Co-operation between the Activin pathway and Tutl in the tiling of Drosophila R7 photoreceptor axons. Activin functions as an autocrine signal to activate its receptor Babo on R7 terminals, which in turn induces the phosphorylation of Smad2. The phosphorylated Smad2 is then transported by Importin α3 into the nucleus to regulate the expression of some unknown target genes, which directly control R7 terminal growth. This Activin-mediated intrinsic growth control functions together with Tutl-mediated homotypic interaction to regulate R7 tiling. Circles, Ig domains; Rectangles, fibronectin type-III repeats.