Megavirus (Megavirus chilensis) — a giant double‑stranded DNA virus

Characteristics

Megavirus particles are visible by light microscopy when aggregated and were initially mistaken for bacteria because of their size and staining properties. The virion is a large icosahedron with surface fibers and contains a complex complement of genes rarely found in typical viruses. Its genome encodes many proteins involved in DNA replication, repair and even elements related to translation, such as tRNAs and aminoacyl‑tRNA synthetase homologs.

Structure and replication

Like other giant viruses, Megavirus has a multilayered capsid and external fibrils that may aid attachment to host cells. It infects free‑living amoebae under laboratory conditions and sets up cytoplasmic viral factories where genome replication and particle assembly occur. These cytoplasmic replication centers are a distinguishing feature shared with related giant viruses.

Discovery and historical context

Megavirus was reported in the early 2010s from a marine or aquatic sample and drew attention because, until the discovery of pandoraviruses in 2013, it held records for particle diameter and for genome size and complexity among described viruses. Its recognition contributed to a shift in virology by demonstrating extremes of viral genome content and challenging simple virus–cell distinctions. For discussion of related discoveries, see pandoraviruses and comparative analyses at phylogenetic resources.

Importance and implications

Megavirus and other giant viruses are important for several reasons: they expand the known genetic repertoire of viruses, provide models to study virus–host interactions with amoebae, and raise questions about the evolution of complex gene sets in viruses. Researchers have used Megavirus genomes to explore horizontal gene transfer, viral evolution, and the origins of translation‑related genes in noncellular entities. Summaries of genomic features can be found via genome records and structural overviews via capsid studies.

Distinguishing facts

• Belongs to the group commonly called giant viruses and is closely related to Mimivirus (APMV).
• Encodes an unusually broad range of genes, including some typically associated with cellular life.
• Its discovery helped stimulate renewed interest in viral diversity and the ecological roles of large DNA viruses.

For basic further reading and curated databases use the linked resources above or consult specialist reviews on giant virus taxonomy and biology (overview, phylogeny).