Modularity of Function among Rickets-expressing neurons in the Wing Expansion Network of Drosophila. Feici Diao¹, Fengqiu Diao¹, Chun-yuan Ting², Chi-hong Lee², Benjamin White¹. 1) NIMH, NIH,Bethesda,MD; 2) NICHD, NIH,Bethesda,MD.

   We have previously shown that wing expansion can be induced in Drosophila by stimulating a single pair of neurons located in the subesophageal ganglion that express the hormone bursicon. These bursicon-expressing neurons trigger not only the behavioral and somatic changes that support wing expansion, but also suppress the inhibitory effects of unfavorable environments. To understand how these diverse functions are coordinated, we have identified the downstream targets of the bursicon-expressing neurons using the T2A-GIFF technique, generating a Gal4 line that expresses in the same pattern as the bursicon receptor encoded by the rickets gene. Expression of a UAS-RK transgene under the control of the rk-Gal4 driver fully rescues the wing expansion deficits of rk4 null mutants. Further, wing expansion can be induced by activating RK-expressing neurons using UAS-dtrpA1. By varying the period of activation between 0 - 10 min, we are able to distinguish two distinct effects: 1) immediate induction of wing expansion behaviors, and 2) suppression of environmental inhibition. To determine whether distinct subsets of RK-expressing neurons mediate these motor and modulatory effects, we coupled rk-Gal4 to the Split LexA system and used 179 VP16AD enhancer-trap lines to drive LexAop-dTrpA1 expression in different subsets of RK-expressing neurons. Consistent with our initial observations, we identified two distinct functional groups of flies, one showing an immediate motor response, but no suppression of environmental inhibition, and the other showing suppression of environmental inhibition, but no immediate motor response. We conclude that at least two distinct groups of RK-expressing neurons act within the wing expansion network, one directly transducing the bursion signal to motor outputs, and the other acting via a positive feedback loop to disinhibit bursicon-expressing neurons. The latter group is likely to help mediate the decision to expand the wings.