Genetic Dissection of Behavior: Modulation of Locomotion by Light in theDrosophila melanogasterLarva Requires Genetically Distinct Visual System Functions

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Abstract
TheDrosophilalarva modulates its pattern of locomotion when exposed to light. Modulation of locomotion can be measured as a reduction in the distance traveled and by a sharp change of direction when the light is turned on. When the light is turned off this change of direction, albeit significantly smaller than when the light is turned on, is still significantly larger than in the absence of light transition. Mutations that disrupt adult phototransduction disrupt a subset of these responses. In larvae carrying these mutations the magnitude of change of direction when the light is turned on is reduced to levels indistinguishable from that recorded when the light is turned off, but it is still significantly higher than in the absence of any light transition. Similar results were obtained when these responses were measured in strains where the larval photoreceptor neurons were ablated by mutations in theglass(gl) gene or by the targeted expression of the cell death genehead involution defective(hid). A mutation in the homeobox genesine oculis(so) that ablates the larval visual system, or the targeted expression of thereaper(rpr) cell death gene, abolishes all responses to light detected as a change of direction. We propose the existence of an extraocular light perception that does not use the same phototransduction cascade as the adult photoreceptors. Our results indicate that this novel visual function depends on the blue-absorbing rhodopsin Rh1 and is specified by thesogene.