Migration and function of glia in the developing Drosophila eye

Although glial cells have been implicated widely in the formation of axon t racts in both insects and vertebrates, their specific function appears to b e context-dependent, ranging from providing essential guidance cues to play ing a merely facilitory role. Here we examine the role of the retinal basal glia (RBG) in photoreceptor axon guidance in Drosophila, The RBG originate in the optic stalk and have been thought to migrate into the eye disc alon g photoreceptor axons, thus precluding any role in axon guidance. Here we s how the following. (1) The RBG can, in fact, migrate into the eye disc even in the absence of photoreceptor axons in the optic stalk; they also migrat e to ectopic patches of differentiating photoreceptors without axons provid ing a continuous physical substratum. This suggests that glial cells are at tracted into the eye disc not through haptotaxis along established axons, b ut through another mechanism, possibly chemotaxis, (2) If no glial cells ar e present in the eye disc, photoreceptor axons are able to grow and direct their growth posteriorly as in wild type, but are unable to enter the optic stalk. This indicates that the RBG have a crucial role in axon guidance, b ut not in axonal outgrowth per se. (3) A few glia close to the entry of the optic stalk suffice to guide the axons into the stalk, suggesting that gli a instruct axons by local interaction.