Ancient Host-Virus Gene Transfer Hints at a Diverse Pre-LECA Virosphere

Area of Science:

• Molecular Biology
• Evolutionary Biology
• Virology

Background:

• The early evolution of eukaryotes and their viruses remains poorly understood.
• Shared enzymes between eukaryotes and Nucleocytoviricota viruses hint at ancient gene exchange, but relationships are unclear.
• Previous studies on polymerases suggest deep-branching clades indicative of ancient viral-eukaryotic gene transfer.

Purpose of the Study:

• To investigate the evolutionary relationships of key enzymes shared between eukaryotes and large DNA viruses.
• To explore the origins of viral DNA polymerases, RNA polymerases, and mRNA-capping enzymes.

Main Methods:

• Updated phylogenetic analysis of eukaryotic and viral family B DNA polymerases.
• Phylogenetic analysis of multimeric RNA polymerases.
• Phylogenetic analysis of mRNA-capping enzymes.

Main Results:

• Viral enzymes formed clades adjacent to eukaryotes, suggesting origins predating the Last Eukaryotic Common Ancestor (LECA).
• Viral replication, transcription, and mRNA capping machinery are ancient, linked to virus factory formation.
• Large DNA viruses may preserve proteins from extinct proto-eukaryotic lineages.

Conclusions:

• The findings suggest a complex pre-LECA virosphere.
• Large DNA viruses likely encode ancient proteins, offering insights into early eukaryotic evolution.
• The study highlights the deep evolutionary history of viral-eukaryotic interactions.