Morphological and functional characteristics of arpa (receptor potential absent) visual mutant of the blowfly (Calliphora erythrocephala)

Abstract

1. We have discovered a previously unreported visual mutant of the blowfly,Calliphora erythrocephala. It shows a reduced or absent visual performance, e.g., in escape and optomotor behavior. The effects of this mutation on the ultrastructure were studied by electron microscopy (Figs. 3–8) and on electrophysiological function, by intracellular recordings (Figs. 1 and 2). 2. The genetic basis of this spontaneous mutation was studied by test crosses of mutant and wild-type flies. The defect appears to be in an autosomal recessive gene (Table 1). 3. Of the mutant stock studied soon after eclosion (n = 18) 35% shows optomotor reactions, whereas only 6% studied in later life (n = 240) shows any optomotor behavior. 4. The absence of the receptor potentials in photoreceptor cells is not directly associated with structural disorders in the early life of these mutant flies, but several types of degenerative changes are manifested in the retinular cells later on. The optomotorically blind specimens have normal (about −60 mV) resting membrane voltages but no detectable receptor cell voltage response to light, indicating a block in phototransduction. The spectral and polarization sensitivities of optomotor-positive flies are normal (Fig. 2). 5. At the beginning of degeneration the number of lysosomes in the receptor cells is increased compared with normal flies, but their number as well as that of other components of the cell interior decrease later on. During the progression of the degeneration, the rhabdomeres shrink while the mitochondria swell and disintegrate (Figs. 6–8). 6. The blocking of phototransduction is proposed to lead to disturbance of the turnover of the rhabdomeres and finally to degeneration of the receptor cells.