Larval neurogenesis in the copepod Tigriopus californicus (Tetraconata, Multicrustacea).

Arthropod early neurogenesis shows distinct patterns that have been interpreted in an evolutionary framework. For instance, crustaceans and Hexapoda form the taxon Tetraconata and share the differentiation of specific neural precursors, the neuroblasts, a character which sets them apart from Chelicerata and Myriapoda. Neuroblasts are relatively large stem cells that generate ganglion mother cells by asymmetric divisions. Ganglion mother cells typically divide once to give rise to neurons and glia cells. In hexapods, neuroblasts segregate from the neuroectoderm before they begin their characteristic proliferative activity. In the crustaceans studied so far, neuroblasts remain in the neuroectoderm. Yet, detailed studies on early neurogenesis of crustaceans at the cellular level are largely restricted to some malacostracan and branchiopod species. Crustaceans are very diverse and likely paraphyletic with respect to hexapods. Hence, knowledge about neural differentiation in other crustacean taxa might contribute to the understanding of evolution of neurogenesis in Tetraconata. Here, we describe the early neurogenesis during naupliar development of the copepod Tigriopus californicus. We show that neuroblasts are present that generate ganglion mother cells, which in turn divide to give rise to neurons of the ventral nerve cord. These two neural precursor cell types and their specific arrangement correspond to what has been found in other crustaceans. One obvious difference concerns the relative size of the neuroblasts, which are not much larger than their progeny. Our results complement the picture of neural differentiation in crustaceans and suggest that superficially located neuroblasts are likely the ancestral condition in Tetraconata.