Asynchronous onset of odorant receptor expression in the developing zebrafish olfactory system.

The functional identity of an olfactory neuron is determined in large part by the odorant receptors it expresses. As an approach toward understanding the events that underlie the specification of olfactory neurons, we have examined the patterns of odorant receptor gene expression in the developing zebrafish. Surprisingly, we find that the onset of specific odorant receptor expression occurs asynchronously in the developing olfactory placode. Our results suggest that odorant receptor expression is not strictly stochastic, but rather is governed by temporally regulated cues during development. Moreover, by restricting the number of receptor genes competent for transcription at different times of development, temporal waves of expression may provide a mechanism for simplifying the regulation of the large odorant receptor gene family.