Despite their different origins, Drosophila glia and hemocytes are related cell populations
that provide an immune function. Drosophila hemocytes patrol the body cavity and act
as macrophages outside the nervous system, whereas glia originate from the neuroepithelium
and provide the scavenger population of the nervous system. Drosophila glia are hence
the functional orthologs of vertebrate microglia, even though the latter are cells
of immune origin that subsequently move into the brain during development. Interestingly,
the Drosophila immune cells within (glia) and outside (hemocytes) the nervous system
require the same transcription factor glial cells deficient/glial cells missing (Glide/Gcm)
for their development. This raises the issue of how do glia specifically differentiate
in the nervous system, and hemocytes in the procephalic mesoderm. The Repo homeodomain
transcription factor and panglial direct target of Glide/Gcm is known to ensure glial
terminal differentiation. Here we show that Repo also takes center stage in the process
that discriminates between glia and hemocytes. First, Repo expression is repressed
in the hemocyte anlagen by mesoderm-specific factors. Second, Repo ectopic activation
in the procephalic mesoderm is sufficient to repress the expression of hemocyte-specific
genes. Third, the lack of Repo triggers the expression of hemocyte markers in glia.
Thus, a complex network of tissue-specific cues biases the potential of Glide/Gcm.
These data allow us to revise the concept of fate determinants and help us to understand
the bases of cell specification. Both sexes were analyzed.