Fig. 5. innexin 2 transcription is induced by Wingless signalling. The ß-Gal expression of the kropf^P16 line in stage 15 embryos of wild-type (A,C), homozygous wingless mutants (B) and in embryos in which ß-Gal has been ectopically expressed by using the twi-Gal4 driver and the UAS-wingless effector lines (see text). Note that reporter gene expression is abolished in the proventriculus region in wingless mutants (compare regions marked by arrows in A,B), whereas it is dramatically expanded in the anterior and posterior region in the Wingless overexpression experiment (compare region marked by arrows in C,D). (E) RT-PCR experiments in Drosophila tissue culture S2 cells (Materials and Methods). Schneider S2 cells were either transfected with an expression vector for Armadillo, the ß-catenin homologue that has been shown to serve as the transducer of the Wingless signalling cascade (pIBArm) or with the vector alone (pIB). innexin 2 mRNA levels were monitored by performing RT-PCR analysis (actin mRNA levels served as an internal control). Note that innexin 2 mRNA levels are increased approximately five times in response to Armadillo when compared with the control reaction. (F) The regionalisation of developing gut tube. The foregut (fg) and the hindgut (hg) are of ectodermal (brown) and the midgut (mg) of endodermal origin (blue). At the ectoderm/endoderm boundaries of the gut, signalling centres arise that control the development of the proventriculus (pv) and the small intestine (si) in the anterior region and in the posterior regions, respectively. A working model suggests that Wingless (green) activates innexin 2 transcription (red) via the signal transducer Armadillo (right) in the boundary regions of the gut. Note, however, that the arrows do not imply direct molecular interactions (see Discussion). This induction may be required for enhanced gap junctional communication during the morphogenetic processes in both gut parts.