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A genetic analysis of Region 31 on Chromosome 2 of Drosophila melanogaster Brock, Jo-Ann Karen


Genes that are subject to position-effect variegation (PEV) are located at newly formed euchromatic/heterochromatic junctions of chromosomal rearrangements. Variegated expression of these genes is believed to result from structural changes in their normal chromatin conformation. Several genetic modifiers of PEV have been identified which can suppress variegated phenotypes. The wild-type products of these genes are thought to be involved in formation or maintenance of chromatin structure, either as structural components, or as assembly or modifying factors. Several dominant suppressors of variegation (Su(var)s) have been localized to the left arm of chromosome 2 (region 31) of Drosophila melanoqaster. A number of intergenic effects are noted when suppressor mutations of this region, mapping to separate loci, are combined in trans. These include male lethality, female sterility and at least two visible phenotypic anomalies: changes in eye colour (possibly resulting from the altered expression of the heterochromatic light gene) and abnormal wing morphology. These effects may occur through additive or synergistic interactions of mutant Su(var) products which alter the integrity of heterochromatin to such an extent that expression of heterochromatic genes is affected. This hypothesis is supported by the fact that altering the heterochromatic content of the cell affects the expression of these abnormal phenotypes. Deletion or addition of a Y chromosome (which presumably acts as a sink for normal Su(var) products) is able to, respectively, ameliorate or exacerbate the intergenic effects. Most of the suppressor genes investigated in this study are hemizygous lethal or female sterile. An additional forty-two EMS-induced mutants were generated in a screen to isolate mutations of essential genes (including suppressor loci) in region 31. These new mutants were characterized genetically. While none of them are dominant suppressors of position-effect variegation, several alleles of at least one Su(var) locus were recovered. This result supports the suggestion that a special event (antimorphic mutation) is necessary to elicit a suppressing phenotype from the Su(var) loci located in region 31.

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