Whole-genome sequencing is a useful approach for identification of chemical-induced lesions, but previous applications involved tedious genetic mapping to pinpoint the causative mutations. We propose that saturation mutagenesis under low mutagenic loads, followed by whole-genome sequencing, should allow direct implication of genes by identifying multiple independent alleles of each relevant gene. We tested the hypothesis by performing three genetic screens with chemical mutagenesis in the social soil amoeba Dictyostelium discoideum. Through genome sequencing, we successfully identified mutant genes with multiple alleles in near-saturation screens, including resistance to intense illumination and strong suppressors of defects in an allorecognition pathway. We tested the causality of the mutations by comparison to published data and by direct complementation tests, finding both dominant and recessive causative mutations. Therefore, our strategy provides a cost-and time-efficient approach to gene discovery by integrating chemical mutagenesis and whole-genome sequencing. The method should be applicable to many microbial systems, and it is expected to revolutionize the field of functional genomics in Dictyostelium by greatly expanding the mutation spectrum relative to other common mutagenesis methods.

Cell surface adhesion receptors play diverse functions in multicellular development. In Dictyostelium, two immunoglobulin-like adhesion proteins, TgrB1 and TgrC1, are essential components with dual roles in morphogenesis and allorecognition during development. TgrB1 and TgrC1 form a heterophilic adhesion complex during cell contact and mediate intercellular communication. The underlying signaling pathways, however, have not been characterized. Here, we report on a mutation that suppresses the tgrB-tgrC1-defective developmental arrest. The mutated gene alg9 encodes a putative mannosyl transferase that participates in N-linked protein glycosylation. We show that alteration in N-linked glycosylation, caused by an alg9 mutation with a plasmid insertion (alg9 ins ) or tunicamycin treatment, can partially suppress the developmental phenotypes caused by tgrC1 deletion or replacement with an incompatible allele. The alg9 ins mutation also preferentially primed cells toward a stalk-cell fate. Despite its effect on development, we found that altered N-linked glycosylation had no discernable effect on TgrB1-TgrC1-mediated allorecognition. Our results show that N-linked protein glycosylation can modulate developmental processes without disturbing cell-cell recognition, suggesting that tgrB1 and tgrC1 have distinct effects in the two processes.