Figure 6.

Proximal–distal axis positional values and formation of compartment borders. In wild-type wings, the positional values along the proximal–distal axis (vertical bars) are correctly established by normal cells and the clones are elongated along the proximal–distal axis. Compartment boundaries (restiction boundaries = RB) in wild-type wings are the consequence of the high positional values reached by the cells in characteristic regions associated with the confrontation of cells expressing different selector genes (represented as squares with differential shading). The clones (horizontal arrows) never cross these compartment boundaries. In nub wings the proximal–distal positional values are not correctly established and nub cells are unable to reach the highest distal values, provoking the growth of the clones perpendicular to the A–P boundary. Clones in nub wings cross the compartment boundaries as a consequence of the inability of nub cells to meet the high positional values required for clonal restrictions.