Abstract
The eyelessโDominant (ey^D^) mutation is a fourth chromosome insertional translocation which affects the eyes, antennae, ocelli, and sexcombs when heterozygous but is a larvalโpupal lethal when homozygous. By use of a crowding technique, it was possible to separate ey^D^ homozygotes and heterozygotes with 100% accuracy at an early stage of larval development. Under these conditions, the ey^D^ homozygotes had a biphasic lethal period since 45% died as first or second instar larvae and 55% died as pupae. The ey^D^/ey^D^ pupal lethal, isolated by this technique, was able to form all the adult structures with the exception of the eyeโantennal disc derivatives. The brain was present but abnormal. In testing the competence of the ey^D^/ey^D^ larval eye discs by means of transplantation experiments, it has been demonstrated that the mutant discs develop nonโautonomously. Histological studies have revealed the existence of neurological defects in both ey^D^ heterozygotes and homozygotes. The ey^D^/ey^D^ lethal larvae lacked detectable optic formation centers and showed an extreme reduction in the number of cells present in the cortex of the brain. The ey^D^/ey^D^ lethal pupae possessed partial formation centers and also had severe reduction in the number of cortical cells. The ey^D^ heterozygotes possessed normal appearing formation centers but they did exhibit a moderate reduction in the number of their cortical cells as compared to wildโtype. These studies have shown that there is a direct correlation between the extent of neurological damage and the time of developmental arrest. It appears that the ey^D^ mutation must adversely affect the neuroblasts at a very early stage of development. As a working hypothesis, it is suggested that the ey^D^ mutation operates via the same basic mechanism of cell death in heterozygous and homozygous animals.