Figure 1.

Immunofluorescent staining of the polytene chromosomes. Polytene chromosomes from (a) D. melanogaster and (b) D. virilis are shown. Top left, phase contrast; others as labeled. Panels on the right provide a close-up of the chromocenter and the dot chromosome. In the merge picture, yellow represents equal staining, red represents more H3K9me staining, and green represents more HP1 staining. The dot chromosome is indicated with an arrow. In D. melanogaster, antibodies for HP1 and H3K9me stain both the chromocenter and the dot chromosome, although the HP1 staining is slightly stronger than the H3K9me staining on the dot. In D. virilis, both antibodies stain the chromocenter but neither stains the dot chromosome.