Drosophila sensory organ cell lineages : asymmetric cell divisions, apoptosis and evolution. V. Orgogozo , Y. Bellaïche , F. Schweisguth. École Normale Supérieure, Paris, France.

   The stereotypic and simple pattern of the Drosophila embryonic peripheral nervous system makes it an ideal system to study the mechanisms of cell fate determination in cell lineages. We describe a cell lineage that generates a single multidendritic (md) neuron (that we named vmd1a) in the ventral region of each abdominal segment A1-A7. This lineage (refered to as md-only lineage) involves two successive asymmetric cell divisions. Following each division, one of the two daughter cells expresses two pro-apoptotic genes, reaper and grim, and subsequently undergoes apoptosis. In reaper- hid- grim- mutant embryos, in which no cell death occurs, the two cells fated to undergo apoptosis divide and together produce an ectopic external sensory (es) organ. This indicates that in the absence of apoptosis, the vmd1a precursor cell follows an md-es lineage (described in Orgogozo et al., Development 128, 631-643). In the md-only lineage, Numb appears to be specifically inherited by the daughter cell that does not undergo cell death. Numb is necessary and sufficient to prevent apoptosis whereas activated Notch is sufficient to trigger death in this lineage. We propose that the md-only lineage is derived from the md-es lineage via cell-specific activation of apoptosis. In segment A8, four precursor cells follow an md-only lineage while their homologous precursor cells follow an md-es lineage in segments A1-A7. Analysis of AbdB mutant indicates that AbdB regulates apoptosis in cell lineages in segment A8. The md-es lineage is very similar to four previously described sensory organ lineages, the adult thoracic bristle, the wing papilla, the poly-innervated labellar taste bristle, and the chordotonal organ lineages, respectively. This suggests that the lineages that produce distinct sensory organs in Drosophila have evolved from the same ancestral lineage through changes in cell type, number of cell divisions, and cell death pattern.