The Role of Compartmentalization in the Activation of Particular Control Genes and in the Generation of Proximo-Distal Positional Information in Appendages

SYNOPSIS. A mechanism is proposed for the activation of particulargenes during interpretation of positional information and duringsequential addition of new structures during outgrowth. Theprimary event is assumed to be an alternation between two states.The alternation of anterior-posterior-anterior compartmentsin insects or bone-joint-bone in the chicken wing is assumedto be a trace of this primary subdivision into two alternativestates. Each (or each second) transition from one state to theother causes a switch from one structure-controlling gene tothe next. This enables a counting of alternations on the DNA-leveland a high resolution in the corresponding activation of controlgenes. The model explains the formation of compartments andsegmental specification in perfect register. The elements ofthe bithorax gene complex become understandable assuming thismechanism. It is shown further that such a "compartmentalization" can beused for the reproducible generation of subpatterns for thefiner subdivision of a developing embryo. By "cooperation ofcompartments," a cone-shaped morphogen distribution can be generated,accounting, e.g., for the circular arrangement of structuresin the fate map of the leg disk of Drosophila. The most distalstructures are formed at the intersection of compartments anddistal transformation occurs whenever cells of all three orfour major compartments are close to each other.