The genetic interactions that help establish the PS0/PS1 parasegmental boundary in Drosophila and Tribolium . In Drosophila (A), expression of the mandibular engrailed (en) stripe is dependent on the activity of buttonhead (btd) . In contrast, in Tribolium (B), expression of the mandibular Tc-en stripe is dependent on the combined activity of Tc-kni in the anterior compartment (PS0) (this study) and Tc-eve in the posterior compartment (PS1) . Cap‘n’collar (Cnc) may limit positive regulation of hh by col to the intercalary segment by physically interacting with Col in the mandibular segment . The interactions between the segment polarity genes in the mandibular segment in Tribolium are assumed from what is known in Drosophila. Black symbols: Possible direct genetic interactions. Gray symbols: Direct interactions shown in .