Differential Gene Expression Analysis and Cytological Evidence Reveal a Sexual Stage of an Amoeba with Multiparental Cellular and Nuclear Fusion

Sex is a hallmark of eukaryotes but its evolution in microbial eukaryotes is poorly elucidated. Recent genomic studies revealed microbial eukaryotes possess genetic toolkit necessary for sexual reproduction. However, the mechanism of sexual development in a majority of microbial eukaryotes including amoebozoans is poorly characterized. The major hurdle in studying sex in microbial eukaryotes is lack of observational evidence, primarily due to its cryptic nature. In this study, we used a tractable fusing amoeba, Cochliopodium, to investigate sexual development using stage specific Differential Gene Expression (DGE) and cytological analyses. Both DGE and cytological results showed that most of the meiosis and sex-related genes are upregulated in Cochliopodium undergoing fusion in laboratory culture. Relative gene ontology (GO) category representations in unfused (single) and fused cells revealed functional skew of the fused transcriptome toward DNA metabolism, nucleus and ligases that are suggestive of a commitment to sexual development. While single cells GO categories were dominated by metabolic pathways and other processes indicative of vegetative phase. Our study provides strong evidence that the fused cells represent a sexual stage in Cochliopodium. Our findings have further implications in understanding the evolution and mechanism of inheritance involving multiparents in other eukaryotes with similar reproductive strategy.