Wrapper, a novel member of the Ig superfamily, is expressed by midline glia and is required for them to ensheath commissural axons in Drosophila

The midline glia are specialized, nonneuronal cells at the midline of the D rosophila central nervous system (CNS). During development, the midline gli a provide guidance cues for extending axons. At the same time, they migrate and help separate the two axon commissures. They then wrap around and ensh eath the commissural axons. In many segments, a few of the glia do not enwr ap the axons, and these cells die. The wrapper gene encodes a novel member of the immunoglobulin (ig) superfamily. Wrapper protein is expressed specif ically on the surface of midline glia. In wrapper mutant embryos, the midli ne glia express their normal guidance cues and migrate normally. However, t hey do not ensheath the commissural axons, and as a result, the glia die. I n the absence of Wrapper, the two axon commissures are not properly separat ed.