Formation and specification of distal leg segments in Drosophila by dual Bar homeobox genes, BarH1 and BarH2

Here, we show that BarH1 and BarH2, a pair of Bar homeobox genes, play esse ntial roles in the formation and specification of the distal leg segments o f Drosophila. In early third instar, juxtaposition of Bar-positive and Bar- negative tissues causes central folding that may separate future tarsal seg ments 2 from 3, while juxtaposition of tissues differentially expressing Ba r homeobox genes at later stages gives rise to segmental boundaries of dist al tarsi including the tarsus/pretarsus boundary. Tarsus/pretarsus boundary formation requires at least two different Bar functions, early antagonisti c interactions with a pretarsus-specific homeobox gene, aristaless, and the subsequent induction of Fas II expression in pretarsus cells abutting tars al segment 5. Bar homeobox genes are also required for specification of dis tal tarsi. Bar expression requires Distalless but not dachshund, while earl y circular dachshund expression is delimited interiorly by BarH1 and BarH2.