Mutations in the proximal region of the optomotor-blind locus of Drosophila melanogaster reveal a gradient of neuroanatomical and behavioral phenotypes.

Mutations in the complex optomotor-blind (omb) gene locus (4C4-6) lead to a number of different phenotypes in various tissues of the adult Drosophila melanogaster fly. At the core of the locus lies a lethal complementation group, named l(1)omb, whose mutations cause larval and pupal lethality. Some 40% of all males hemizygous for lethal omb alleles develop to the pharate adult stage. These flies can be rescued from the pupal case and show a severe disturbance in optic lobe development. The recessive viable allele In(1)ombH31 reduces the optomotor response in walking flies and during stationary flight of tethered flies. At the neuroanatomical level, these animals lack a subset of lobula plate giant neurons (LPGNs), which are thought to mediate optomotor behavior. Chromosomal aberrations deleting the proximal, non transcribed part of the locus complement the lethality, but still cause neuroanatomical and optomotor defects. Analysis of different allelic combinations of such mutations, in which increasing amounts of DNA downstream of the transcribed region are removed, reveals a step gradient of increasing severity of the neuroanatomical defects and behavioral phenotypes. On this basis the 3'-regulatory region is divided into three domains each having specific effects on optic lobe development.