Vps26¹ mutant photoreceptor cells exhibit a progressive loss of ERG amplitudes compared to controls. Upon rearing these flies in the dark, Vps26¹ mutants exhibit nearly normal ERG responses and the ERG amplitude and on- and off-transients in Vps26¹ mutant photoreceptor cells are slightly reduced when compared to controls. Newly eclosed Vps26¹ flies exhibit ommatidia that display an overall wild-type morphology, although rhabdomere loss is observed at low frequency in Vps26¹ mosaic clones.

In a 12-hour light/dark cycle, Vps26¹ flies show a loss of on- and off-transients, indicating a loss of synaptic transmission. They also exhibit a reduced depolarisation by 2 day, and a progressive loss of the ERG amplitude with age. At day 21, the light-induced photoreceptor cell depolarisation in Vps26¹ mutant photoreceptor cells is almost completely absent. Three-week-old dark/dark-reared Vps26¹ mutants exhibit nearly normal ERG responses, similar to newly eclosed flies. The photoreceptor cell morphology of Vps26¹ mutants is severely affected in flies kept in light/dark conditions for 21 days.

Vps26¹ flies raised in a light/dark cycle for 4 days show obvious ultrastructural defects in photoreceptor cells when compared to controls. These defects include a dramatic increase in late endosomes and lysosomes. However, photoreceptor cells in dark-reared Vps26¹ flies do not show an increased number of late endosomes or lysosomes when compared to controls.

Chronic light exposure (for several days) severely affects the endolysosomal system in Vps26¹ mutants.

A reduction in dietary vitamin A levels in both Vps26¹ and control flies leads to a decrease in rhabdomere size. In addition, vitamin A degeneration leads to a very strong suppression in photoreceptor cell degeneration in Vps26¹ mutants at day 10 in a light/dark cycle.