Abstract
Cnidarian nervous systems are traditionally described as diffuse nerve nets lacking true organization. However, there are examples of stereotypical structure in the nerve nets of multiple cnidarian species that suggest nerve nets are organized. We previously demonstrated that the NvashA target gene NvLWamide-like is expressed in a small subset of the Nematostella nerve net and speculated that observing a few neurons within the developing nerve net would provide a better indication of potential stereotypy. Here we document NvLWamide-like expression more systematically. NvLWamide-like is initially expressed in the typical neurogenic salt and pepper pattern within the ectoderm at the gastrula stage, and expression expands to include endodermal salt and pepper expression at the planula larval stage. Expression persists in both ectoderm and endoderm in adults. We generated an NvLWamide-like::mCherry transgenic reporter line to visualize the neural architecture. NvLWamide-like is expressed in six neural subtypes identifiable by neural morphology and location. Upon completing development the numbers of neurons in each neural subtype are minimally variable between animals and the projection patterns of each subtype are consistent. Between the juvenile polyp and adult stages the number of neurons for each subtype increases. We conclude that cnidarian nerve nets are organized, develop in a stereotyped fashion, and that one aspect of generating the adult nervous system is to modify the juvenile nervous system by increasing neural number proportionally with size.
