Daily rhythmic changes of cell size and shape in the first optic neuropil in Drosophila melanogaster

Daily rhythms of changes in axon size and shape are seen in two types of mo nopolar cell-L1 and L2-that are unique cells within each of the modules or cartridges of the first optic neuropil or lamina in the fly's optic lobe, I n the fruit fly Drosophila, L1 and L2's axons swell at the beginning of bot h day and night, with larger size increases occurring at the beginning of n ight. Later, they shrink during the day and night, respectively. Simultaneo usly, they change shape from an inverted conical form during the day to a c ylindrical one at night. This is because the axonal cross section of L1 inc reases during the night, especially at proximal depths of the lamina, close st to the brain, whereas the axon of L2 increases in size at distal lamina depths. The cross-sectional areas of the L1 cell and of an individual cartr idge both change under constant darkness (DD), indicating the circadian ori gin of changes observed under day/night (LD) conditions. We sought to see w hether such changes impart a net change to the entire lamina's volume or sh ape that is visible by light microscopy, but oscillations in the volume or the curvature of the whole lamina neuropil are found neither in LD nor in D D, These size changes are discussed in relation to previous findings in the housefly Musca, with respect to differences in L1 and L2 between the two s pecies, and to differences in the time course of their circadian changes. ( C) 1999 John Wiley & Sons, Inc.