The structural brain mutant Vacuolar medulla of Drosophila melanogaster with specific behavioral defects and cell degeneration in the adult.

The mutant Vam (Vacuolar medulla) has vacuoles in the distal medulla, caused by age-dependent cell degeneration in the lamina and the medulla. Lamina monopolar neurons L1 and L2 degenerate, but whether the degeneration is confined only to these cells is uncertain. The cell degeneration commences at eclosion and the vacuoles begin appearing about 1/2 h after eclosion. This is accompanied by the disappearance of the electroretinogram transients and a loss of the optomotor response. Vam males or homozygous Vam females one day after eclosion or later, show no measurable optomotor response to horizontal or vertical movement and no landing response. However, they are able to turn towards dark stripes larger than 20 degrees in width. This indicates that motion-dependent visual responses are not a prerequisite for landmark fixation in Drosophila. The mutant's apparent defects suggest that the loss of motion-dependent visual functions is due to the loss of certain cell types linking the lamina to the distal medulla. The role of lamina neurons in optomotor responses and fixation behavior is discussed.