BackgroundThe zona limitans intrathalamica (ZLI) and the isthmus organizer (IsO) are two major secondary organizers of vertebrate brain development. These organizers are located at the interface of the expression domains of key patterning genes (Fezf-Irx and Otx-Gbx, respectively). To gain insights into the evolutionary origin of the ZLI, we studied Fezf in bilaterians.ResultsIn this paper, we identified a conserved sequence motif (Fezf box) in all bilaterians. We report the expression pattern of Fezf in amphioxus and Drosophila and compare it with those of Gbx, Otx and Irx. We found that the relative expression patterns of these genes in vertebrates are fully conserved in amphioxus and flies, indicating that the genetic subdivisions defining the location of both secondary organizers in early vertebrate brain development were probably present in the last common ancestor of extant bilaterians. However, in contrast to vertebrates, we found that Irx-defective flies do not show an affected Fezf expression pattern.ConclusionsThe absence of expression of the corresponding morphogens from cells at these conserved genetic boundaries in invertebrates suggests that the organizing properties might have evolved specifically in the vertebrate lineage by the recruitment of key morphogens to these conserved genetic locations.

The visual system of insects is a multilayered structure composed externally by the compound eye and internally by the three ganglia of the optic lobe: lamina, medulla and the lobula complex. The differentiation of lamina neurons depends heavily on Hedgehog (Hh) signaling, which is delivered by the incoming photoreceptor axons, and occurs in a wave-like fashion. Despite the primary role of lamina neurons in visual perception, it is still unclear how these neurons are specified from neuroepithelial (NE) progenitors. Here we show that a homothorax (hth)-eyes absent (eya)-sine oculis (so)-dachshund (dac) gene regulatory cassette is involved in this specification. Lamina neurons differentiate from NE progenitors that express hth, eya and so. One of the first events in the differentiation of lamina neurons is the upregulation of dac expression in response to Hh signaling. We show that this dac upregulation, which marks the transition from NE progenitors into lamina precursors, also requires Eya/So, the expression of which is locked in by mutual feedback. dac expression is crucial for lamina differentiation because it ensures repression of hth, a negative regulator of single-minded, and thus dac allows further lamina neuron differentiation. Therefore, the specification of lamina neurons is controlled by coupling the cell-autonomous hth-eya-so-dac regulatory cassette to Hh signaling.

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