A novel developmental mechanism in Dictyostelium revealed in a screen for communication mutants.

We performed a screen for signaling genes by selecting mutant strains of Dictyostelium that fail to develop spores in a pure population but sporulate well in chimerae with wild type cells. We found 9 strains whose sporulation was induced up to 10 million-fold in chimerae. Most strains were also able to sporulate in chimerae with each other, but 2 pairs failed to do so, suggesting that the genes in each pair participate in the production of 1 signal. One of the pairs, comD and comB, is described in detail. Sequence analysis revealed that both genes encode putative membrane proteins. ComD is predicted to have 15 transmembrane domains, and ComB has a region of high similarity to the Rab family of small GTPases and 1 transmembrane domain. Similarities between the developmental regulation and cell-type specificity of the genes' expression, the terminal developmental morphology, and the expression pattern of cell-type specific markers in the mutants suggest that comD and comB participate in 1 signal production pathway. This idea is also supported by a high similarity between the global transcriptional profiles of the mutant strains. Differences between the mutant phenotypes late in development suggest that comD and comB participate in separate processes as well. comD has a cell-autonomous role in the specialization of a novel prespore cell type, whereas comB has a cell-autonomous role in prestalk A cell differentiation.