Glial cells are one of the major components of the central nervous system. In the Drosophila central brain, different subtypes of glial cells form specific architectures. Neural networks of holometabolous insects such as Drosophila remodel during metamorphosis. How do the glial architectures change in response to the neural remodeling? Here we report that the larval glial architecture associating with neuropil in the brain is replaced with newly formed architecture during metamorphosis. Two subtypes of neuropil-associated glia, ensheathing glia and astrocyte-like glia, are found in both larval and adult brains. Neuropils of both larval and adult brains are wrapped by ensheathing glia and filled with astrocyte-like glia. However, we found that the larval and adult neuropil-associated glial architectures are composed of different cell population. Larval neuropil-associated glial cells change their molecular and morphological features in the early pupal stage. On the other hand, immature glial cells appear at the interface between cell-body region and neuropil in the late larval stage. As the number of immature glial cells increases, they spread over in the interface. When these immature glial cells were genetically labeled in larval stage, labels were specifically found in both subtypes of neuropil-associated glial cells in the adult brains, indicating that these immature glial cells are precursors of adult neuropil-associated glia.
In order to understand how the new neuropil-associated glial architecture is formed over the old one, we analyzed development of the glial precursors and their lineage in detail. Using the cell tracing system combined with the cell cycle marker, FUCCI, and cell death marker, PARP::VENUS, cell proliferation and apoptosis were analyzed in the glial precursor lineage, respectively. We found that these glial precursors generate immature glial cells excessively by cell proliferation and the immature cells are distributed confluently at the interface. Then, excess cells are removed by apoptosis. Finally, survived cells differentiate into two glial subtypes and form adult neuropil-associated glial architecture. These results suggest that the adult glial architecture is established through the overproduction and elimination of immature glial cells. We would like to discuss the potential molecular mechanism regulating the adjustment of the glial precursor lineage.