Pathogens of evolutionary importance: Scientists have discovered viruses that infect a mysterious group of microbes that may have been the ancestors of all complex life forms. As the researchers explain, the characteristics of these particularly complex pathogens fit a hypothesis within the framework of the endosymbiont theory: Viruses could also have formed a component in the formation of the first eukaryotic cells.
Basically, our planet produced life forms billions of years ago, as evidenced by finds of biological deposits. But these organisms remained at a rather modest level of development for a long time: They were archaea and bacteria – prokaryotes with a comparatively simple cell structure without a cell nucleus and organelles. But then at some point there was an evolutionary drumbeat that was to have a lasting impact on the development of life: the first forms of life with a complex cell structure emerged – the eukaryotes. The higher living beings then developed from them, from plants to animals to humans.
Target the mysterious Asgard Archaea
Scientists currently assume that the first eukaryotes arose from the fusion of several microbes into a hybrid organism. According to this endosymbiont theory, archaea once ingested bacteria, which then developed into cell organelles, including mitochondria and plastids, within them. Research results indicate that the first eukaryotes were descendants of a special group of archaea, whose descendants still exist today: the so-called Asgard archaea have characteristics that distinguish them as the closest known prokaryotic relatives of the eukaryotes. Asgard archaea have been discovered in deep sea sediments and hot springs around the world. However, they remain mysterious because they are difficult to grow and study in the laboratory.
In their current study, the scientists led by Brett Baker from the University of Texas at Austin have once again investigated the Asgard archaea. Their results are based on examining genetic material from the microbes’ habitat, from which their genomes can be reconstructed. In their analyses, the researchers focused primarily on finding genetic traces of viruses that infect the Asgard archaea. To do this, they deliberately rummaged through the so-called Crispr arrays of the microbes, which act like a kind of archive and store pieces of the genetic material of viruses that previously caused an infection in them.
Did cell nuclei evolve from viruses?
In this way, the scientists were able to identify viral pathogens in this evolutionarily important group of archaea for the first time: they discovered and characterized six relatively large double-stranded DNA virus genomes. “The exciting thing is that these are completely new types of viruses that are different from those we have previously known from archaea,” says Baker. Specifically, the genetic analyzes showed that the newly discovered viruses have some characteristics that are similar to those of viruses that infect eukaryotes. Among other things, they have the ability to copy their own DNA and to use their host’s machinery for the production of proteins in a special way.
These results appear particularly interesting against a special background, the researchers explain: “There are suspicions that viruses could also have contributed to the emergence of complex cellular life,” says first author Ian Rambo from the University of Texas at Austin. He is referring to a hotly debated hypothesis called viral eukaryogenesis. It states that, in addition to bacteria and archaea, viruses could also have contributed to the development of eukaryotes. According to this, the cell nucleus could have developed from a large DNA virus, in which the pathogen took over genes from the host genome.
The new discovery may not confirm the hypothesis of viral eukaryogenesis, but it offers some interesting new leads, the scientists point out. “This study opens a door to better elucidate the origin of eukaryotes and to understand the role of viruses in the ecology and evolution of Asgard archaea,” says Rambo. His colleague Valerie De Anda sums up the importance even further: “The study contributes to the understanding of the role that viruses may have played in eukaryogenesis”.
Source: University of Texas at Austin, professional article: Nature Microbiology, doi: 10.1038/s41564-022-01150-8