DETECTION OF SUBTLE PHENOTYPES - THE CASE OF THE CELL-ADHESION MOLECULE CSA IN DICTYOSTELIUM

Dictyostelium amoebae aggregate irate a multicellular organism by cAMP -driven chemotaxis and cell-cell adhesion. Cell adhesion is mediated b y an EDTA-sensitive and an EDTA-resistant adhesion system. The latter is developmentally regulated and triggered by hemophilic interactions of the membrane glycoprotein csA; on disruption of the encoding,gene, EDTA-resistant contacts fail to form. Nevertheless, csA-null cells und er usual laboratory conditions aggregate normally and complete develop ment. By using experimental conditions that reproduce more closely the habitat of Dictyostelium amoebae, evidence is provided that csA is re quired for development and that its expression confers a selective adv antage to populations of wild-type cells over csA-null mutants. The la tter display reduced cell-cell adhesion, increased adhesiveness to the substratum and slower motility, which lead to their sorting out from aggregating wild-type cells. It is proposed that the experimental cond itions commonly used in the laboratory are not stringent enough to ass ess the developmental role of csA and other proteins. The assay descri bed can be used to detect subtle phenotypes, to reexamine the developm ental role off apparently nonessential genes, and to test the validity off recent models ore emergence and maintenance of apparent genetic r edundancy.