Expanding Asgard members in the domain of Archaea shed new light on the origin of eukaryotes

Abstract

The hypothesis that eukaryotes originated from within the domain Archaea has been strongly supported by recent phylogenomic analyses placing Heimdallarchaeota from the Asgard superphylum as the closest known archaeal sister-group to eukaryotes. At present, only six phyla are described in the Asgard superphylum, which limits our understanding of the relationship between eukaryotes and archaea, as well as the evolution and ecological functions of the Asgard archaea. Here, we describe five previously unknown phylum-level Asgard archaeal lineages, tentatively named Tyr-, Sigyn-, Freyr-, Njord- and Balderarchaeota. Comprehensive phylogenomic analyses further supported the origin of eukaryotes within Archaea to form a 2-domain tree of life and a new Asgard lineage Njordarchaeota was identified as the potential closest branch with the eukaryotic nuclear host lineage rather than Heimdallarchaeota that were previously considered as the closest archaeal relatives of eukaryotes. Metabolic reconstruction of Njordarchaeota suggests a heterotrophic lifestyle, with potential capability of peptides and amino acids utilization. This study largely expands the Asgard superphylum, provides additional evidences to support the 2-domain life tree and sheds new light on the evolution of eukaryotes.