Cultured Asgard Archaea and a Virus-Host System: Live Clues to Eukaryogenesis

Live-cell microscopy captured active crawling motility in Asgard archaea for the first time, with protrusions extending and retracting at 1.5 to 5.3 micrometres per minute and dynamics arrested by actin inhibitors. Knowledge of Asgard archaea had come mostly from genomes assembled from environmental DNA, and the few cultures showed only static complex morphology; this preprint moves protrusions from structural description to real-time behaviour and pharmacologically intervenable function. Based on live-cell microscopy of cultures from two lineages, Loki and Hodarchaea, with selected actin inhibitors as intervention controls; the abstract reports the extension and retraction speed range.

An Asgard archaeal virus was cultured for the first time: a 16 kbp integrated provirus infecting Ca. Lokiarchaeum ossiferum B36 can excise, replicate independently and form virus particles, and network analysis places it in a new family, Fylgjaviridae. Asgard archaeal viruses had previously been described only through metagenomic reconstructions, never observed in a living host; this work turns the virus-host relationship from sequence inference into an experimentally tractable culture system. Based on cultivation of the new B36 strain, observation of provirus excision and particle formation, and network analysis of shared protein clusters with other archaeal viruses; the abstract gives no quantitative titre or infection kinetics.

Host B36 encodes antiviral defence systems including Septu, Wadjet and a type II CBASS system, which differ from those of the related strain B35. This suggests defence repertoires differ between strains of the same species, providing concrete material for studying defence evolution and developing genetic tools in Asgard archaea. Derived from a genome-level comparison of defence systems between strains B36 and B35, i.e. sequence-level evidence.

Protrusions may help capture and retain methane-producing partners and align with the inside-out model of eukaryogenesis proposed in 2014. Protrusion attachment and surface spikes observed in cultures link a previously genome-level hypothesis about eukaryogenesis to observable cell behaviour. Based on microscopy of M. peptidophilum and F. multiprotrusionis cultures and researcher commentary on the model; these are correlative observations rather than causal proof.

The results apply to a small number of laboratory-cultured Asgard archaeal strains, particularly the Loki and Hodarchaea lineages and strain B36; conclusions about crawling motility and protrusion function rest on attachment to glass surfaces and specific inhibitor conditions, and the virus-host conclusions rest on a single culture system plus sequence network analysis. For readers tracking the cell biology of eukaryogenesis or developing archaeal genetic tools, these cultures offer a tractable starting point.

Both preprints have not yet been peer reviewed, and the abstracts do not provide full figures, sample sizes or statistical detail, so the causal chain between protrusion function and eukaryogenesis remains speculative; quantitative features of virus particle formation, the actual role of defence systems during infection, and whether the culture systems can be stably reproduced by other laboratories are questions to watch.