Morphology of the Chromosomes of Drosophila ananassae

Abstract

1) The chromosome complement of D. ananassae, studied in the neurocytes, consists of three pairs of V-shaped autosom.es and the sex chromosomes. These are a pair of V-shaped X-chromosomes in the female, an X and a J-shaped Y-chromosome in the male. The chromosomes may be distinguished by relative, sizes and characteristic constrictions.
2) The fourth chromosomes and the Y-chromosome appear totally heteropyknotic in resting and early prophase stages of mitotic cells. Short heteropyknotic regions lie adjacent to the spindle attachment regions of the X-chromosomes and the four longer autosomes.
3) In salivary gland nuclei there are but six chromosome arms with euchromatic sections. Four of these represent the paired arms of the longer autosomes, the other two the X-chromosome arms. The fourth chromosomes are represented in salivary gland nuclei by a small bipartite mass of heterochromatin which forms part of the chromocenter.
4) In mitotic prophases of female larvae the fourth chromosomes are associated with the nucleolus, which separates a small satellite-like portion from the remainder of the long arm of these chromosomes. In the male the Y-chromosome forms a third member of the group associated with the nucleolus.
5) The fourth chromosome-nucleolus-satellite relationship is. also evident, in salivary gland nuclei, the satellite appearing as a banded body associated with the nucleolus, and also connected with the fourth chromosomes by chromatic strands.
6) The chromocenter of mitotic cells and of the salivary gland cells is not amorphous, but discloses, under suitable conditions, the limits of the component chromosomes.