SUPPRESSORS OF POSITION-EFFECT VARIEGATION IN DROSOPHILA-MELANOGASTERAFFECT EXPRESSION OF THE HETEROCHROMATIC GENE LIGHT IN THE ABSENCE OFA CHROMOSOME REARRANGEMENT

Suppressors of position-effect variegation (Su(var)s) in Drosaphila me lanogaster are usually studied in the presence of chromosomal rearrang ements, which exhibit variegated expression of euchromatic genes moved near to, or heterochromatic genes moved away from, centromeric hetero chromatin. However, the effects of Su(var) mutations on heterochromati c gene expression in the absence of a variegating re-arrangement have not yet been defined. Here we present a number of results which sugges t that Su(var) gene products can interact to affect the expression of the light gene in its normal heterochromatic location. We initially ob served that eye pigment was reduced in several Su(var) double mutants; the phenotype resembled that of light mutations and was more severe w hen only one copy of the light gene was present. This reduced pigmenta tion could be alleviated by a duplication for the light gene or by a r eduction in the amount of cellular heterochromatin. In addition, the v iability of most Su(var) double mutant combinations tested was greatly reduced in a genetic background of reduced light gene dosage, when ex tra heterochromatin is present. We conclude that Su(var) gene products can affect expression of the heterochromatic light gene in the absenc e of any chromosamal rearrangements. However, it is noteworthy that mu tations in any single Su(var) gene have little effect on light express ion; we observe instead that different pairings of Su(var) mutations a re required to show an effect on light expression. Interestingly, we h ave obtained evidence that at least two of the second chromosome Su(va r) mutations are gain-of-function lesions, which also suggests that th ere may be different modes of interaction among these genes. It may th erefore be possible to use this more sensitive assay of Su(var) effect s on heterochromatic genes to infer functional relationships among the products of the 50 or more known Su(var) loci.