Figure 6.

SDR is constitutively secreted in the hemolymph and is critical under adverse food conditions. (A) InR, dilp5, and SDR expression levels were analyzed by qRT–PCR in different food conditions. Early third instar larvae just after molting were used (set to 0 d). (B) Constitutive production of SDR into the hemolymph. SDR protein levels in the hemolymph were analyzed in the sample shown in A. (C) SDR mutants promote body growth regardless of the nutrient conditions. The relative concentrations of yeast contents in food are indicated (2×, 1×, 1/3×, and 1/6×). (D,E) SDR mutants grown on the restricted food conditions exhibit an elevated mortality rate during metamorphosis. A defined number of newly hatched first instar larvae was seeded on normal food (D) or low-nutrient food (10% food) (E), and the number of pupae (puparium formation) and eclosed flies (eclosion) was counted. All of the values are the mean and SD (n = 3 [A,B], n > 4 batches [C], or n = 7–10 vials [D,E]). (*) P < 0.01 (Student's t-test). (F) A proposed model showing the role of SDR in growth control. Glia-derived SDR binds to circulating Dilps in the hemolymph and negatively regulates systemic growth by antagonizing insulin/IGF signaling in peripheral tissues. The function of SDR is likely complementary to and redundant with that of Imp-L2.