In both vertebrates and invertebrates, neuronal progenitor and stem cells divide to give rise to lineally-related groups of neurons that adopt similar morphologies and are thought to act in the same or parallel circuits to regulate specific behaviors. Animal behavior, therefore, appears encoded, to a significant extent, within the population of stem cells. We take a lineage-based approach to study how a complex nervous system is assembled during development.

The precise and largely stereotyped connectivity patterns of neurons underlie simple knee-jerk-like reflexes and complex behavior, like playing the violin. While we have a good understanding of the conserved genetic and molecular mechanisms that drive the initial steps of nervous system formation, we possess a far more rudimentary knowledge of those that drive neural circuit formation and animal behavior. By focusing on the development and function of the Drosophila adult ventral nerve cord (VNC), which controls behaviors, such as walking, flying, and grooming, our research leverages the power of the fly model system to dissect the genetic and cellular basis of neural circuit formation and behavior.

Like the vertebrate spinal cord, the Drosophila adult VNC is composed of segmentally repeated pools of lineally related neurons. In Drosophila, these pools of neurons are termed hemilineages (see below) and are the basic developmental and functional unit of the VNC.

The adult fly VNC is composed of ~16,000 neurons, all of which arise from a set of 30 segmentally repeated paired neural stem cells and one unpaired neural stem cell (called neuroblasts [NBs]). Each NB creates a unique set of progeny via two rounds of proliferation: a brief embryonic phase of a few hours generates neurons of the larval VNC, and an extended postembryonic phase, lasting a few days, generates neurons of the adult VNC . The extended period of neurogenesis and neuronal maturation during adult VNC formation allows many experimental manipulations that are not feasible during the rapid period of neurogenesis in the embryo.

NBs divide in a stem-cell manner to renew themselves and create a secondary precursor cell, which divides via Notch-mediated asymmetric cell division to generate two neurons with distinct identities (See Figure 1). After many rounds of cell divisions, each NB produces two distinct hemilineages of neurons, termed the Notch-ON or ‘A’ hemilineage and the Notch-OFF or ‘B’ hemilineage. Due to the elimination of some NBs and hemilineages during development, only 34 of the 62 possible hemilineages per hemisegment survive post-embryonically to form the adult VNC. These lineages interconnect in a stereotyped manner to govern a large repertoire of behaviors, such as walking, grooming, and flight. Currently, however, we lack a clear understanding of the behaviors each lineage regulates, the neural circuits each lineage resides in, and most of all the genes that act within each lineage to regulate its connectivity and associated behaviors. Addressing these knowledge gaps is the focus of our research.

To build the descriptive foundation needed to dissect the genetic basis of behavior, we created a high-resolution, descriptive understanding of all neuronal lineages of the adult VNC. We mapped the embryonic stem cell origin, axonal projection pattern, transcription factor expression, and neurotransmitter usage of all 34 hemilineages. We uncovered that hemilineages in the VNC resemble cardinal classes of neurons in the vertebrate spinal cord: Neurons in individual cardinal classes and hemilineages typically acquire similar fates and morphologies and act in the same or a parallel circuit. Both flies and vertebrates then employ a lineage-based mechanism to organize CNS connectivity. The simplicity of the fly CNS and power of Drosophila genetics make the adult fly VNC a powerful system in which to clarify how lineally related sets of neurons form neural circuits and drive animal behavior.