Drosophila Cryptochrome: Variations in Blue.

CRYPTOCHROMES (CRYs) are structurally related to ultraviolet (UV)/blue-sensitive DNA repair enzymes called photolyases but lack the ability to repair pyrimidine dimers generated by UV exposure. First identified in plants, CRYs have proven to be involved in light detection and various light-dependent processes in a broad range of organisms. In Drosophila, CRY's best understood role is the cell-autonomous synchronization of circadian clocks. However, CRY also contributes to the amplitude of circadian oscillations in a light-independent manner, controls arousal and UV avoidance, influences visual photoreception, and plays a key role in magnetic field detection. Here, we review our current understanding of the mechanisms underlying CRY's various circadian and noncircadian functions in fruit flies.

Drosophila PSI controls circadian period and the phase of circadian behavior under temperature cycle via tim splicing.

The Drosophila circadian pacemaker consists of transcriptional feedback loops subjected to post-transcriptional and post-translational regulation. While post-translational regulatory mechanisms have been studied in detail, much less is known about circadian post-transcriptional control. Thus, we targeted 364 RNA binding and RNA associated proteins with RNA interference. Among the 43 hits we identified was the alternative splicing regulator P-element somatic inhibitor (PSI). PSI regulates the thermosensitive alternative splicing of timeless (tim), promoting splicing events favored at warm temperature over those increased at cold temperature. Psi downregulation shortens the period of circadian rhythms and advances the phase of circadian behavior under temperature cycle. Interestingly, both phenotypes were suppressed in flies that could produce TIM proteins only from a transgene that cannot form the thermosensitive splicing isoforms. Therefore, we conclude that PSI regulates the period of Drosophila circadian rhythms and circadian behavior phase during temperature cycling through its modulation of the tim splicing pattern.

Neural Network Interactions Modulate CRY-Dependent Photoresponses in Drosophila.

Light is one of the chief environmental cues that reset circadian clocks. In Drosophila, CRYPTOCHROME (CRY) mediates acute photic resetting of circadian clocks by promoting the degradation of TIMELESS in a cell-autonomous manner. Thus, even circadian oscillators in peripheral organs can independently perceive light in Drosophila However, there is substantial evidence for nonautonomous mechanisms of circadian photoreception in the brain. We have previously shown that the morning (M) and evening (E) oscillators are critical light-sensing neurons that cooperate to shift the phase of circadian behavior in response to light input. We show here that light can efficiently phase delay or phase advance circadian locomotor behavior in male Drosophila even when either the M- or the E-oscillators are ablated, suggesting that behavioral phase shifts and their directionality are largely a consequence of the cell-autonomous nature of CRY-dependent photoreception. Our observation that the phase response curves of brain and peripheral oscillators are remarkably similar further supports this idea. Nevertheless, the neural network modulates circadian photoresponses. We show that the M-oscillator neurotransmitter pigment dispersing factor plays a critical role in the coordination between M- and E-oscillators after light exposure, and we uncover a potential role for a subset of dorsal neurons in the control of phase advances. Thus, neural modulation of autonomous light detection might play an important role in the plasticity of circadian behavior.SIGNIFICANCE STATEMENT Input pathways provide circadian rhythms with the flexibility needed to harmonize their phase with environmental cycles. Light is the chief environmental cue that synchronizes circadian clocks. In Drosophila, the photoreceptor CRYPTOCHROME resets circadian clocks cell-autonomously. However, recent studies indicate that, in the brain, interactions between clock neurons are critical to reset circadian locomotor behavior. We present evidence supporting the idea that the ability of flies to advance or delay their rhythmic behavior in response to light input essentially results from cell-autonomous photoreception. However, because of their networked organization, we find that circadian neurons have to cooperate to reset the phase of circadian behavior in response to photic cues. Our work thus helps to reconcile cell-autonomous and non-cell-autonomous models of circadian entrainment.

Perinatal and juvenile social environments interact to shape cognitive behaviour and neural phenotype in prairie voles.

