Nucleolar Scaling in Adult C. elegans: A Concentration Dependent Phase Transition

Abstract

Cells actively organize their contents to achieve normal cell functions. They contain many membrane bound structures that allow for the intricate control of the distribution of biochemical molecules within the cells. Recently, it has been found that non-membrane bound organelles also play integral roles in cellular organization. These bodies represent regions of high concentration of certain biomolecules that dynamically exchange particles with their surroundings. The nucleolus is a non-membrane bound organelle that plays integral roles in cell growth and development through its functions in rRNA transcription and ribosome assembly. The nucleolus has recently been found to exhibit liquid-like properties, and its assembly has been shown to be governed by a concentration dependent phase transition in C.elegans embryos. This study extends those findings into mature adult C. elegans. We apply positive pressure to worms to decrease cellular and nuclear volume through a custom microfluidic platform and study the consequences for nucleolar assembly. We find that nucleolar volume scales directly with nuclear and cell volumes under applied pressure. We find that nucleolar concentration, total nuclear concentration, and saturation concentration are maintained constant through the export of nucleolar components from nuclei to the cytoplasm. We apply a concentration dependent phase transition model to calculate nucleolar size and find that it accurately predicts nucleolar volume under applied pressure. Therefore, nucleolar scaling behavior is consistent with a fixed concentration regime of the phase transition model.