Homoeotic and atavic mutations in insects

A comparative analysis of the known types of homoeotic mutants in insects uncovers the existence of invariants in the homoeotic transformations. Mutations change individual segments and individual compartments into one another leaving other developmental coordinates unchanged. Clonal analysis of some favorable mutants shows that the homoeotically transformed organ or region (allotype) and the organ or region it mimics (telotype) are developmentally identical. From this it is concluded that the function of the wildtype allele is to repress the developmental characteristic of the telotype in the affected organ or region (autotype). Since homoeotic mutations are specific to compartments and double mutants show an additive effect, it is concluded that their wildtype genes act in a combinatorial way, defining a given developmental pathway. The establishment and maintenance of a given step in this pathway may be affected by mutations in different loci. Genetic and developmental analyses of these mutations suggest that they are related in a sequence of regulatory steps. The consideration of these findings leads to an operational model for explaining genetic control of developmental pathways during ontogenesis and evolution.