Diversity of expression of engrailed-like antigens in zebrafish.

We have studied developmental expression of zebrafish engrailed-like (Eng) antigens. Many cell types are reproducibly labeled by two antibodies that recognize the Eng homeodomain, but other cells are labeled by only one or the other, suggesting a hitherto unrecognized complexity of Eng proteins. Expression patterns vary remarkably according to cell type and location. In the undifferentiated primordia of the brain and of each myotome, expression by a stripe of cells spatially subdivides the primordium at a location where a morphological boundary forms later, suggesting expression may be required for development of the boundaries. Supporting this hypothesis, trunk myotomal cells that express Eng are absent in spt-1 mutant embryos, just where the myotomal boundaries fail to form. Another pattern is present in rhombomeres, pharyngeal arches, and the pectoral girdle. In each of these cases, cells (neuron, muscle, cartilage) generating a subset of a series of repeated elements selectively express Eng. These subsets then form specialized derivatives, suggesting Eng homeoproteins are involved in determining the specializations. Epidermal expression is present in the ventral half of the pectoral fin rudiment, precisely 'compartmentalizing' the fin. Neuronal cells at a certain dorsoventral level in each hindbrain and spinal cord segment selectively express Eng, suggesting segmental control of neuronal identity. Specific expression patterns are observed in taste buds, otic vesicles and teeth. Thus we propose that eng genes function in diverse cell types in zebrafish, but play selector roles that can be classified into a few basic types.