Structural daily rhythms in GFP-labelled neurons in the visual system of Drosophila melanogaster

In the visual system of Drosophila melanogaster, two classes of interneurons in the first optic neuropil, or lamina, the monopolar cells L1 and L2, show rhythmic circadian changes in the shape and size of their axons. In the present study we have used the GAL4-UAS system to target the GFP expression to the L2 cells in D. melanogaster and to examine morphological changes in the cell body, nucleus, axon and dendritic spines. Our results showed that in addition to changes in the caliber of its axon, L2 also shows daily changes in the morphology of its dendritic spines, differences which are most pronounced at the beginning of the night. There are also changes in the sizes of the cells' nuclei in the lamina cortex, which are largest at the beginning and in the middle of day, in females and males, respectively. In contrast to the axon and dendrites, L2's soma does not change size significantly during the day or night. The observed changes clearly indicate the cyclical modulation of the structure of the L2 interneurons. These changes seem to be regulated by a circadian clock, which exhibits certain differences between the sexes.