Abstract text (incl. figure legends and references)

Axonal wrapping starts in the developing Drosophila larva and is similar to the non-myelinating Schwann cell in the vertebrate PNS. Myelin is a more sophisticated form of axonal wrapping in vertebrates but has not been reported for Drosophila so far. Nevertheless, myelin-like structures have been described in several invertebrate species, e.g., in earthworms, shrimps, prawns, and cockroaches. Here we demonstrate that myelin-like structures also exist in the Drosophila nervous system. Abutting the myelinated axon segments, we found a prominent lacunar system formed by glial cell processes. The glial lacunar system comprises an exceptionally wide, liquid-filled extracellular space that possibly constitutes an ion reservoir meeting the high demand for ions needed for sustained firing of action potentials. We therefore analysed the localization of the only voltage-gated sodium channel encoded by the Drosophila genome termed Para. Antibody labelling indicates that Para predominantly localizes in motor axons at a position that coincides with the localization of the glial lacunae. To further test this we labelled the endogenous Para protein by inserting an Apex2 encoding exon, which allows to detect Para protein localization in the electron microscope. Our findings will be discussed in the light of the evolution of fast and precise signal transduction in the peripheral nervous system.