Proneural function of neurogenic genes in the developing Drosophila eye

The R8-photoreceptor equivalence group in Drosophila: fate choice precedes regulated Delta transcription and is independent of Notch gene dose

It has been suggested that lateral specification of cell fate by Notch signaling depends on feedback on Notch (N) and Delta (Dl) transcription to establish reciprocal distributions of the receptor and its ligand at the protein level. In Drosophila neurogenesis the predicted reciprocal protein distributions have not been observed. Either this model of lateral specification or the description of N and/or Dl protein distributions must be incomplete. We have reexamined R8 photoreceptor specification in the developing eye to resolve this question for this example of lateral specification. N and Dl protein levels were assessed in the cell as a whole and at the cell surface, where these proteins were mostly found at the intercellular cell junctions. Protein levels did not correspond to Notch signaling in wild type. However, Dl transcription and protein levels did correlate with altered N signaling in mutant genotypes. Our findings suggest the difference relates to the speed of lateral specification in vivo. The time required for N signaling to inhibit ato expression was at most 90 min, but changes in the Dl protein distribution in mutant genotypes arose more slowly. N expression was little regulated by N signaling, but protein encoded by the Nts1 allele was temperature-sensitive for appearance at the cell surface. Some aspects of the pattern of Dl protein appeared to be due to endocytosis. We conclude that feedback of N signaling on Dl transcription does occur but is too slow to account for the pattern of R8 specification. Studies of ommatidia mosaic for a Notch duplication, or for the Nts1 allele at semi-restrictive temperatures, found that cells beginning with less N activity were not necessarily predisposed to be selected for R8 differentiation. Our data argue that other signals may be responsible for the pattern of R8 cell fate allocation by N. Potential relevance to other neurogenic regions is discussed.

Xiro3 encodes a Xenopus homolog of the Drosophila Iroquois genes and functions in neural specification

We have identified in Xenopus and in the mouse two highly related genes, Xiro3 and Irx3 respectively, that encode a Drosophila Iroquois-related homeobox transcription factor. Xiro3 in Xenopus and Irx3 in the mouse are expressed early in the prospective neural plate in a subset of neural precursor cells. In Xenopus, injection of Xiro3 mRNA expands the neural tube and induces ectopic neural tissue in the epidermis, based on the ectopic expression of early neural markers such as Xsox3. In contrast, the differentiation of the early forming primary neurons, as revealed by the expression of the neuronal marker N-tubulin, is prevented by Xiro3 expression. Activation of Xiro3 expression itself requires the combination of a neural inducing (noggin) and a posteriorizing signal (basic fibroblast growth factor). These results suggest that Xiro3 activation constitutes one of the earliest steps in the development of the neural plate and that it functions in the specification of a neural precursor state.

Hedgehog is an indirect regulator of morphogenetic furrow progression in the Drosophila eye disc

Pattern formation in the eye imaginal disc of Drosophila occurs in a wave that moves from posterior to anterior. The anterior edge of this wave is marked by a contracted band of cells known as the morphogenetic furrow, behind which photoreceptors differentiate. The movement of the furrow is dependent upon the secretion of the signalling protein Hedgehog (Hh) by more posterior cells, and it has been suggested that Hh acts as an inductive signal to induce cells to enter a furrow fate and begin differentiation. To further define the role of Hh in this process, we have analysed clones of cells lacking the function of the smoothened (smo) gene, which is required for transduction of the Hh signal and allows the investigation of the autonomous requirement for hh signalling. These experiments demonstrate that the function of hh in furrow progression is indirect. Cells that cannot receive/transduce the Hh signal are still capable of entering a furrow fate and differentiating normally. However, hh is required to promote furrow progression and regulate its rate of movement across the disc, since the furrow is significantly delayed in smo clones.

Pattern formation in the basilar papilla: evidence for cell rearrangement

The avian basilar papilla is composed of hair and supporting cells arranged in a regular pattern in which the hair cells are surrounded and isolated from each other by supporting cell processes. This arrangement of cells, in which the apical borders of hair cells do not contact one another, may be generated by contact-mediated lateral inhibition. Little is known, however, about the way in which hair and supporting cells are organized during development. Whole mounts double-labeled with antibodies to the 275 kDa hair-cell antigen and the tight junction protein cingulin were therefore used to examine the development of cell patterns in the basilar papilla. Hair cells that contact each other at their apical borders are seen during early development, especially on embryonic days (E) 8 and 9, but are no longer observed after E12. Hair and supporting cell patterns were analyzed in three different areas of the papilla at E9 and E12. In two of these regions between E9 and E12, the ratio of supporting cells to hair cells does not change significantly, whereas there is an increase in both the number of supporting cells around each hair cell and the number of hair cells that each supporting cell contacts. In the third region examined, there is a dramatic rise in the number of supporting cells around each hair cell, which although accompanied by a small, significant increase in the ratio of supporting cells to hair cells cannot be accounted for by an increase in supporting cell numbers. These data show that a rearrangement of hair and supporting cells with respect to one another may be a fundamental process underlying the development of a regular pattern in the basilar papilla.