Expression of Pis^{hs.T:Hsap\MYC} between the embryonic and the late pupal stages by daily heat-shock treatments allows Pis¹ flies to survive to adult stages. When newly-eclosed Pis¹; Pis^{hs.T:Hsap\MYC} flies are dark adapted for 2 minutes and then exposed to three pulses of orange light, the ERG amplitudes produced are similar to wild-type flies. However, when the same flies are exposed to this light regimen after 3 days without heat shock, the ERG amplitude resulting from the second light pulse is decreased compared with the first. When flies are tested after 7 days without heat shock, amplitudes are further decreased and a very small photoresponse is observed during exposure to the third light pulse.

The ERG recordings of newly-eclosed Pis¹; Pis^{hs.T:Hsap\MYC} flies exposed to 2 minutes of orange light, followed by a 3 s dark adaptation are not significantly different to those produced by wild-type flies. 3-day-old Pis¹; Pis^{hs.T:Hsap\MYC} flies (which have no further heat shock after eclosion) show a much smaller ERG amplitude after the 2 min light adaptation, which is restored after 1.5-2 minutes of dark adaptation. 7-day-old Pis¹; Pis^{hs.T:Hsap\MYC} flies require a minimum of 2 min of dark adaptation to recover the light response.

When newly-eclosed Pis¹; Pis^{hs.T:Hsap\MYC} flies are reared in the dark without additional heat shocks for 7 days and subsequently exposed to a 12 h light/dark cycle the eyes undergo a gradual loss of rhabdomeres. By 14 days, the rhabdomeres are either missing or dramatically smaller than in wild type and are distorted in appearance. This rhabdomere degeneration phenotype is not seen when newly eclosed Pis¹; Pis^{hs.T:Hsap\MYC} flies are maintained in the dark. Induction of daily heat-shock treatments after day 7 in Pis¹; Pis^{hs.T:Hsap\MYC} flies reared in a light/dark cycle prevents rhabdomere loss.