Fig. 7. Clonal analysis of Su(H) during SOP development of the machrochaete. (A-E) An example of a large clone that encompasses part of the PPA, tr2/APA+tr1 and DC clusters. (A) Overview. Expression of the SOP-E is shown in blue and that of Hnt in red. The clones are labelled by the absence of the GFP marker. The arrowhead indicates the region that is shown at higher magnification in B-E. (B) Expression of Hnt is detected in single cells highlighted by the arrowhead and arrows. The cell highlighted by the arrowhead expresses high levels of Hnt. (C) Channel revealing the clone area by the absence of GFP. Arrowhead indicates a large GFP-positive cell that is located in the mutant territory. (D) Expression of the SOP-E is detected in groups of cells. (E) The pseudo-colour composite of the single channels shown in B-D. Green, GFP expression; red, Hnt expression; blue, SOP-E expression. The picture reveals that the Su(H) mutant cells of the DC cluster do express the SOP-E but not Hnt. Hnt is restricted to the green wild-type cell at the boundary of the clone. In the two other clusters, only one mutant cell (arrows in B,E) weakly express Hnt. This shows that the activity of Su(H) is required for the SOP to express Hnt. However, the requirement for Su(H) among cells of a proneural cluster seems to vary, as indicated by the weak expression of Hnt in a single mutant cell in the two cluster labelled by the arrows. Another important observation revealed by this figure is that wild-type cells can develop as SOP, even if located adjacent to Su(H) mutant cells. The arrowheads in B-E label a wild-type cell that, as the SOP-E expression reveals, is part of the otherwise Su(H) mutant DC cluster. This cell is the only cell that expresses Hnt, indicating that it has chosen the SOP fate, although surrounded by mutant cells. Hence, the Su(H) mutant cells are not able to inhibit the cells from adopting the SOP fate. The expression of Hnt in one cells of each of the two mutant clusters (arrows) also suggest that cells at specific positions in the clusters have a higher inclination to adopt the SOP fate. (F,G) Cells at specific positions within a proneural cluster are determined to adopt the SOP fate. (F) Expression of Hnt in cells of the DC cluster of the late third larval instar stage, where the posterior part consists of Su(H) mutant cells. The expression of SOP-E in this disc is shown in the insert. (G) Expression of the SOP-E (blue), hnt (red) in the same disc shown in F. The Su(H) mutant area is labelled by the absence of GFP (green). Two hnt-positive cells are detectable at the aDC position. These cells have switched off the SOP-E, indicating that these cells are the second order precursors of the aDC. In the mutant territory a single hnt-expressing cell is detectable. This cell is located at the position of the pDC and still expresses the SOP-E. The pDC develops earlier than aDC during normal development. Hence, it appears that the Su(H) mutant SOP has arrested its development at an early stage. For further information, see text. The phenotype of Su(H) mutant clones varies also among proneural clusters. An example is the SC cluster shown in H,I. (H) Expression of neur^A101 in a Su(H) mutant clone that includes the SC cluster. Many, if not all cells of the cluster express this early marker. (I) By contrast, only a fraction of these cells also express Hnt, often at low levels. (J) Expression of Hnt in kuz^ES24 mutant proneural clusters. Expression of Hnt is shown in red, the clones are revealed by the absence of the green GFP marker. Hnt is expressed in probably all mutant cells of the proneural clusters. This suggests that, as in the case of homozygous kuz mutants, the cells of kuz mutant clusters do not arrest their development as SOP.

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