Two in vivo images showing the first apterous-expressing cells in the wing discs of two flies expressing UAS CD8-GFP under the control of apGAL4.

The wing disc outlines are indicated by the lighter areas. A: In this wing disc, only a single cell shows apterous expression, indicating that although apterous is a marker for the dorsal compartment, the D/V boundary is not established simultaneously with the first apterous expression. B: two cells show apterous-expression. The rather typical position of the first apterous-expressing cells near the tracheal fork is not in accordance with a direct control of apterous via wingless and vein. The scale bar is 50 µm.