Calcium Enables Photoreceptor Pigment Migration in a Mutant Fly

Upon bright illumination, pigment granules in the photoreceptors of the fly compound eye migrate transversely toward the rhabdomeres (Kirschfeld & Franceschini, 1969). The granules absorb light passing down the rhabdomeric light guide, thereby reducing the light available to the visual pigment. This light-regulating mechanism is found in a number of invertebrate eyes (Stavenga, 1979) and has been termed the 'longitudinal pupil' (Franceschini, 1972). Pigment migration is triggered by the absorption of light by the visual pigment (Franceschini, 1975) but is not directly caused by the accompanying depolarization of the photoreceptors (Kirschfeld & Vogt, 1980). Kirschfeld & Vogt showed that the extracellular application of the calcium-sequestering agent EGTA to the fly retina disables pigment migration. They suggested that an increase in intracellular free Ca concentration, which is known to accompany strong illumination of invertebrate photoreceptors (Brown & Blinks, 1974; Muijser, 1979), triggers pigment movement. Here I present evidence in support of this hypothesis by using Ca to promote pigment migration in a transduction mutant of the fly Lucilia which does not normally exhibit pigment migration: this is the converse to Kirschfeld & Vogt's experiment. Pigment migration was monitored in restrained, living flies by observing lightinduced changes in reflection from the deep pseudopupil (Franceschini & Kirschfeld, 1971). In wild-type Lucilia, the pupil reflects yellow light after adaptation with bright white light. The time constant of the reflection change from the dark-adapted to the light-adapted state is about 7 s at room temperature (Fig. 1A). The receptor potential in the wild-type fly has a characteristic initial transient followed by a steady-state plateau (Fig. IB). As reported by Kirschfeld & Vogt (1980), infiltration of the retina with EGTA (0-1M dissolved in 0-15M-NaCl and neutralized to pH7 with NaOH) blocks pigment migration without decreasing the amplitude of the receptor potential. The pigment remains in the 'dark-adapted' position independent of the light regime. A drop of solution, the dimensions of which were measured under the dissection microscope and the volume of which was calculated to be about 1 nl, was absorbed into the eye, which has a volume of about 250 nl, through a small hole cut in the cornea. EGTA takes effect about 1 min after infiltration and can be reversed by subsequent