Fig. 4. Bar is the limiting factor for the development of the fourth tarsal segment. (A) Leg of an InB^M2 mutant. This mutation produces partial loss of Bar function (Kojima et al., 2000). In these mutants, 33% of the legs show a weak fusion between the fourth and fifth tarsal segment, with the joint not properly differentiated (inset, arrowhead). (B) Leg of a InB^M2; ap^UGO /+ mutant. Removal of a copy of ap enhances the mutant phenotype observed in InB^M2 mutants. Tarsus four and five are shorter and are fused in 61% of the legs (inset, arrowhead; compare with A). (C) The ap mutant phenotype is also enhanced by reducing Bar function in InB^M2; ap^UGO /apGal4 flies (compare with Fig. 2A). Tarsus four is completely absent (inset), and even the joint between tarsus five and three is affected (arrowhead). (D) Overexpression of Bar does not repress Ap expression in the fourth tarsal segment (arrow). (E) Leg of an apGal4;UAS-Bar fly. Overexpression of Bar prevents the development of the fourth tarsal segment. (F) Overexpression of Ap rescues completely the phenotype caused by overexpression of Bar in the ap domain (compare with E). (G) Ectopic expression of a hybrid molecule, consisting in the LIM domains of Lim3 and the Ap homeodomain, completely rescues the loss of tarsus four phenotype produced by overexpression of Bar (compare with E). (H) Rescue of the apGal;UAS-Bar phenotype is also achieved by overexpression of Chip. (I) Ectopic expression of Chip lacking the LIM interaction domain is not able to rescue the dominant-negative effect produced by Bar overexpression. (J) Ectopic expression of the Chip-Ap hybrid protein partially rescues the apGal4;UAS-Bar phenotype, suggesting that Ap interacts with Chip to form dimers in tarsus four. However, the hybrid protein does not rescue as efficiently as the Ap and Chip wild-type proteins (compare with F, G and H).