An extraretinally expressed insect cryptochrome with similarity to the blue light photoreceptors of mammals and plants

Photic entrainment of insect circadian rhythms can occur through either ext raretinal (brain) or retinal photoreceptors, which mediate sensitivity to b lue light or longer wavelengths, respectively. Although visual transduction processes are well understood in the insect retina, almost nothing is know n about the extraretinal blue light photoreceptor of insects. We now have i dentified and characterized a candidate blue light photoreceptor gene in Dr osophila (DCry) that is homologous to the cryptochrome (Cry) genes of mamma ls and plants. The DCry gene is located in region 91F of the third chromoso me, an interval that does not contain other genes required for circadian rh ythmicity. The protein encoded by DCry is similar to 50% identical to the C RY1 and CRY2 proteins recently discovered in mammalian species. As expected for an extraretinal photoreceptor mediating circadian entrainment, DCry mR NA is expressed within the adult brain and can be detected within body tiss ues. Indeed, tissue in situ hybridization demonstrates prominent expression in cells of the lateral brain, which are close to or coincident with the D rosophila clock neurons. Interestingly, DCry mRNA abundance oscillates in a circadian manner in Drosophila head RNA extracts, and the temporal phasing of the rhythm is similar to that documented for the mouse Cry1 mRNA, which is expressed in clock tissues. Finally, we show that changes in DCry gene dosage are associated predictably with alterations of the blue light resett ing response for the circadian rhythm of adult locomotor activity.