The Drosophila toll gene functions zygotically and is necessary for proper motoneuron and muscle development.

Toll is a maternally required Drosophila gene that encodes a transmembrane protein with an important function in embryonic dorsal-ventral patterning. The Toll protein is widely expressed zygotically, but its roles in late embryo-genesis have not been described in detail. We have examined the expression of Toll protein in the late embryonic central nervous system and somatic musculature. Toll is expressed in a dynamic pattern in teh musculature, initially in several muscle fibers in each hemisegment, with a later narrowing of expression to a single muscle fiber pair. Zygotic Toll mutants were used to investigate the development consequences of loss of Toll expression. We found that loss of one or both copies of the Toll gene leads to widespread defects in motoneuron number and muscle patterning. Loss of motoneurons prevents certain muscle fibers from receiving their wild-type innervation. Denervation in the mutants results in collateral sprouting from nearby nerve branches and leads to the appearance of ectopically placed motor endings. The limited expressivity observed suggests that Toll is only one of several genes required for proper motoneuron and muscle specification.