THE ALL-OR-NONE RULE IN MORPHOGENETIC ACTION OF JUVENILE-HORMONE ON INSECT EPIDERMAL-CELLS

We have investigated ultrastructural changes in the integuments of lar val-adult and larval-pupal intermediates produced by exogenous applica tion of juvenile hormone (JH) analogues in Pyrrhocoris apterus (Hemipt era), and Galleria mellonella and Manduca sexta (Lepidoptera). Ultrast ructural analysis of the epidermis of these intermediates always revea led the presence of only two types of epidermal cell, which produced m orphologically perfect cuticles of the previous and future development al stages. There were no intermediate cuticles at the level of individ ual cells. It has been determined that a single epidermal cell constit utes the lowest elementary unit in the perception and realization of t he developmental messages conveyed by JH to its target tissues. Furthe r investigations revealed that the responses of individual epidermal c ells to JH were strictly autonomous and qualitative, i.e. they were ex ecuted according to the 'yes-or-no' or 'all-or-none' rule. The neighbo uring epidermal cells could realize independently, side-by-side, the q uite dissimilar +JH (somatic growth) or -JH (metamorphosis) developmen tal programmes, although each of them formed biochemically, functional ly, and ontogenetically different structures. The qualitative on-and-o ff signal given by JH for induction of the stationary (+JH) developmen tal cycle was limited to relatively short, genetically determined, and stage-specific developmental periods of cellular susceptibility to JH . The mosaic mixtures of the heterochronic, larval-pupal or adult epid ermal cells, which we found in different proportions on the bodies of the intermediates, revealed two variable, development-related factors: (i) the presence or absence of a minimum effective concentration of J H, and (ii) positive or negative sensitivity of a particular epidermal cell to JH.