Distance and pairing effects on the brownDominant heterochromatic element in Drosophila.

We examined the behavior of the brownDominant (bwD) heterochromatic insertion moved to different locations relative to centric heterochromatin. Effects were measured as the degree of silencing of a wild-type brown eye pigment gene by bwD across a tandem duplication. A series of X-ray-induced effects were recovered at high frequency. Cis-acting enhancers were obtained by relocation of the duplication closer to autosomal heterochromatin. Enhancers were also recovered on the homologous chromosome when it was similarly rearranged, revealing a novel interhomologue effect whereby interactions occur between genetic elements near opposite ends of a chromosome arm rather than between paired alleles. Cis-acting suppressors were obtained as secondary rearrangements in which the duplication was moved farther away from heterochromatin. Suppression was correlated with loss of cytological association between bwD and the polytene chromocenter. Surprisingly, the distance from bwD to the chromocenter was not correlated with the strength of enhancement or suppression. We propose that bwD fails to coalesce with the chromocenter when its position along the chromosome places it beyond a threshold distance from heterochromatin, and this threshold depends upon the configuration of both the chromosome carrying bwD and its paired homologue.