The physical state of the nucleus tracks its function across eukaryotes

The nucleus is consistently less dense than the surrounding cytoplasm, a difference proposed to arise from selective nuclear import. The functional significance of low nuclear density, however, remains unknown. Because density is a bulk property of the cell, acute perturbations inevitably have pleiotropic consequences. We therefore turned to the natural experiments offered by the tree of life where nuclei vary in genome size, chromatin organisation, import selectivity and transcriptional state, and asked whether low nuclear density is tied to particular nuclear architectures or functions. Using dinoflagellates, Giardia and ciliates, we show nuclear density is uncoupled from genome size. In ciliates, two nuclei in one cytoplasm reach opposite densities relative to it, governed by nuclear import rather than transcription. We thus confirm nuclear import as a major determinant of nuclear density and reveal nuclei that break the low-density rule.