A possible morphogen controlling differentiation in Dictyostelium.

The complex morphology of a higher organism is generated partly by such developmental processes as cell movement and cohesion but also by a social interaction between cells in small areas of embryonic tissue known as morphogenetic fields. The initially similar cells within such a field organize themselves and differentiate, forming a discrete spatial pattern which is remarkably independent of field size and which can regenerate after some part is removed. Although it is believed that a cell signalling system must underlie this behaviour, the putative signals--or morphogens--have so far proved elusive. Perhaps the simplest known morphogenetic field arises within the multicellular aggregate formed by developing cells of the slime mould Dictyostelium discoideum. As the amorphous aggregate transforms into a cylindrical slug, a simple pattern emerges, with prestalk cells differentiating in the anterior and prespores in the posterior. One great difficulty in identifying any morphogen has been to predict properties that could form the basis of a bioassay. However, in Dictyostelium it is almost essential that the morphogens should dictate to cells their choice of differentiation pathway. We have described previously a crude factor termed DIF which stimulates the differentiation of isolated amoebae into stalk cells. We now show that purified DIF also inhibits spore formation and so switches cells to stalk cell formation. Thus, we believe that DIF is a morphogen which regulates the choice of differentiation pathway of cells in the Dictyostelium slug.