Genetic Relationships between the MutationsspadeandSternopleuraland thewinglessGene inDrosophilaDevelopment

Abstract

InDrosophila melanogaster,there are cases in which gene products contributing to the same developmental event may derive from closely adjacent transcription units and may even sharecis-regulatory sequences. Correct recognition of such genomic organization is central to an understanding of developmental mechanisms. The adult phenotypes of combinations between the mutationsspade, Sternopleural,andwinglesssuggest that they are lesions in functionally related genes within the same chromosomal region.winglessmutations fail to complement the recessive mutationspade.Thespademutation, as previously shown, behaves as a lesion in a regulatory site ofwingless,sited 5′ to the transcription unit, and is concerned with particular postembryonic functions ofwingless.While showingwingless-like phenotypes in combination withSternopleural,even lethal alleles ofwinglesscomplement the recessive lethality ofSternopleuralalleles. Mutations inSternopleuralincrease the severity ofwinglessphenotypes in manywingless-dependent processes during postembryonic development, and this interaction can occur when the only functional copies ofSporwgare located in either opposing chromosomes or the same chromosome. This is inconsistent with previous attempts to defineSpas a regulatory allele ofwgand explain the phenotypes that result from combinations ofSpandwgby means of transvection. We have analyzed a new EMS-induced allele ofSternopleuralthat is more severe than the original allele, which also argues forSpbeing a separate, mutable genetic locus rather than a regulatory allele ofwg.Finally, we have a revertant ofSternopleural(SpRv1) that behaves as a genetic null allele ofwg,but causes ventral-to-dorsal transformations in combination withwgP,which is not observed in combinations ofwgnull alleles withwgP.BecausewgPis the result of an inversion and because inversions inhibit transvection, the increased severity observed inSpRv1/wgPin comparison towgnull/SpRv1animals cannot be explained by an absence of transvection. Therefore, the twoSternopleuralmutations most reasonably define an independent gene located 3′ to thewinglessgene and having strong functional synergism with it.