Embryonic development of the nervous system in the planarian Schmidtea polychroa.

The development of a nervous system is a key innovation in the evolution of metazoans, which is illustrated by the presence of a common developmental toolkit for the formation of this organ system. Neurogenesis in the Spiralia, in particular the Platyhelminthes, is, however, poorly understood when compared with other animal groups. Here, we characterize embryonic neurogenesis in the freshwater flatworm Schmidtea polychroa and analyze the expression of soxB and a set of proneural bHLH genes, which are gene families with a well-established role in metazoan early neural development. We show that the nervous system is fully de novo assembled after the early embryo ingests the maternal nutrients. At early stages of neurogenesis, soxB1 genes are expressed in putative neural progenitor cells, whereas soxB2 and neural bHLH genes (achaete-scute, neuroD and beta3) are associated with late neurogenesis and the specification of neural subpopulations of the central and peripheral nervous system. Our findings are consistent with the role of proneural genes in other bilaterians, suggesting that the ancestral neural-specific gene regulatory network is conserved in triclads, despite exhibiting a divergent mode of development.