Hierarchical regulation of odorant receptor gene choice and subsequent axonal projection of olfactory sensory neurons in zebrafish.

In both Drosophila and mice, olfactory sensory neurons (OSNs) expressing a given odorant receptor (OR) project axons to specific glomeruli in the antennal lobe or olfactory bulb (OB), developing a topographic odor map. To gain insights into the modes of OR expression and axonal projection in zebrafish, we generated a bacterial artificial chromosome transgenic line carrying an OR gene cluster in which two OR-coding sequences, OR111-7 and OR103-1, were replaced with yellow fluorescent protein (YFP) and cyan fluorescent protein (CFP), respectively. In the transgenic embryos, YFP and CFP signals appear in small populations of OSNs at an early stage of development when OR expression is first observed. Time-lapse imaging of living embryos revealed that both YFP- and CFP-expressing OSNs project axons to the medial portion of the OB. This pattern of axonal projection is maintained in the adult transgenic fish, in which fluorescently labeled OSN axons target a topographically fixed cluster of glomeruli in the medial OB. Because the OR-coding sequences were replaced with fluorescent reporter genes, we examined which OR genes are expressed in YFP/CFP-expressing OSNs and found that the OR choice is mostly restricted to OR members within the same subfamily of the cluster. Furthermore, we found that the one receptor-one neuron rule is not always applicable to zebrafish OSNs and that multiple receptors-one neuron is true for a subpopulation of OSNs in both wild-type and transgenic fish. These data demonstrate two distinct modes of OR expression and suggest a model of the hierarchical regulation of OR gene choice and subsequent axonal projection in the zebrafish olfactory system.