Hemilineage-specific expression of the RHG genes Grim and Reaper sculpt neural network composition during Drosophila neurogenesis

During development, nervous systems generate an incredible diversity of different cell types. One mechanism that appears critical for controlling whether a specific subtype is present or absent, and ensuring that neuronal numbers are appropriate for specific regions, is programmed cell death. Here we show that the precisely patterned hemilineage-based cell death found in the ventral nerve cord of Drosophila melanogaster is controlled by the transcription of the pro-apoptotic RHG genes reaper and grim. Using smFISH we show that reaper and grim, but not hid, are expressed in postembryonic lineages during neurogenesis. By generating novel T2A-GAL4 knock-in tools we have been able to map reaper and grim expression on a lineage-by-lineage basis and show diverse expression patterns within different hemilineage populations. Analysis of null mutants for both genes reveal a division of labour with distinct combinations being important in sculpting the nervous system. Investigation of a specific sexually dimorphic lineage reveals temporal patterning of hemilineage-specific programmed cell death through Grim and Reaper function underpins the development of sex-specific circuitry in the VNC. Our findings suggest that the precisely patterned expression of these pro-apoptotic genes is a critical fate determinant that generates segmental-specific circuity and instructs the appropriate assembly of sexually dimorphic neural circuits in the VNC.