An RNA polymerase II mutation in Drosophila melanogaster that mimics ultrabithorax

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Abstract
An EMS-induced, sex-linked recessive lethal mutation that in heterozygous condition mimics the third-chromosome dominant mutant Ultrabithorax-130 (Ubx 130) has been discovered in Drosophila melanogaster. This Ultrabithorax-like (Ubl) mutant, when heterozygous, adds several hairs to and enlarges the apical segment (capitellum) of the haltere. Ubl fails to complement Ubl n [previously called l(1)L5], a recessive lethal null allele located in section 10C of Bridges' (1938) map of the polytene X chromosome at map position 35.7. Ubl behaves as an antimorph: heterozygous deficiencies for section 10C do not display the Ubl dominant phenotype. Ubl shows a dosage effect: the maximum expression occurs in females with the genotype Ubl/Ubl; Dp Ubl +, in which the capitellum is about three times as large as that of Ubl/ +, with two or more rows of bristles. These flies are poorly viable and sterile when mated to Ubl; Dp Ubl + males, but produce a few offspring when mated to Ubl + males. Ubl displays a complex series of interactions with loci other than Ubx and elicits expression of specific mutant phenotypes when it is heterozygous in trans with certain nonallelic deficiencies and recessive mutations. Greenleaf et al. (1980) have demonstrated that Ubl is allelic with an α-amanitin-resistant mutation that affects RNA polymerase II; therefore, the interactions observed between Ubl and other loci may result from an inability of heterozygous Ubl flies to undergo normal transcription.