Social environments experienced at different developmental stages profoundly shape adult behavioural and neural phenotypes, and may have important interactive effects. We asked if social experience before and after weaning influenced adult social cognition in male prairie voles. Animals were raised either with or without fathers and then either housed singly or in sibling pairs. Males that were socially deprived before (fatherless) and after (singly housed) weaning did not demonstrate social recognition or dissociate spatial from social information. We also examined oxytocin and vasopressin receptors (OTR and V1aR) in areas of the forebrain associated with social behaviour and memory. Pre- and post-wean experience differentially altered receptor expression in several structures. Of note, OTR in the lateral septum-an area in which oxytocin inhibits social recognition-was greatest in animals that did not clearly demonstrate social recognition. The combination of absentee fathers on V1aR in the retrosplenial cortex and single housing on OTR in the septohippocampal nucleus produced a unique phenotype previously found to be associated with poor reproductive success in nature. We demonstrate that interactive effects of early life experiences throughout development have tremendous influence over brain-behaviour phenotype and can buffer potentially negative outcomes due to social deprivation.

Social recognition is context dependent in single male prairie voles.

Single males might benefit from knowing the identity of neighbouring males when establishing and defending boundaries. Similarly, males should discriminate between individual females if this leads to more reproductive opportunities. Contextual social cues may alter the value of learning identity. Knowing the identity of competitors that intrude into an animal's territory may be more salient than knowing the identity of individuals on whose territory an animal is trespassing. Hence, social and environmental context could affect social recognition in many ways. Here we test social recognition of socially monogamous single male prairie voles, Microtus ochrogaster. In experiment 1 we tested recognition of male or female conspecifics and found that males discriminated between different males but not between different females. In experiment 2 we asked whether recognition of males is influenced when males are tested in their own cage (familiar), in a clean cage (neutral) or in the home cage of another male (unfamiliar). Although focal males discriminated between male conspecifics in all three contexts, individual variation in recognition was lower when males were tested in their home cage (in the presence of familiar social cues) compared to when the context lacked social cues (neutral). Experiment 1 indicates that selective pressures may have operated to enhance male territorial behaviour and indiscriminate mate selection. Experiment 2 suggests that the presence of a conspecific cue heightens social recognition and that home-field advantages might extend to social cognition. Taken together, our results indicate social recognition depends on the social and possibly territorial context.

Human cryptochrome exhibits light-dependent magnetosensitivity.

Humans are not believed to have a magnetic sense, even though many animals use the Earth's magnetic field for orientation and navigation. One model of magnetosensing in animals proposes that geomagnetic fields are perceived by light-sensitive chemical reactions involving the flavoprotein cryptochrome (CRY). Here we show using a transgenic approach that human CRY2, which is heavily expressed in the retina, can function as a magnetosensor in the magnetoreception system of Drosophila and that it does so in a light-dependent manner. The results show that human CRY2 has the molecular capability to function as a light-sensitive magnetosensor and reopen an area of sensory biology that is ready for further exploration in humans.

Animal cryptochromes mediate magnetoreception by an unconventional photochemical mechanism.

Understanding the biophysical basis of animal magnetoreception has been one of the greatest challenges in sensory biology. Recently it was discovered that the light-dependent magnetic sense of Drosophila melanogaster is mediated by the ultraviolet (UV)-A/blue light photoreceptor cryptochrome (Cry). Here we show, using a transgenic approach, that the photoreceptive, Drosophila-like type 1 Cry and the transcriptionally repressive, vertebrate-like type 2 Cry of the monarch butterfly (Danaus plexippus) can both function in the magnetoreception system of Drosophila and require UV-A/blue light (wavelength below 420 nm) to do so. The lack of magnetic responses for both Cry types at wavelengths above 420 nm does not fit the widely held view that tryptophan triad-generated radical pairs mediate the ability of Cry to sense a magnetic field. We bolster this assessment by using a mutant form of Drosophila and monarch type 1 Cry and confirm that the tryptophan triad pathway is not crucial in magnetic transduction. Together, these results suggest that animal Crys mediate light-dependent magnetoreception through an unconventional photochemical mechanism. This work emphasizes the utility of Drosophila transgenesis for elucidating the precise mechanisms of Cry-mediated magnetosensitivity in insects and also in vertebrates such as migrating birds.