Distribution of Heterochromatin in the Chromosomes of Drosophila pallidipennis

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Abstract
D. pallidipennis is known from a single locality in Brazil. Its mitotic metaphases show one pair of dot, 4 pairs of rod, and 1 pair of large V-shaped chromosomes, the last being the X- and Y-chromosomes. The heterochromatic sections, which constitute the bulk of the sex chromosomes, appear precociously during prophase. Later, constrictions may be noted between euchromatic and heterochromatic regions and in the latter themselves. In late prophase, the euchromatic regions of the sex chromosomes lose their chromomeric structure before the euchromatic chromosomes. In salivary gland nuclei, attachment of all auto-somes to the large chromocenter is rare. In some cells the bases of all 4 autosomes may be united with each other and with a small mass which seems to be heterochromatin. The several heterochromatic regions of the basal portion of the salivary X-chromosome may be attracted to each other or to the chromocenter. The boundary between heterochromatic and euchromatic regions in the basal portions of the X-chromosome may vary strikingly, the euchromatic regions becoming heterochromatic in appearance. The dot chromosomes are not apparent in the salivary nuclei and may be heterochromatic and included in the chromocenter. Of the 5 euchromatic strands, that of the X-chromosome is next to the shortest. D. pallidipennis is unusual among Drosophila because of the large amt. of heterochromatin and its concentration on the X- and Y-chromosomes. Several mechanisms to account for this conc. are discussed in regard to the possibility of evolution proceeding against natural selection.