Phylogenomic analyses reveal that Panguiarchaeum is a clade of genome-reduced Asgard archaea

The Asgard archaea are a diverse archaeal phylum that includes the host lineage from which eukaryotes evolved. Due to the importance of the Asgard archaea for our understanding of cellular evolution and eukaryotic complexity, recent efforts have focused on better characterizing the genomic diversity of this lineage and its relatives from the TACK archaea. Newly discovered lineages, the Njordarchaeales and Panguiarchaeales, are difficult to place, branching either with Korarchaeota or Asgard archaea. The phylogenetic position of these taxa is important because they include genome-reduced and potentially symbiotic lineages, which may help to inform our understanding of genome evolution and the evolution of symbiotic interactions within Asgard archaea and their relatives. To resolve the placement of Njordarchaeales and Panguiarchaeales in the archaeal tree, we performed a range of phylogenetic analyses revealing that the Njordarchaeales and Pangiuarchaeales constitute the newly-identified class Njordarchaeia within the Asgard archaea. Members of the Njordarchaeia exhibit hallmarks of adaptations to (hyper-) thermophilic lifestyles, which may contribute conflicting signals to phylogenetic analyses. Panguiarchaeum appears to be metabolically distinct from its relatives, displaying reduced metabolic potential and various auxotrophies. Gene tree-species tree reconciliation recovers a complex common ancestor of Asgard archaea that encoded the Wood Ljungdahl pathway, however the subsequent loss of this pathway during the reductive evolution of Panguiarchaeum may have been associated with the switch to a symbiotic lifestyle based on H2 syntrophy. Our analyses identify Panguiarchaeum as the first described Asgard archaea lineage with streamlined genomes that show indications for a symbiotic lifestyle.