Fig. 7.

Model for the neurogenic pathways defined in this work. SoxC was previously identified as being dependent on Six3. SoxC expression is activated in isolated neural precursors by unidentified factors and cell signaling mechanisms (dotted arrow). Cells expressing SoxC are dividing (circular arrow) and at least some express Delta and are subject to lateral inhibition. SoxC is required for the development of neurons expressing serotonin and Tph (upper pathway) and also for all other neurons expressing SynB (lower pathway). SoxC is required for expression of Brn1/2/4 in both pathways; SoxC is transiently expressed and some cells are observed to contain both mRNAs. Brn1/2/4 also is transiently expressed, and in later embryos some Brn1/2/4-expressing cells also express the differentiated marker SynB. Z167 is expressed only in developing serotonergic neurons (upper pathway) and is essential for their development. Its position in the hierarchy relative to Brn1/2/4 remains to be defined, but Z167 is likely to function in parallel or downstream.