Marseilleviruses: An Update in 2021

Abstract

The family Marseilleviridae was the second family of giant viruses that was described in 2013, after the family Mimiviridae. Marseillevirus marseillevirus, isolated in 2007 by coculture on Acanthamoeba polyphaga, is the prototype member of this family. Afterward, the worldwide distribution of marseilleviruses was revealed through their isolation from samples of various types and sources. Thus, 62 were isolated from environmental water, one from soil, one from a dipteran, one from mussels, and two from asymptomatic humans, which led to the description of 67 marseillevirus isolates, including 21 by the IHU Méditerranée Infection in France. Recently, five marseillevirus genomes were assembled from deep sea sediment in Norway. Isolated marseilleviruses have ≈250 nm long icosahedral capsids and 348-404 kilobase long mosaic genomes that encode 386-545 predicted proteins. Comparative genomic analyses indicate that the family Marseilleviridae includes five lineages and possesses a pangenome composed of 3,082 clusters of genes. The detection of marseilleviruses in both symptomatic and asymptomatic humans in stool, blood, and lymph nodes, and an up-to-30-day persistence of marseillevirus in rats and mice, raise questions concerning their possible clinical significance that are still under investigation.

Keywords: Marseilleviridae; Pimascovirales; amoeba; giant virus; human; marseillevirus; megavirales.

FIGURE 1

Scanning electron microscopy of marseillevirus T19 particles. Micrographs of viral particles from culture supernatant. Technical settings and scale bars are indicated in the images.

FIGURE 2

Worldwide distribution of isolated marseilleviruses.

FIGURE 3

Replication cycle of marseilleviruses. A summary of the different pathways of marseillevirus entry and new virion assembly in amoeba, and their release pathways.

FIGURE 4

Evolution of the size of the core genome and pangenome of marseilleviruses. The analysis was made by a reciprocal best BLAST hit strategy with significant thresholds for amino acid identity and coverage of 30 and 70%, respectively. The core genome is represented by the histogram column. Each lineage is labeled by a specific color: blue for lineage A, green for lineage B, orange for lineage C, purple for lineage D, and red for lineage E. The pangenome is highlighted by the red curve. The order for inclusion of each marseillevirus in the analysis corresponds to the order shown in Supplementary Table 1, as follows: mrsv_2: Cannes 8 virus; mrsv_3: Melbournevirus; mrsv_4: Tokyovirus A1; mrsv_5: Senegalvirus; mrsv_6: Phoenicianvirus; mrsv_7: Marseillevirus G648; mrsv_8: Marseillevirus G649; mrsv_9: Marseillevirus G650; mrsv_10: Marseillevirus Shangai 1; mrsv_11: Lausannevirus; mrsv_12: Noumeavirus; mrsv_13: Port-miou virus; mrsv_14: Kurlavirus BKC-1; mrsv_15: Tunisvirus; mrsv_16: Insectomime virus; mrsv_17: Marseillevirus N1; mrsv_18: Marseillevirus N16; mrsv_19: Marseillevirus N36; mrsv_20: Marseillevirus N40; mrsv_21: Marseillevirus N50; mrsv_22: Marseillevirus N57; mrsv_23: Marseillevirus N60A; mrsv_24: Marseillevirus N60B; mrsv_25: Marseillevirus NAQ2; mrsv_26: Marseillevirus AM2; mrsv_27: Marseillevirus AM21; mrsv_28: Brazilian marseillevirus; mrsv_29: Golden marseillevirus; mrsv_30: Marseillevirus LCMAC 101; mrsv_31: Marseillevirus LCMAC 102; mrsv_32: Marseillevirus LCMAC 103; mrsv_33: Marseillevirus LCMAC 201; mrsv_34: Marseillevirus LCMAC 202.

FIGURE 5

Phylogenetic tree based on the major capsid protein (MCP). The analysis was performed using the Maximum Likelihood method (ML) with a JTT substitution matrix and 1,000 replicates. MCP sequences of marseilleviruses are color-labeled according to their classification into lineages: blue corresponds to lineage A, green to lineage B, orange to lineage C, purple to lineage D, and red to lineage E.