Abdominal-B and caudal inhibit the formation of specific neuroblasts in the Drosophila tail region

The central nervous system of Drosophila melanogaster consists of fused segmental units (neuromeres), each generated by a characteristic number of neural stem cells (neuroblasts). In the embryo, thoracic and anterior abdominal neuromeres are almost equally sized and formed by repetitive sets of neuroblasts, whereas the terminal abdominal neuromeres are generated by significantly smaller populations of progenitor cells. Here we investigated the role of the Hox gene Abdominal-B in shaping the terminal neuromeres. We show that the regulatory isoform of Abdominal-B (Abd-B.r) not only confers abdominal fate to specific neuroblasts (e.g. NB6-4) and regulates programmed cell death of several progeny cells within certain neuroblast lineages (e.g. NB3-3) in parasegment 14, but also inhibits the formation of a specific set of neuroblasts in parasegment 15 (including NB7-3). We further show that Abd-B.r requires cooperation of the ParaHox gene caudal to unfold its full competence concerning neuroblast inhibition and specification. Thus, our findings demonstrate that combined action of Abdominal-B and caudal contributes to the size and composition of the terminal neuromeres by regulating both the number and lineages of specific neuroblasts.