Abstract
In multicellular organisms morphogenesis results from organized cell proliferation and spatial cell differentiation. A genetic approach to development has the two-fold task of explaining how the genetic information is selected and how it expresses itself in developmental operations. The genetic and developmental analysis of morphogenetic mutants in Drosophila has thrown some light on both problems. Experiments on genetic mosaics confirm the idea that the genetic information is expressed within the cell itself. The behaviour of those morphogenetic mutants in mosaics suggests the existence of at least two classes of morphogenetic genes. Genes of a first group (cyto-differentiation genes) would include those controlling cell behaviour relevant to morphogenesis and common to most developing systems: mitotic rate, mitotic orientation, cell recognition and cuticular differentiation. Those of a second group (selector genes) seem to control developmental pathways and share several operational characteristics. A functional scheme is advanced showing how selector genes may become activated and control development. We postulate that inductor molecules interfere with the products of activator genes which are selector specific. In this way signals extrinsic to the genome become translated into genetic ones. The activation, or repression, of selector genes occurs once in development and remains clonally irreversible. This, possibly, is the genetic basis of a stable state of determination. However, the products of these activated selector genes are required throughout subsequent development in order to maintain a certain pathway. They may function by activating in turn the corresponding cytodifferentiation genes leading to changes in developmental operations.
