Abstract
BackgroundIn the first optic neuropil (lamina) of the fly's visual system, monopolar cells L1 and L2 and glia show circadian rhythms in morphological plasticity. They change their size and shape during the day and night. The most pronounced changes have been detected in circadian size of the L2 axons. Looking for a functional significance of the circadian plasticity observed in axons, we examined the morphological plasticity of the L2 dendrites. They extend from axons and harbor postsynaptic sites of tetrad synaptic contacts from the photoreceptor terminals.Methodology/Principal FindingsThe plasticity of L2 dendrites was evaluated by measuring an outline of the L2 dendritic trees. These were from confocal images of cross sections of L2 cells labeled with GFP. They were in wild-type and clock mutant flies held under different light conditions and sacrified at different time points. We found that the L2 dendrites are longest at the beginning of the day in both males and females. This rhythm observed under a day/night regime (LD) was maintained in constant darkness (DD) but not in continuous light (LL). This rhythm was not present in the arrhythmic per01 mutant in LD or in DD. In the clock photoreceptor cryb mutant the rhythm was maintained but its pattern was different than that observed in wild-type flies.Conclusions/SignificanceThe results obtained showed that the L2 dendrites exhibit circadian structural plasticity. Their morphology is controlled by the per gene-dependent circadian clock. The L2 dendrites are longest at the beginning of the day when the daytime tetrad presynaptic sites are most numerous and L2 axons are swollen. The presence of the rhythm, but with a different pattern in cryb mutants in LD and DD indicates a new role of cry in the visual system. The new role is in maintaining the circadian pattern of changes of the L2 dendrite length and shape.
Highlights
The visual system of different fly species show circadian rhythms in many physiological and structural processes
Circadian rhythm in morphology of dendrites The results obtained in the present study showed that the L2 cell dendrites, postsynaptic sites of tetrad synapses in the first optic neuropil of D. melanogaster, show circadian plasticity in morphology
Under the day/night regime LD 12:12, the L2 dendrites, measured as the cross-sectional outline of the dendritic tree are the longest at the beginning of day and shorter later during the day and at night. This daily rhythm in changes of size of the dendritic tree was maintained in DD. This indicates that the circadian plasticity of the L2 dendrites is controlled endogenously by a circadian clock
Summary
The visual system of different fly species show circadian rhythms in many physiological and structural processes They have been detected in the retina, in changes of the electroretinogram amplitude [1], and in the first optic neuropil (lamina). The molecular clock in most clock cells is based on circadian transcription and translation feedback loops with two main core genes, per and timeless (tim) It is entrained by light and other environmental cues to drive daily rhythms in output processes [11]. In the first optic neuropil (lamina) of the fly’s visual system, monopolar cells L1 and L2 and glia show circadian rhythms in morphological plasticity They change their size and shape during the day and night. They extend from axons and harbor postsynaptic sites of tetrad synaptic contacts from the photoreceptor terminals
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