Introduction

Changes to virus taxonomy (the universal scheme of virus classification of the International Committee on Taxonomy of Viruses [ICTV]) take place annually and are the result of a multi-stage process. In accordance with the ICTV Statutes (https://ictv.global/statutes.asp), taxonomic proposals (TaxoProps) submitted to the ICTV Executive Committee (EC) undergo a review process that involves input from the ICTV Study Groups (SGs) and Subcommittees (SCs), other interested virologists, and the EC. After final approval by the EC, proposals are presented for ratification to the full ICTV membership by posting on the ICTV website (https://ictv.global/), followed by an electronic vote.

The latest set of proposals approved by the EC was made available on the ICTV website in February 2021 (see https://ictv.global/proposal/MSL36/ for all proposals combined into a single zip file, and use the links provided in the References to access individual proposals). A list of proposals was then emailed on 3rd February 2021 to the 176 members of ICTV, namely the EC Members, Life Members, ICTV SC Members including the SG chairs, and ICTV National Representatives. Members were then requested to vote on whether to ratify the taxonomic proposals. Voting closed on 5th March 2021.

Changes to virus taxonomy, to the International Code of Virus Classification and Nomenclature, and to the ICTV Statutes ratified in 2021

All proposals were ratified by ICTV members, in every case receiving an absolute majority of votes; at least 60% of eligible voters approved each proposal. A summary of the taxonomy changes enacted by ratification of the proposals is provided in Table 1. Each proposal is cited and listed in the References to acknowledge the authors’ efforts and to provide links to the specific proposal on the ICTV website [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262,263,264,265,266,267,268,269,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295,296,297]. These documents remain available for those who wish to see the full details of the proposals. A ratified non-taxonomic proposal 2020.002G [3] specifies a novel procedure for expedited correction of unintended errors that may occur during proposal writing but clearly do not reflect the intention of the proposers, as described in the submitted proposal documentation. Examples of such errors include assignment of a lower-rank taxon to an incorrect higher rank, inadvertent introduction of duplicate taxon names, or assignment of a misspelled taxon name. Since the changes ratified by ICTV take effect immediately, this mechanism was used to correct seven proposals that relate to proposals ratified in March 2021 but were found to contain errors upon cross-checking [18, 53, 54, 66, 254, 255, 297].

Table 1 Summary of taxonomic changes approved in March 2021
Full size table

A notable change approved this year is the adoption of a uniform binomial format for naming of virus species [1]. Previously, virologists have not been required to follow the convention of species naming that has been established in other fields of biology, whereby the species name consists of the genus name separated by a space from a species epithet that identifies the species within that genus. Over the last several decades, the case for a standardized nomenclature for virus species has been acknowledged by the ICTV and by many other virologists, but a uniform format of the species names had not been adopted (see references [298,299,300,301,302,303,304] for an incomplete but relatively representative account of the opinion spectrum). The case for a binomial nomenclature was presented by the ICTV Executive Committee in 2018 and 2020, and the advantages and disadvantages of different naming formats within the binomial framework were discussed [305, 306]. The EC then invited comments on this position from the virology community. Comments from ICTV SGs were also solicited and received by the EC SCs. Debates of the merits of various forms of binomial nomenclature took place in social media (e.g., 307) and in journal publications [308, 309]. Taking these discussions and comments into account, ICTV ratified the adoption of a standardized binomial system, which may follow either Latinized or free form formats [1]. The ICTV SGs are now charged with conversion of all existing virus species names to the new binomial form; the process has started already and is expected to be completed in time to be considered by an EC meeting planned for 2023. It is worth emphasizing that this system concerns only the names of virus species but does not affect the names of viruses.

Another general proposal [2] was the abolition of the requirement for a type species—a designated nomenclatural type for each genus. The ICTV noted that the requirement for type species in virus taxonomy was largely historical. Most importantly, the ICVCN defines virus species as follows: “A species is a monophyletic group of viruses whose properties can be distinguished from those of other species by multiple criteria", suggesting that the emphasis on “defining” a genus has been shifting from the properties of any particular member to the closeness of molecular and ecological relationships between all genus members. The ICTV concluded that the concept of a type species no longer serves a clear and useful purpose in virology and voted to abolish it.

One more proposal [4] has clarified the remit of the ICTV. In addition to viruses, ICTV has been classifying viroids and virus satellites, though none of these mobile genetic elements, including viruses themselves, have been operationally defined, leaving the limits of the ICTV scope unclear. This has been clarified now by giving operational definitions of viruses sensu stricto, viriforms, viroids, and satellites. These entities together constitute the virosphere and are within the remit of the ICTV responsibilities for their classification, taxonomy and nomenclature. The historical name of the organization—the International Committee on Taxonomy of Viruses—remains unchanged.

Conclusion

All proposals submitted to ratification were ratified by an absolute majority of the ICTV, and the changes are now part of official ICTV taxonomy. An up-to-date list of all approved taxa, which now includes more than 9000 virus species, can be found on the ICTV website: see https://ictv.global/proposal/MSL36/ for the Master Species List and https://ictv.global/vmr/ for the Virus Metadata Resource.