How neural stem cells generate the correct number and type of differentiated neurons in appropriate places is an important question in developmental biology. Although nervous systems are diverse across phyla, many taxa have a larva that forms an anterior concentration of serotonergic neurons, or apical organ. The number of neurons in these organs is highly variable. Previous work demonstrated that the sea star embryo initially has a pan-neurogenic ectoderm, but the genetic mechanism that directs only a subset of these cells to generate serotonergic neurons in a particular location had not been resolved. Here, we show that neurogenesis in the sea star larvae begins with soxc-expressing multipotent progenitors. These give rise to restricted progenitors that express lhx2/9. Soxc- and lhx2/9-expressing cells are capable of undergoing both asymmetric divisions, which allow for progression towards a particular neural fate, and symmetric proliferative divisions. Importantly, we show that nested concentric domains of gene expression along the anterior-posterior (AP) axis, which have been observed in a great diversity of metazoans, control neurogenesis in the sea star larva by promoting particular division modes and progression towards becoming a neuron. This work, therefore, explains how spatial patterning in the ectoderm controls progression of neurogenesis in addition to providing spatial cues for proper neuron location. Modification to the sizes of these AP territories provides a simple mechanism to explain the diversity of neuron number found among apical organs.
- Received January 15, 2016.
- Accepted September 23, 2016.