Genetic Analysis of Cell-type Proportioning in Dictyostelium
Dictyostelium discoideum is a soil amoeba which undergoes a simple multicellular development resulting in a fruiting body consisting of spores held atop a cellular stalk. The proportioning mechanism that establishes the ratio of prestalk to prespore cells in Dictyostelium is robust. The proper ratio of cell types is maintained in the migrating slug: if part of the slug is removed or destroyed, the remaining cells are able to re-establish the proper cell-type proportions. In this way, the development of this simple soil amoeba recapitulates several crucial aspects of the regulative development of vertebrate embryos. We are using molecular genetic analyses to dissect the proportioning mechanisms of this system in the hope of uncovering general principles by which groups of cells operate to organize themselves into tissues.
Genetic screens are being used to identify genes which control the prespore/prestalk ratio. Random plasmid insertion mutations are being screened for alterations in this early proportioning mechanism. In one screen, mutations are being generated in a strain that arrests development just after the time of the initial prepsore/prestalk divergence, and which contains a green fluorescent protein marker gene which is expressed specifically in prestalk cells. This allows a direct visual screen, in live cells, for alterations in cell-type proportioning.
Members of a membrane transporter gene family suspected of being involved in the proportioning process are also being investigated. The mdrA gene, a close homolog of the human multiple drug resistance transporter genes, has been cloned and deleted from the Dictyostelium genome. The developmental consequences of this mutation are being investigated.