Approximately 55% of toy^hdl mutants exhibit wild-type-like brains. Loss of toy causes deformation of the embryonic brain hemispheres, with 22% showing severe commissural and longitudinal defects with their brain commissures markedly distorted, and another 22% showing mild morphological defects in their brain hemispheres and the major axonal tracts.

Approximately one third of toy^hdl homozygotes die during the larval period, with the remaining dying at the late pupal stage as pharate adults inside the pupal case. More than half of the toy^hdl homozygous pharates exhibit a remarkable headless phenotype at 25-28[o]C. This is due to a head inversion defect as the brains of these animals are embedded in the thorax. toy^hdl pharates exhibit severe defects in the development of major brain structures such as the mushroom bodies. Rare escapers exhibit severe defects in the mushroom bodies where they are often missing part or all of the lobes.

Approximately one-third of homozygous toy^hdl mutants die during embryonic or larval stages. The remaining two-thirds reach the pupal stage, where many develop as far as pharate adults. At this stage they die since they are unable to escape from the pupal case. Almost no escapers eclose at 25[o]C or at higher temperatures. Removal of the pupal cases shows flies with variable head defects, ranging from almost normal heads with small compound eyes, over 'half-heads' and 'cleft-heads', to completely headless flies where the proboscis is protruding from the thorax.

The proportion of flies with normal ocelli among the toy^hdl homozygous mutant escapers is 18% at 18[o]C.

Dissected toy^hdl eye-antennal discs grown at 21[o]C exhibit a variety of morphologies. Fully normal eye-antennal discs are found, as well as defects ranging from size reduction, deformation, lack of parts, to complete loss of discs. Asymmetric loss of disc portions is often seen. The level of cell death seen in the developing eye disc appears wild-type. The antennal part of the discs exhibit massive cell death. In those cases where the antennal part is absent, cell death seems to spread into the anterior edge of the eye portion reaching the region where the frons and ocelli precursor cells are localized.

65% of homozygous mutants when raised at 28^oC, die inside their pupal case headless, developing no structures derived from the eye-antennal discs, while less extreme phenotypes had partially developed heads, 'half-heads' and cleft heads. The weakest phenotype is a 'small eye' phenotype. About 1/3 of mutants die during the larval stages and 2/3 during the pupal stage. The relative proportions of phenotypes observed among homozygous toy^hdl flies depends on the temperature at which they are raised. At 18^oC about 3% survive to apparently wild-type fertile adults, 65% of pharates have normal heads and about 3% are headless. The remaining pharates develop partial heads. The temperature sensitive period is from stage 12 to 16. The larval lethal phenotype is not temperature sensitive.

Lethality occurs during larval or pupal development.

Lethal phase is during the larval to pupal stages.

toy^hdl has lethal | pharate adult stage phenotype, suppressible | partially by Scer\GAL4^toy1/ey^UAS.cHa

toy^hdl has lethal | pharate adult stage phenotype, suppressible by Scer\GAL4^toy1/BacA\p35^UAS.cHa

toy^hdl has increased cell death | embryonic stage phenotype, suppressible by Scer\GAL4^toy1/BacA\p35^UAS.cHa

toy^hdl has increased cell death | larval stage phenotype, suppressible by Scer\GAL4^toy1/BacA\p35^UAS.cHa