Expression of zebrafish bHLH genes ngn1 and nrd defines distinct stages of neural differentiation.

Two zebrafish bHLH genes, neurogenin-related gene I (ngn1) and neuroD (nrd), have been isolated. ngn1 expression is initiated at the end of gastrulation in the neural plate and defines broad domains of cells that probably possess an ability to develop as neurons. This finding suggests that ngn1 may play a role during determination of cell fate in neuroblasts. ngn1 and pax-b are expressed in a mutually exclusive manner. nrd expression follows that of ngn1 in restricted populations of cells selected from ngn1-positive clusters of cells. The earliest nrd-positive cells in the brain and the trunk are a subset of the primary neurons. ngn1 is not expressed in the eye. Here, nrd transcription is activated at 25 hours postfertilization in the ventral retina. Expression of islet-1 occurs in nrd-positive cells after expression of nrd, and the expression of the two genes partially overlaps in time. These observations suggest that during eye development nrd expression may follow expression of some other neurodetermination gene(s). This supports the idea that expression of nrd is a necessary step leading toward overt neuronal differentiation.