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

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Abstract
SYNOPSIS. A mechanism is proposed for the activation of particular genes during interpretation of positional information and during sequential addition of new structures during outgrowth. The primary event is assumed to be an alternation between two states. The alternation of anterior-posterior-anterior compartments in insects or bone-joint-bone in the chicken wing is assumed to be a trace of this primary subdivision into two alternative states. Each (or each second) transition from one state to the other causes a switch from one structure-controlling gene to the next. This enables a counting of alternations on the DNA-level and a high resolution in the corresponding activation of control genes. The model explains the formation of compartments and segmental specification in perfect register. The elements of the bithorax gene complex become understandable assuming this mechanism. It is shown further that such a “compartmentalization” can be used for the reproducible generation of subpatterns for the finer subdivision of a developing embryo. By “cooperation of compartments,” a cone-shaped morphogen distribution can be generated, accounting, e.g., for the circular arrangement of structures in the fate map of the leg disk of Drosophila. The most distal structures are formed at the intersection of compartments and distal transformation occurs whenever cells of all three or four major compartments are close to each other.

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