Some fly sensory organs are gliogenic and require glide/gcm in a precursorthat divides symmetrically and produces glial cells

In flies, the choice between neuronal and glial fates depends on the asymme tric division of multipotent precursors, the neuroglioblast of the central nervous system and the IIb precursor of the sensory organ lineage. In the c entral nervous system, the choice between the two fates requires asymmetric distribution of the glial cell deficient/glial cell missing (glide/gcm) RN A in the neuroglioblast. Preferential accumulation of the transcript in one of the daughter cells results in the activation of the glial fate in that cell, which becomes a glial precursor. Here we show that glide/gcm is neces sary to induce glial differentiation in the peripheral nervous system. We a lso present evidence that glide/gcm RNA is not necessary to induce the fate choice in the peripheral multipotent precursor. Indeed, glide/gcm RNA and protein are first detected in one daughter of IIb but not in IIb itself. Th us, glide/gcm is required in both central and peripheral glial cells, but i ts regulation is context dependent. Strikingly, we have found that only sub sets of sensory organs are gliogenic and express glide/gcm, The ability to produce glial cells depends on fixed, lineage related, cues and not on stoc hastic decisions. Finally, we show that after glide/gcm expression has ceas ed, the IIb daughter migrates and divides symmetrically to produce several mature glial cells, Thus, the glide/gcm-expressing cell, also called the fi fth cell of the sensory organ, is indeed a glial precursor. This is the fir st reported case of symmetric division in the sensory organ lineage. These data indicate that the organization of the fly peripheral nervous system is more complex than previously thought.