Refining the taxonomy of pithovirus-related giant DNA viruses within the order Pimascovirales

Abstract

The first member of the family Pithoviridae (Pithovirus sibericum) was isolated from ancient Siberian permafrost and characterized in 2014. Since then, many relatives have been isolated, characterized, and classified as members of the genera Alphapithovirus, Alphacedratvirus, and Alphaorpheovirus. In addition, one complete circular genome sequence was assembled from metagenomic data (hydrivirus). All of these viruses form distinctive giant elongated ovoid particles, up to 2 µm in length, but they differ significantly in the size of their genome, their nucleotide composition, and their gene content. Based on their shared ovoid virion shape, common replication strategy, and core gene similarity, we recently proposed to update their taxonomic status by classifying them in three distinct families (Pithoviridae, Orpheoviridae, and Hydriviridae) within a new suborder, the Ocovirineae, to separate them clearly from the other more distant families (Marseilleviridae, Ascoviridae, Iridoviridae) of the order Pimascovirales. This new taxonomy, validated by the last ICTV Ratification vote held in March 2025, extends the previous partition from three clades to four (to include hydrivirus) while keeping the genera Alphacedratvirus and Alphapithovirus in the same family, Pithoviridae (but split into two subfamilies), due to their much greater similarity to each other than to orpheovirus and hydrivirus.