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Characterization of a cluster of dominant suppressors of position effect variegation including effects on heterochromatic variegating rearrangements in Drosophila melanogaster Hedrick, Amy L.


The mosiac, cell-autonomous expression of genes resulting from chromosomal rearrangement and relocation next to broken heterochromatin is termed position effect variegation (PEV). Since the gene is inactivated due to chromatin changes, this system allows the genetic study of chromatin structure and function using mutations which rescue the mosaic phenotype. These mutations called suppressors of variegation, Su(var)s, must influence chromatin structure. The genetic characterization of several groups of Su(var)s has been undertaken in this study using Drosophila melanogaster. Variegation of the light gene, located in heterochromatin, is enhanced by several Su(var) mutations on chromosome two. This opposite effect suggests that products of these Su(var)s are essential for functioning heterochromatin and deleterious for euchromatic environments. Other Su(var)s have slight or no effects on the same variegating rearrangements, demonstrating functional differences, among the Su(var)s tested. A group of Su(var)s located within 4 map units near the centromere of chromosome three was characterized using deficiency mapping, new compound autosome formation and inter se complementation based on newly established homozygous phenotypes. Two Su(var)s mapped to 87B on 3R, while one Su(var) maps to 3L according to compound mapping. Inter se complementation, in combination with mapping data, suggests that four seperate loci make up this group of Su(var)s. Eight of nine Su(var)s are extremely sensitive to heterochromatic deletions as shown by their responses to loss of 2R heterochromatin, as well as the Y chromosome. In contrast, Su(var)A130 is insensitive to both forms of heterochromatic deficiencies. Su(var)s show complicated reactions to maternal verses paternal source effects. Six of nine Su(var)s show a female-specific temperature sensitive maternal effect. Some maternal and paternal effects are observed at 22 C. Su(var)A57 is maternal semi-lethal and suppressed at 29 C. This characterization has better defined these mutants, making them ammenable to molecular study.

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