Distribution of Heterochromatin in the Chromosomes of Drosophila pallidipennis

The chromosomes of Drosophila pallidipennis are exceptional in several respects. One of its six chromosome pairs is longer at metaphase than all other chromosomes combined. The large chromosomes are X- and Y-chromosomes, they consist chiefly of heterochromatin, and at mitotic prophases they condense long before the euchromatic autosomes and the euchromatic part of the X become visible. In the salivary gland cells, the autosomes (except the small dot-like pair) show no obvious heterochromatin at the centromeres, but some interstitial heterochromatic sections are present. Several fairly large interstitial heterochromatic sections occur also within the predominantly euchromatic part of the X-chromosome. The boundary between the hetero- and euchromatic sections of the X is variable: many discs that appear euchromatic in some cells are heterochromatic in other cells.