INTRODUCTION Glia play diverse supportive roles in the adult nervous system, including wrapping and insulating neurons, providing them with nourishment, maintaining their ionic homeostasis, and helping to establish and maintain the blood-brain and blood- nerve barrier. During development, glia help control the patterning of neuronal differentiation and, in particular, guide axons to their targets (Klaes et al., 1994; Klämbt et al., 1996). The specific role that glia play in the formation of axon pathways appears to depend on the context. In Drosophila, the glia along the midline of the central nervous system (CNS) provide complex guidance cues for commissural axons (Klämbt et al., 1991; Tear et al., 1993). Midline glia bear not only attractive cues, such as the Netrins, which attract commissural fibers to the midline, but also repulsive cues, such as Slit, which prevent commissural fibers from recrossing the midline (Seeger et al., 1993; Mitchell et al., 1996; Tear et al., 1996; Kidd et al., 1999). However, the role of glia in the formation of other axonal pathways is less clear. On the one hand, studies of mutants such as glial cells missing (gcm), pointed (pnt) and reversed polarity (repo) in Drosophila suggest an ancillary role (Klaes et al., 1994; Halter et al., 1995; Hosoya et al., 1995; Jones et al., 1995). The analysis of gcm mutants, which lack all glia except for those in the midline, demonstrates that most axon pathways in the embryo, and specifically the longitudinal axon tracts, can develop without glia, albeit with greater variability, and suggests a merely facilitory role for glia in the formation of non-commissural pathways (Hosoya et al., 1995; Jones et al., 1995). By contrast, toxin-induced ablation of longitudinal glia early in development leads to a complete loss of the longitudinal axon tracts, suggesting that longitudinal glia are strictly required for growth cone guidance in the formation of these axon tracts (Hidalgo et al., 1995). Genetic and toxin ablation studies agree that glia have an essential role in the maintenance of established axon pathways. In order for glial cells to fulfill their role in axon guidance or even maintenance, they have to be positioned correctly with respect to the neurons. For many glial cell populations, this means migration over many cell diameters within a restricted period of time during development. Examples of migrating glial cells include the midline glia in the Drosophila CNS and the oligodendrocytes of the optic nerve in vertebrates (Small et al., 1987; Klämbt et al., 1991). The mechanisms involved in glial migration are not well understood, and only a few cellular and molecular components have so far been identified. In vertebrates, the fibroblast growth factor (FGF) receptor as well as several components involved in cell adhesion have been implicated in oligodendrocyte motility (Milner et al., 1996; Payne et al., 1996; Osterhout et al., 1997). Similarly, in Drosophila, the FGF receptor Breathless (Btl) has been implicated in the migration of a subset of midline glia in the embryo, suggesting that activation of the Ras signaling pathway is involved in the migration process (Klämbt et al., 1992; Fried-Reichman et al., 1994). In this study, we report on the migration and function of glial cells in the developing adult eye of Drosophila. A previous 3285 Development 126, 3285-3292 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV8599 Although glial cells have been implicated widely in the formation of axon tracts in both insects and vertebrates, their specific function appears to be context-dependent, ranging from providing essential guidance cues to playing 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 along photoreceptor axons, thus precluding any role in axon guidance. Here we show 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 migrate to ectopic patches of differentiating photoreceptors without axons providing a continuous physical substratum. This suggests that glial cells are attracted into the eye disc not through haptotaxis along established axons, but through another mechanism, possibly chemotaxis. (2) If no glial cells are 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, but 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 glia instruct axons by local interaction. Key words: Drosophila, Visual system development, Glia, Migration, Axon guidance SUMMARY Migration and function of glia in the developing Drosophila eye Radha Rangarajan, Qizhi Gong and Ulrike Gaul* Laboratory of Developmental Neurogenetics, Rockefeller University, 1230 York Avenue, New York, NY 10021, USA *Author for correspondence (e-mail: gaul@rockvax.rockefeller.edu) Accepted 12 May; published on WWW 5 July 1999