Lecture Notes: Giant Viruses and Their Origins

Introduction to Giant Viruses

• Giant viruses are mysterious microbes originally considered potentially a new domain of life.
• Differ from normal viruses by having hundreds of genes, suggesting a possible reclassification.

Hypothesis of Gene Theft

• Recent study suggests giant viruses are normal viruses that grew larger by stealing host genes.
• New findings challenge the unique status of giant viruses.

History and Discovery

• Mimivirus was the first discovered giant virus in 2003, found in a Bradford, UK water tower.
  • It has 1018 genes, much larger than other viruses like HIV which has nine genes.
• More giant viruses have been discovered, some nearly as large as bacterial cells.

Unique Genetic Makeup

• 50-90% of giant virus genes are unique, not found in other organisms or even among themselves.
• Speculations arose about them being descendants of a new life form beyond known domains (archaea, bacteria, eukaryotes).

Klosneuvirus Discovery

• Discovered by a team led by Frederik Schulz at the DOE Joint Genome Institute in a wastewater treatment tank in Austria.
• Found by analyzing metagenomic data; host remains unidentified.
• Classified in the mimivirus family after genome comparison.

Genetic Common Traits

• Klosneuvirus shares 700 genes with cellular organisms, more so than with other viruses.
• Suggests genes were acquired from different eukaryotic organisms, not from a single cellular ancestor.
• Likely hosts: protists, amoebas, ciliates.

Implications of Gene Acquisition

• Klosneuvirus likely acquired genes over time, aiding in replication efficiency.
• Genes might encode for protein-making machinery components.
• Schulz argues all giant viruses stem from small virus ancestors.

Scientific Debate

• Schulz states findings don't necessitate rewriting life's evolutionary story.
• Opposition from scientists like Jean-Michel Claverie argues against the study's methods and conclusions.
  • Claims about chimeric genomes and biological laws countering small viruses growing larger.

Conclusion

• Findings suggest re-evaluation of what constitutes viral capabilities, without altering life’s evolutionary history.
• Ongoing debate about the evolutionary path of giant viruses and their classification.

References

• Published in Science, DOI: 10.1126/science.aal4657
• Additional readings on giant viruses and microbial life.