Dynamic domains of gene expression in the early avian forebrain

The expression domains of genes implicated in forebrain patterning often sh are borders at specific anteroposterior positions. This observation lies at the heart of the prosomeric model, which proposes that such shared borders coincide with proposed compartment boundaries and that specific combinatio ns of genes expressed within each compartment are responsible for its patte rning. Thus, genes such as Emx1, Emx2, Pax6, and qin (Bf1) are seen as bein g responsible for specifying different regions in the forebrain (diencephal on and telencephalon). However, the early expression of these genes, before the appearance of putative compartment boundaries, has not been characteri zed. In order to determine whether they have stable expression domains befo re this stage, we have compared mRNA expression of each of the above genes, relative both to one another and to morphological landmarks, in closely st aged chick embryos. We find that, between HH stage 8 and HH stage 13, each of the genes has a dynamic spatial and temporal expression pattern. To test for autonomy of gene expression in the prosencephalon, we grafted tissue f rom this region to more caudal positions in the neural tube and analyzed fo r expression of Emx1, Emx2, qin, or Pax6. We find that gene expression is a utonomous in prosencephalic tissue from as early as HH stage 8. In the case of Emx1, our data suggest that, from as early stage 8, presumptive telence phalic tissue also is committed to express this gene. We propose that early patterning along the anteroposterior axis of the presumptive telencephalon occurs across a field that is subdivided by different combinations of gene s, with some overlapping areas, but without either sharp boundaries or stab le interfaces between expression domains. (C) 2001 Academic Press.