Rare microbial relict sheds light on an ancient eukaryotic supergroup

During the past decade, our understanding of eukaryotic evolution has increased immensely. Newly recognized eukaryotic supergroups have been established^1-3, and most enigmatic orphan lineages have had their relationships resolved^4-6. Studies on unicellular protist eukaryotes have also been key to understanding the evolution of mitochondria, the fundamental organelles of the eukaryotic cell, which originated from an alphaproteobacterial ancestor. The retention of ancestral alphaproteobacterial pathways in some protist lineages reveals that the mitochondrion of the last eukaryotic common ancestor was more metabolically versatile than are the highly derived mitochondria that are found in most modern eukaryotes^7,8. Here we report the discovery of such a unicellular eukaryote, Solarion arienae gen. et sp. nov., an inconspicuous, free-living heterotrophic protist with two morphologically distinct cell types and a novel type of predatory extrusome. We assign Solarion to the new phylum Caelestes. Together with Provora, hemimastigophoreans and Meteora, they form a new eukaryotic supergroup, Disparia. Moreover, S. arienae has some noteworthy traits associated with the mitochondrial genome; in particular, the mitochondrially encoded secA gene, a remnant of an ancestral alphaproteobacterial protein secretion pathway, which has been lost almost entirely in extant mitochondria^9,10. The discovery of S. arienae broadens our understanding of early eukaryotic evolution and facilitates the study of proto-mitochondrial metabolic remnants, shedding light on the complexity of ancestral eukaryotic life.