EVIDENCE FOR REGULATORY VARIANTS OF THE DOPA DECARBOXYLASE AND ALPHA-METHYLDOPA HYPERSENSITIVE LOCI IN DROSOPHILA

J. Lawrence Marsh ¹ and T. R. F. Wright ²

¹ Developmental Biology Center and Department of Developmental and Cell Biology, University of California, Irvine, California 92717
² Department of Biology, University of Virginia, Charlottesville, Virginia 22901

We have analyzed two variants of Drosophila melanogaster (R^S and R^E) which lead to the dual phenotype of elevated DDC activity and increased resistance to dietary alpha-methyldopa relative to Oregon-R controls. Both phenotypes show tight genetic linkage to the dopa decarboxylase, Ddc, and l(2)amd genes (i.e., < 0.05 cM distant). We find that low (Oregon-R), medium (R^S) and high (R^E and Canton-S) levels of DDC activity seen at both pupariation and eclosion in these strains are completely accounted for by differences in accumulation of DDC protein as measured by immunoprecipitation. Genetic reconstruction experiments in which Ddc ⁺ and amd⁺ gene doses are varied show that increasing DDC activity does not lead to a measurable increase in resistance to dietary alphamethyldopa. This suggests that the increased resistance to dietary alpha-methyldopa is not the result of increased DDC activity but, rather, results from increased l(2)amd⁺ activity. Both cytogenetic and molecular analyses indicate that these overproduction variants are not the result of small duplications of the Ddc and amd genes, nor are they associated with small (100 bp) insertions or deletions. Measurements of DDC activity in wild-type strains of Drosophila reveal a unimodal distribution of activity levels with the Canton-S and R^E strains at the high end of the scale, the Oregon-R control at the low end and R^S near the modal value. We conclude that accumulated changes in a genetic element (or elements) in close proximity to the Ddc⁺ and amd⁺ genes lead to the coordinated changes in the expression of the Ddc and amd genes in these strains.