Sexual differentiation of identified motor terminals in Drosophila larvae.

In Drosophila, we have found that some of the motor terminals in wandering third-instar larvae are sexually differentiated. In three out of the four body-wall muscle fibers that we examined, we found female terminals that produced a larger synaptic response than their male counterparts. The single motor terminal that innervates muscle fiber 5 produces an EPSP that is 69% larger in females than in males. This is due to greater release of transmitter from female than male synaptic terminals because the amplitude of spontaneous miniature EPSPs was similar in male and female muscle fibers. This sexual difference exists throughout the third-instar: it is seen in both early (foraging) and late (wandering) third-instar larvae. The sexual differentiation appears to be neuron specific and not muscle specific because the same axon produces Is terminals on muscle fibers 2 and 4, and both terminals produce larger EPSCs in females than males. Whereas, the Ib terminals innervating muscle fibers 2 and 4 are not sexually differentiated. The differences in transmitter release are not due to differences in the size of the motor terminals. For the terminal on muscle fiber 5 and the Is terminal on muscle fiber 4, there were no differences in terminal length, the number of branches, or the number of synaptic boutons in males compared to females. These sexual differences in neuromuscular synaptic physiology may be related to male-female differences in locomotion.