How serially homologous neuroblasts produce different temporal cohorts along the Drosophila larval body axis

Abstract

Neural stem cells are inherently flexible, producing different lineages of neurons in different contexts; yet in vivo, development must be highly regulated because aberrant stem cell development leads to microcephalies, tumors, and neurodevelopmental disorders. However, the full complement of in vivo flexibility in neural stem cell development remains poorly understood. Within the Drosophila CNS, 30 types of neural stem cells or neuroblasts repeat in each segment with segment-specific differences in development. We studied NB3-3 neuroblast in each of the 14 segments where it is present. In different segments, NB3-3 produces different types of neurons in different numbers but always produces Even-skipped Lateral (EL) neurons. Prior work showed that in abdominal segments, ELs comprise two types of temporal cohorts. Temporal cohorts are small sets of neurons within a lineage that are born within a tight time window. In the abdomen, EL neurons in the early-born temporal cohort all receive mechanosensory input, whereas EL neurons in the late-born temporal cohort all receive proprioceptive input. Herein, we find that NB3-3 generates an early-born EL temporal cohort in all segments. In specific regions, NB3-3 neuroblasts produce additional types of temporal cohorts, including but not limited to the late-born EL temporal cohort. We show that variations in NB3-3 division pattern determine the number and type of temporal cohorts. Later in development, the number of neurons within a temporal cohort is refined by a variety of post-mitotic mechanisms: regulation of molecular identity markers, cell death, and differentiation status, acting in complex combinations in different segments. Post-mitotic refinement of neuron numbers occurs most often for neurons of the early-born EL temporal cohort. Further, neurons of the early-born EL temporal cohort are incorporated into different circuits, depending on segment. Together, our results show how a single neural stem cell type can flexibly generate segment-specific lineages and contribute to various circuits. We offer the following framework for understanding how a given type of neural stem cell generates diverse temporal cohorts: a stem cell type invariably generates one type of temporal cohort, regardless of location or pattern of division. For neurons in this invariant temporal cohort, post-mitotic mechanisms introduce diversity in number. The stem cell, on a segment-by-segment basis, can also produce additional types of temporal cohorts simply by altering its division pattern. This framework is valuable for motor circuit development, the development of sexual dimorphisms, nervous system evolution, and in vitro stem cell applications.