Disruption of the sporulation-specific gene spiA in Dictyostelium discoideum leads to spore instability.

The spiA gene of Dictyostelium is expressed specifically in prespore cells and spores during culmination, the final stage of development during which prespore and prestalk cells undergo terminal differentiation to form spores and stalk. We have used homologous recombination to delete this gene and have characterized the resulting phenotype. The spiA- strains develop normally and produce spores that are indistinguishable from those of wild-type strains by transmission and scanning electron microscopy. Mutant spores have normal viability when assayed soon after the completion of development, but, as the spiA- spores age, they lose viability more rapidly than those of the spiA+ parent. The drop in viability is more pronounced when spores are submerged in dilute buffer at a concentration that does not allow germination; after 11 days submerged, the viability of spiA- spores is 10(5)-fold reduced, whereas that of the parent is decreased only 10-fold. Reinserting an intact copy of the spiA gene into a spiA- strain restores the stability of its spores. The product of the spiA gene, Dd31, was identified on Western blots as a 30-kD protein using an antibody raised against a fusion protein containing a portion of the coding sequence. Dd31 is associated with the inner face of spore coat fragments in a detergent-resistant manner. This location is consistent with its observed role in maintaining stability of the spores.