Different as the cells from animals, plants, fungi and protozoa can be, they all share one prominent feature: a nucleus. They have other organelles, too, like the energy-producing mitochondria, but the presence of a nucleus — a well-defined porous pouch full of genetic material — is what inspired the biologist Édouard Chatton in 1925 to coin the term eukaryotes, which referred to living things with a “true kernel.” All the rest he labeled prokaryotes, for life “before kernel.” This dichotomy between nucleated and nonnucleated life became fundamental to biology.No one knows exactly how the nucleus evolved and created that division. Growing evidence has persuaded some researchers, however, that the nucleus might have arisen through a symbiotic partnership much like the one believed to have produced mitochondria. A crucial difference, though, is that the partner responsible for the nucleus might not have been a cell at all, but a virus.“What we [eukaryotes] are is a classic case of what they call emergent complexity,” explained Philip Bell, the head of research for the yeast biotechnology company MicroBioGen. Bell proposed a viral origin for the eukaryotic nucleus back in 2001 and refreshed the theory in September. “It’s three organisms that came together to make a new community, which eventually integrated to such an extent that it became, effectively, a new life-form.”The discovery of archaeal viruses which construct nucleus-like and membrane-based structures in archaeal cells will be the strongest evidence of the viral origin for the nucleus.He and other researchers take their confidence from findings such as the demonstration that giant viruses build “viral factories” inside prokaryotic cells — compartments that, much like the nucleus, uncouple the processes of transcription (reading genes) and translation (constructing proteins). “I think it’s now the strongest model,” he said.Most researchers who study the origins of eukaryotes might not agree with him; some still describe it as an idea on the fringe. But proponents of a viral origin point out that several recent discoveries line up conveniently with a viral model — and they believe that conclusive evidence in their favor is finally within reach.

A Viral Gift or Grift

Scientists generally think eukaryotes first came on the scene between 2.5 billion and 1.5 billion years ago, when evidence suggests that a bacterium took up residence inside a different kind of prokaryote, an archaeon, and became its mitochondrion. But a deeper mystery surrounds the emergence of the nucleus; no one even knows whether that ancient archaeon was already a kind of proto-eukaryote with a nucleus, or whether the nucleus came later.Any origin story for the eukaryotic nucleus needs to explain several of its features. There’s the nature of the structure, for starters: its nested inner and outer membranes, and the pores that connect its interior to the rest of the cell. There’s also the curious way it compartmentalizes the expression of genes within itself but leaves the construction of proteins outside. And a truly persuasive origin story must also explain why the nucleus exists at all — what evolutionary pressures pushed those ancient cells to wall up their genomes.For most of the past century and more, conjectures about the origin of the nucleus failed to answer at least one of those questions. But around the turn of the 21st century, two researchers independently came up with the idea that viruses were responsible for the nucleus.In Japan, Masaharu Takemura (then a research associate at Nagoya University) was studying the biochemistry of DNA polymerases — enzymes that cells use to copy DNA — when he became interested in their evolution. “I performed a phylogenetic analysis of DNA polymerases including eukaryotic, bacterial, archaeal and viral ones,” Takemura, now a molecular biologist and virologist at Tokyo University of Science, recalled in an email. His analysis revealed that one group of viruses (the poxviruses) had DNA polymerases that were surprisingly similar to one of the major classes of polymerases from eukaryotes. He hypothesized that the eukaryotic enzyme originated as a contribution from some ancient poxvirus.

Media contact:
Mercy
Microbiology: Current Research