Drosophila cryptochrome achieves high effective light sensitivity by integrating photon information over extreme time periods. Pooja G Vinayak, Jamie Coupar, S. Emile Hughes, Preeya Fozdar, Jack Kilby, Jay Hirsh. University of Virginia, Department of Biology, Charlottesville, VA.

   Drosophila melanogaster show extreme dim light sensitivity for entrainment to 12:12 hour light/dark schedules (Hirsh et al, 2010). To better understand this light sensitivity, we use a related paradigm, shifting circadian phase by administration of dim light pulses in the subjective night. Here we show that flies show graded responses to varying intensities of dim light pulses. However, light sensitivity shows a surprising increase with pulse duration up to durations of 360 minutes, implying that photic integration is occurring over a time scale of hours. To explicitly test for photic integration, we exposed flies to light pulses containing equal numbers of photons given over time intervals between 0.1 and 100 minutes. To account for minimum and maximum possible phase shifts, these pulses were given in separate experiments during both the early and late subjective night respectively. Flies respond to late-night pulses with increasing phase advance amplitude as time intervals increase, showing a surprising intensity/duration relationship. However, early-night pulses result in phase delays that are characterized by increased amplitude and light sensitivity relative to phase advances. Furthermore, phase delay amplitude remains constant with increasing time intervals for light pulses of equal photon numbers, implying that the process of temporal integration is different during the early and late subjective night. The large amplitude phase advances that result from this temporal integration depend critically on the circadian photopigment cryptochrome, with little input from visual photoreceptors in the eyes. These results show that cryptochrome has inherently low photic sensitivity, but achieves high effective sensitivity by integrating photon information over extremely long times. Our findings provide a general mechanism by which a non-image forming photopigment can achieve or exceed the light sensitivity of image forming visual photopigments.