Multiple regimes of constrained chromosome motion are regulated in the interphase Drosophila nucleus

Background: Increasing evidence indicates specific changes in the three-dimensional organization of chromosomes in the cell nucleus during the cell cycle and development. These changes may be linked to changes in both the coordinated regulation of gene transcription and the timing of chromosome replication. While there is cytological evidence for short-range diffusive motion of chromosomes during interphase, the mechanisms for large-scale chromosome remodeling inside the nucleus remain unknown. Results: Chromosome motion was tracked in Drosophila spermatocyte nuclei by 3D fluorescence microscopy. The Lac repressor/lac operator system was used to label specific chromosomal sites in live tissues, allowing extended observation of chromatin motion in different cell cycle stages. Our results reveal a highly dynamic chromosome organization governed by two types of motion: a fast, short-range component over a 1-2 s time scale and a slower component related to long-range chromosome motion within the nucleus. The motion patterns are consistent with a random walk. In early G2, short-range motion occurs within a small, approximately 0.5 μm radius domain, while long-range motion is confined to a much larger, chromosome-sized domain. Progression through G2 as cells approach meiotic prophase is accompanied by a complete arrest of long-range chromosome motion. Conclusions: Our analysis provides direct evidence for cell cycle-regulated changes in interphase chromatin motion. These changes are consistent with changes in local and long-range constraints on chromosome motility. We propose that dynamic interactions between chromosomes and internal nuclear structures modulate the range and rate of interphase chromatin diffusion and thereby regulate large-scale nuclear chromosome organization.