Modes of morphogen cooperation for limb formation in vertebrates and insects

Diffusing morphogens in cooperation can control gene expression in developi ng limbs. Additive cooperation corresponds to the Boolean operator OR and i mplies the equivalent action of the (suitably scaled) concentrations of two morphogens, either by their alternative binding to the same receptor or by another way of convergence of their effects during the signal transduction procedure. This cooperation can explain the spatial and temporal collinear ities of the expression of herd genes in the vertebrate limb bud. A multipl icative cooperation of morphogens (corresponding to the Boolean operator AN D), produced at the DPP and WG domains in the Drosophila leg imaginal disc, may account for the expression domains observed for Dll and dac. A molecul ar interpretation of the multiplicative morphogen cooperation is proposed. Some experiments are suggested for further testing of the model. (C) 1999 A cademic Press.