Photopigment and receptor properties in Drosophila compound eye and ocellar receptors

A review of the spectral sensitivity and the rhodopsin and metarhodopsin characteristics in three compound eye receptor types (R1-6, R7, and R8) and ocellar receptors is presented (Fig. 1). Photopigment properties were determined from measures of conversion efficiency. The photopigments of R1-6 were studied using in vivo microspectrophotometry in the deep pseudopupil of white-eyed flies. These studies yielded a refined estimate of the R1-6 metarhodopsin spectrum (Fig. 2). The quantum efficiency relative to the spectral sensitivity estimate of the rhodopsin spectrum was factored out. The quantum efficiency of rhodopsin is about 1.75 times that of metarhodopsin. The peak absorbance of metarhodopsin was estimated to be about 2.6 times that of rhodopsin. The mechanism of the two-peaked R1-6 spectral sensitivity and metarhodopsin spectrum is discussed in terms of evidence that there is only one rhodopsin in R1-6 and that vitamin A deprivation preferentially lowers ultraviolet sensitivity. The prolonged depolarizing afterpotential is reviewed from the standpoint of the internal transmitter hypothesis of visual excitation. A careful comparison of the intensity-responsivity for photopigment conversion and its adaptional consequences is made (Fig. 3).