Spatial arrangement of homologous chromosomes during anaphase in early embryos of Drosophila melanogaster studied by three-dimensional fluorescence microscopy

Using a computer-controlled fluorescence microscope system, spatial arrangement of homologous chromosomes was analyzed in fixed and living embryos of Drosophila melanogaster at the syncytial blastoderm stage. In fixed embryos, chromosomes were stained with a DNA-specific fluorescent dye, 4′,6-diamidino-2-phenylindole; the arrangement of anaphase chromosomes was determined with their identification by high-resolution analysis. In living embryos, the behavior of chromosomes during anaphase was examined by the use of a strain carrying a long translocated chromosome as a cytological marker to identify the chromosome while the chromosomes were stained by microinjection of rhodamine-conjugated histones into the embryos. Chromosome arrangement was also examined in nuclei that had swollen artificially under anoxic conditions for better spatial separation of individual chromosomes in such nuclei. All these experiments consistently showed that homologous chromosomes were not associated with each other in syncytial blastoderm embryos of Drosophila melanogaster. Our studies also demonstrated that cytological tools greatly facilitate the dissection of nuclear structures when used in combination with imaging technology.