Glia in the chiasms and medulla of the Drosophila melanogaster optic lobes.

Different classes of glia cells in the optic lobes of Drosophila melanogaster were defined by the enhancer trap technique, using expression of the lacZ reporter gene. At both the outer and inner optic chiasms, there are stacks of glia, arrayed from dorsal to ventral, interpersed between the crossings of axonal fiber bundles. The giant glial cells of both the outer and inner chiasms are similar with respect to their nuclear shapes and positions, indicating similar functions of these cell types. Another class of glia is found in the medulla neuropil. Their cell bodies anchor in the most distal region of the neuropil, and their processes extend into the deeper neuropil layers. Birth dating using BrdU shows that both groups of chiasm glia are born early in larval life; they may participate in the development of the optic lobe. The medulla glia are born later and may be involved primarily in adult functions. In the wild type, and in mutants with structurally altered optic lobes, the numbers of tract-associated glial cells in the outer and inner optic chiasms seem to vary with the number of visual columns, whereas the complement of medulla neuropil glia correlates with the volume of the optic lobe.

Genetic and developmental analysis of irreC, a genetic function required for optic chiasm formation in Drosophila.

Irregular chiasm C (irreC) is an X-linked genetic function necessary for the correct projection of visual fibers in the optic chiasms of Drosophila optic ganglia. In addition to a severe disorganization of the inner optic chiasm irreC mutants display a subtle phenotype in the outer optic chiasm, in which some bundles of axons that leave the posterior equatorial part of the lamina on their way to the anterior medulla take a long detour before eventually finding their specific targets in the medulla neuropile. Deletion and recombination mapping of two irreC alleles (one P-element induced, the other associated with an inversion) have yielded a precise cytogenetic location in 3C4-5. A complex complementation pattern between roughest (rst) and irreC alleles indicates that both genetic functions are structurally and/or functionally closely interrelated. Flies in which the irreC locus is completely deleted by overlapping deficiencies are viable and their defects in the optic chiasms are similar to those seen in the two alleles. The defects in the outer and inner optic chiasms are not epigenetically connected and mosaic analyses have shown them to be independent from the genotype of the compound eye. Although the larval visual nerve looks normal, we have found that in the optic lobes of irreC mutants a group of early differentiating larval neurons is misplaced, suggesting a pioneering function of these cells during organization of the outer optic chiasms.

Pre-existing neuronal pathways in the developing optic lobes of Drosophila.

We have identified a set of larval neurones in the developing adult optic lobes of Drosophila by selectively labelling cells that have undergone only a few mitoses. A cluster of three cells is located in each of the optic lobes near the insertion site of the optic stalk. Their axons fasciculate with fibres of the larval optic nerve, the Bolwig's nerve, and then form part of the posterior optic tract. These cells are likely to be first order interneurones of the larval visual system. Unlike the Bolwig's nerve, they persist into the adult stage. The possibility of a pioneering function of the larval visual system during formation of the adult optic lobe neuropil is discussed.