Expression of usp^hs.PO in embryos does not result in a dorsal open phenotype.

Expression of usp^hs.PO using a single 30 minute 37^oC heat pulse during the first larval instar stage is sufficient to rescue more than 70% of usp² animals to the third instar stage. These rescued animals fail to wander near the end of the third instar stage, maintain their larval shape, become motionless at the surface of the food, fail to respond to touch stimulus and do not evert their anterior spiracles. After several hours in this abnormal "stationary stage", the animals begin to apolyze from their third instar cuticle, retracting from both the anterior and posterior ends. Apolysis is complete by 24 hours after becoming stationary. A supernumerary cuticle covers the posterior two-thirds of the animal. This cuticle is thick, well infiltrated with tracheae and segmentally ridged along the body. Most of these rescued animals die by 72 hours after the stationary phase. Imaginal discs appear normal in rescued third instar larvae and begin to evert following the stationary phase, but arrest their development at a point normally seen 1 hour after puparium formation. The gastric caecae retract, although this occurs gradually over 24 hours. A slight compaction of the larval midgut is seen, but the larval cells do not die and the adult midgut does not form. The number of midgut imaginal cells does not change significantly in the 24 hours following the stationary phase. Larval salivary gland development is normal until the end of the third instar stage, however destruction of the larval salivary gland does not occur and the gland persists until the animal dies.

Heat induced expression can rescue usp^- mutants to adulthood and multiple heat shocks do not decrease the viability of otherwise wild type flies.

Basal expression of usp^hs.PO in usp mutant/Y males delays the lethal phase, so that most of the individuals die in the second instar phase, instead of between the first and second instar stage. Periodic expression of usp^hs.PO by heat shock allows survival up to adulthood in both usp mutant/Y males and usp mutant female homozygotes. The males are fertile, the females are partially sterile. 30-50% of the eggs laid by these rescued females have patches of abnormally thin chorion.

usp^hs.PO is a suppressor of lethal phenotype of svp^1.hs

usp^hs.PO is a suppressor of lethal phenotype of svp^2.hs