In the adult olfactory bulb (OB), particular chemical classes of odorants preferentially activate glomeruli within loosely defined regions, resulting in a coarse and fractured "chemotopic" map. In zebrafish, amino acids and bile acids predominantly stimulate glomeruli in the lateral and medial OB, respectively. We studied the development of these spatial response maps in zebrafish. At 3 d postfertilization (dpf), the OB contained protoglomerular structures that became refined and more numerous during subsequent days. In a transgenic zebrafish line expressing the Ca 2ϩ indicator protein inverse pericam, mainly in mitral cells, odor responses in the OB were first detected at 2.5-3 dpf. Already at this stage, amino acids and bile acids evoked activity predominantly in the lateral and medial OB, respectively. Twophoton Ca 2ϩ imaging using a synthetic indicator was used to reconstruct activity patterns at higher resolution in three dimensions. Responses to amino acids and bile acids were detected predominantly in the lateral and medial OB, respectively, with little overlap. Between 2.5 and 6 dpf, the number of odor-responsive units increased, but the overall spatial organization of activity persisted. Hence, a coarsespatialorganizationoffunctionalactivitymapsisestablishedveryearlyduringOBdevelopmentwhenglomeruliarenotyetdifferentiated. This spatial organization is maintained during development and growth of neuronal circuits and may have important functions for odor processing in larvae, for the differentiation of glomeruli, and for the refinement of activity maps at later developmental stages.

Background: Investigating the architecture of gene regulatory networks (GRNs) is essential to decipher the logic of developmental programs during embryogenesis. In this study we present an upstream survey approach, termed trans-regulation screen, to comprehensively identify the regulatory input converging on endogenous regulatory sequences.

There was an error published in Development 138, 755-765.On p. 756, the actin:GAL4 transgenic strain was erroneously attributed to Scheer and Campos-Ortega (1999). This line [which now appears on ZFIN with designation Tg(actc1b:GAL4) i269 ] should have been attributed to Sudipto Roy and Franco di Giovine who generated it in the laboratory of Philip W. Ingham.The authors apologise to readers for this mistake.

There was an error published in Development 138, 755-765.On p. 756, the actin:GAL4 transgenic strain was erroneously attributed to Scheer and Campos-Ortega (1999). This line [which now appears on ZFIN with designation Tg(actc1b:GAL4) i269 ] should have been attributed to Sudipto Roy and Franco di Giovine who generated it in the laboratory of Philip W. Ingham.The authors apologise to readers for this mistake.