(16e) The Role of rDNA in the Nucleolus Formation: A Seeding Process?

The nucleolus is a membrane-less organelle, which plays a pivotal role in different cellular functions including ribosomal biogenesis. During embryogenesis, this organelle forms de novo through a process that requires the activation of rDNA transcription and the recruitment of nucleolar components to the nucleolus organizer regions. However, the interdependency of these two events is not clear. We developed quantitative approaches that allow us to study this question in wild-type and mutant Drosophila melanogaster embryos. Our results show that the assembly of the nucleolar proteins is not necessary for the transcription of rDNA. Moreover, in the absence of rDNA, the nucleolar proteins studied are able to form high concentration assemblies, but as a stochastic event. This shows that rDNA controls the timing and position of these assemblies probably through a nucleation mechanism.