Theodor-Boveri-Institut für Biowissenschaften, Lehstuhl für Genetik, Universität Würzburg, 97074 Würzburg,
Evidence is accumulating that the nervous system of invertebrates, once considered hard-wired, depends, at least to some extent, on the appropriate sensory stimuli to develop. We investigated the optic lobes and mushroom bodies (MBs) of Drosophila melanogaster in order to find out whether rearing in different light regimes affects their size. The four neuropil regions of the optic lobes and the calyces of the MBs were measured. Monocular deprivation (i.e. the painting of one eye) affected the lamina, medulla, lobula plate and calyces in four-day old flies such that the volume underneath the painted eye was significantly reduced by up to 6%. As a control, painted flies were kept in constant darkness to exclude any effects of the painting itself. The laminae of these flies showed a more robust reduction in volume which could be as much as 30%. An electron microscopy study (in collaboration with I. Meinertzhagen, Halifax) revealed that the changes in the lamina are largely due to an increase in the distal portion of the photoreceptor cell axons. In addition, the large monopolar cells L1 and L2 as well as glia cells were shown to change in size, although their contribution to the overall volume changes is rather small. Moreover, the data suggest that the lamina cartridges of flies grown in permanent darkness are nearly cylindrical and those of flies raised in constant light conical. Due to this change in shape the radius of the lamina calotte changes by a factor of three.
The volume of the lamina increases during the first 24 hours post eclosion and it grows more in the light than in darkness. When flies are kept in the dark for the first 12 hours of their adult life and are then brought back to constant light for the next 3 ½ days, the lamina is almost as small as the lamina of flies raised for four days in constant darkness. 12 hour dark shifts at a later time are less effective. This finding suggests a critical period for lamina development during day one of the imago. The lamina depends on visual stimulation to maintain its size during the first five days after eclosion. Dark-rearing for one day or more at any stage during that period can decrease its volume to the level of flies raised in constant darkness. A lamina that is once reduced in size seems not to recover.
Since Drosophila offers many biochemical mutants, we use some of them to approach the question which molecular mechanisms underly the volume changes. For example, mutations affecting the cAMP cascade such as dunce and amnesiac show plasticity of the lamina and the lobula plate, but not of the MB calyces. This suggests that the two types of plasticity are regulated by different molecular mechanisms and only the plasticity of the MBs requires the cAMP cascade.