regions (NORs) after each mitosis, and is responsible for inducing de novo nucleologenesis at cycle 13. Both X and Y chromosome NORs are active at cycle 13, as they contain Pol I and the pre-rRNA early-processing protein, Fibrillarin. These findings propose a model of de novo nucleologenesis during Drosophila melanogaster embryogenesis that is unique from other species, and suggests that further research should be done to clarify the specifics of nucleolar formation in different organisms.