ZEBRAFISH TRKC1 AND TRKC2 RECEPTORS DEFINE 2 DIFFERENT CELL-POPULATIONS IN THE NERVOUS-SYSTEM DURING THE PERIOD OF AXONOGENESIS

We identified previously fire distinct trk genes in the zebrafish. The structures of two of these, TrkC1 and TrkC2, are most similar to mamm alian TrkC. Detailed sequence comparisons reported here indicate that although the similarities to TrkC are greatest in those regions of the extracellular domain implicated in ligand binding, the two sequences also differ significantly in these regions. Whole-mount in situ hybrid ization experiments in the early embryo revealed full-length trkC1 but no trkC2 transcripts in the cranial ganglia and in a subset of Rohon- Beard neurons. At the same time, full-length trkC2 but no trkC1 transc ripts were detected laterally in the spinal cord, in the caudal hindbr ain, in reticulospinal neurons of rhombomeres 4, 5, and 6, and in the midbrain. Both types of transcripts were expressed in clusters of cell s in the dorsal telencephalon and the nucleus of the tract of the post optic commissure. These results suggest distinct functions of trkC1 an d trkC2 in nervous system development. The expression patterns define two different neuronal populations in the Zebrafish. (C) 1998 Academic Press.