Fig. 8. Role of PaxB in cell sorting and slug movement. To assess whether paxB^– strains are defective in sorting we let 5% Ax2 cells expressing the paxB/lacZ construct synergize with paxB^– cells. As can be seen during aggregation (A) the cells are randomly intermingled but at the late aggregate stage (B) the Ax2 cells start to sort to the centre of the cell masses until they are found in the tips of forming slugs (C) and small culminates (D). This clearly shows that paxB^– cells are defective in cell sorting. (E,F) Sorting of 50% paxB^– cells in Ax2 and of another Ax3 paxB knockout mutant in Ax3. As can be seen the majority of the paxB^– cells both in Ax2 and Ax3 sort to the back of the slug indicating that they may be defective in moving in cell masses. (G) a panel showing three culminants from synergy experiments where Ax2 cell synergised with GFP expressing Ax2 cells (left culminant), PaxB-GFP expressing cells synergised with Ax2 (middle culminant) and paxB^– cells expressing a PaxB-GFP fusion protein under the control of a constitutive Actin15 promoter. As can be seen Ax2 cells do not sort in Ax2, while paxB^– cells are confined mostly to the lower cup and basal disk when allowed to develop in an Ax2 environment and that expression of paxB under the control of an string actin15 promoter does rescue development somewhat, i.e. now cells are also found in upper cup, but few cells are found in the tip or in the spore mass. (H) Migration assay comparing the migration of slugs towards a localised light source indicated by an arrow. This experiment compares the migration of Ax2 cells (left), paxB^– cells (middle) and paxB^–[A15/paxB-gfp] cells (right). Ax2 cells show a strong directed migration towards the light. The paxB^– cells are completely unable to migrate while the paxB^–[A15/paxBgfp] cells show a substantial rescue of migration ability, however pooling many experiments shows that migration is not as efficient as that of wild-type cells, implying that proper level of expression of PaxB may be important.