What are the current top five largest viruses by genome size?

The viruses with the biggest genomes discovered so far are all giant viruses that infect amoebas or other protists. Ongoing research keeps updating the record for the largest viral genomes.

Top five largest viruses by genome size:

• Pandoravirus salinus- Discovered in 2013, this virus still holds the record with a genome of about 2.5 megabases (Mb), or 2,500,000 base pairs. It belongs to the Pandoravirus genus, a group known for massive genomes.
• Hydrivirus permafrostis- A relative of the Pithoviruses, this virus was identified through metagenomic studies. It has a circular genome of ~1.6 Mb.
• Tupanvirus deep ocean- Found in deep-sea sediments, this virus carries a 1.516 Mb genome. Tupanviruses are remarkable because they contain the most complete set of genes for protein translation ever seen in viruses.
• Orpheovirus IHUMI-LCC2- This virus infects the amoeba Vermamoeba vermiformis and has a genome of about 1.47 Mb. It is related to the Pithoviruses.
• Bodo saltans virus (BsV)- A cousin of Mimivirus, BsV’s genome is nearly 1.4 Mb in size.

Why They’re Called “Giant Viruses”

• Physical size: Unlike most viruses, these cannot pass through the fine filters used historically to separate viruses from bacteria. This is why they were overlooked for decades.
• Gene content: They carry unusually large double-stranded DNA genomes with hundreds to thousands of genes. Some, like Tupanvirus, even have genes for functions normally found only in cellular life, such as DNA repair and translation machinery.