Heterochromatin of the Drosophila melanogaster Y chromosome as modifier of position effect variegation: the time of its action.

Summary Addition of heterochromatin suppresses while subtraction enhances position effect variegation. The heterochromatin-sensitive period has been determined in white/white-apricot variegated eyes of Y^Sw^a/w^a; Dp (1;3) w^265-58 flies. When such larvae, carrying a Y-short (Y^S) arm at the distal end of one X chromosome, are X-rayed, mitotic recombination leads to one daughter cell with two Y^S arms and an adjacent daughter cell with no Y^S arm. When induced after clonal initiation, the frequency of dark clones developing from daughter cells with two Y^S arms is significantly higher than the frequency of dark clones in the rest of the eye; and this frequency is. even higher when induced before clonal initiation. The modifying action of the Y-heterochromatin is exerted, therefore, during and after clonal initiation. Surprisingly, the frequency of dark clones developing from cells with no Y^S arm is not lower than the frequency of dark clones in the rest of the eye.