Heterochromatin in mitotic chromosomes of Drosophila nasuta.

The heterochromatin in mitotic cells of larval neural ganglia of Drosophila nasuta has been analysed by C-banding and by fluorescence studies. All chromosomes, except the 'dot'-like 4th chromosome pair, carry large blocks of heterochromatin which are darkly stained by C-banding, and which fluoresce uniformly brightly with Hoechst 33258, quinacrine mustard, and acridine orange. These heterochromatic segments make up about 40% of the total metaphase chromosome length. The heterochromatic segments also fluoresce brightly with all the dyes at the anaphase stage. In interphase nuclei, all the heterochromatic segments form a common, compact and homogeneous mass which fluoresces brightly with the three fluorochromes used. The size of the bright chromocentre is similar with all the dyes. It is suggested that the heterochromatic segments of different chromosomes are relatively homogeneous in their DNA base sequences, which are likely to be A-T rich in view of their bright fluorescence with Hoechst 33258 as well as quinacrine mustard.