Odorant Receptor Gene Regulation in the Zebrafish, Danio rerio

The sense of smell enables an organism to extract vital information from the composition of its odor environment. Each olfactory receptor neuron faces the daunting task of selecting one odorant receptor (OR) gene from the vast array of about 100 and 1,000 OR genes in fish and mammals, respectively. Thus far, the control mechanism of this highly differential gene expression is largely unknown. Silke Argo evaluates several features of OR genes in the olfactory system of a microsmatic vertebrate, the zebrafish, Danio rerio, in order to assess the regulation of OR gene expression: The temporal expression pattern of endogenous zebrafish OR (ZOR) genes is investigated by a detailed quantitative analysis. The genomic arrangement of ZOR genes is elucidated by a linkage analysis and screen of a zebrafish genomic DNA library. Moreover, the author identifies two putative ZOR gene promoter regions and demonstrates the functionality by means of their ability to drive expression of the GFP reporter gene in vivo. Finally, for the first time the axonal pattern of olfactory receptor neurons that are transgenic for a particular ZOR gene promoter/GFP construct is shown. Based on these observations a two-step model for a mechanism of OR gene regulation is suggested, which comprises as a first step the determination of a group of OR genes to be accessible for activation and as a second step a stochastic selection that achieves the exclusive expression of a single OR gene of the group. Restriction of the large array of odorant receptors to groups of genes might facilitate the setup and maintenance of the functional connections between the olfactory epithelium and the olfactory bulb and thus may constitute a general mechanism of vertebrate OR gene regulation. The author: born in 1968; studied biology and teaching profession for biology and German at Eberhard-Karls-Universität Tübingen, Germany, and at California Polytechnic State University of San Luis Obispo, USA; Diploma thesis at Max-Planck-Institute for Developmental Biology in Tübingen, Germany; PhD at the Institute for Genetics at the University of Cologne, Germany.