GRADIENTS IN THE EXPRESSION OF CELL-SURFACE GLYCOPROTEINS IN A SIMPLE TISSUE, THE DICTYOSTELIUM-DISCOIDEUM SLUG

Gradients in diffusible morphogens have long been postulated to have a role in the establishment of polarity and pattern formation in developing animal embryos. In the cellular slime mould Dictyostelium discoideum, several morphogens have been described and their interactions and location in the multicellular 'slug' stage are being studied. Here we report that two cell surface antigens, which identify prespore and prestalk cells, respectively, are present at different levels on the surface of cells depending on their position within the slug. This is the first evidence of the existence of gradients in cell surface molecules in Dictyostelium discoideum tissues. The level of prespore glycoprotein PsA on the surface of prespore cells increased progressively along the length of the slug, so that the rear cells had the highest surface density of this glycoprotein. Since the pspA gene, which encodes PsA, is cAMP-regulated, this raises the possibility that expression of PsA reflects an underlying morphogenetic cAMP gradient. The level of the MUD9 antigen, which is prestalk-enriched, decreased on the surface of prestalk cells towards the rear of the slug. As this correlated with a decrease in size of these cells along the length of the slug, it is possible that the surface density of this antigen is approximately constant. We have also identified a population of prestalk-like cells that lack both the PsA glycoprotein and the MUD9 antigen. These cells primarily occurred in the posterior of the slug and could represent the population of cells that form the basal disc.