Anelloviruses have circular negative-sense single-stranded genomes ranging in size from 1.6 to 3.9 kb. In general, anelloviruses have one large open reading frame (ORF), generally referred to as ORF1 or VP1, and a few smaller ORFs [1,2,3]. There are currently 30 genera in the family Anelloviridae (Table 1) [4, 5]. Members of the genus Gyrovirus have been identified infecting various avian species and detected in fecal samples of various mammals [4], and a report discusses their identification in reptiles [6], whereas those in the remaining 29 genera have been identified in mammal species samples [4, 5].

Table 1 Summary of the changes to the species names in the family Anelloviridae
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The ORF1/VP1 protein is the most conserved protein, a single jelly-roll capsid protein, encoded by anelloviruses, despite a size variation, which is attributed to insertions of varying length in the conserved jelly-roll domain (between β-strands H and I) [7]. Based on the phylogeny of this conserved ORF1/VP1, 30 genera have been established to classify various anelloviruses (Table 1, Fig. 1). Furthermore, based on the analysis of the nucleotide pairwise identities of the orf1 gene of anelloviruses, a 69% species demarcation threshold has been established [4, 5]. Within the 30 genera in the family Anelloviridae, there are currently 155 established species (Table 1), which are listed on the International Committee on Taxonomy of Viruses (ICTV) site within the master species list (https://ictv.global/msl) and virus metadata resources (https://ictv.global/vmr) [8].

Fig. 1
figure 1

Maximum-likelihood phylogenetic tree of the representative ORF1/VP1 protein sequences of anelloviruses from each species. The accession number of the representative sequence is provided next to the binomial species name. The phylogenetic tree is rooted with gyrovirus sequences. Branch support is shown for branches with >0.6 aLRT support.

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In 2021, the International Committee on Taxonomy of Viruses (ICTV) [8] proposal TaxoProp 2018.001G.R.binomial_species was ratified [9]. This requires all species names to be in binomial format comprising a “genus + epithet” with various style options for the epithets (as either freeform or Latinized; see Siddell et al. [10] and Postler et al. [11]). The species name is different from the name of a virus [12].

Here, we provide an update on the revised species names (using a freeform alphanumeric epithet where possible) for all species in the family Anelloviridae (Table 1). Nine species in the genus Gyrovirus have previously been named in binomial freeform alphanumeric epithet format with the five-letter epithet derived from the host species followed by a number [4]. Given that viruses in the previous species "Chicken anemia virus" (genus Gyrovirus) have been associated with disease, we have renamed this species Gyrovirus chickenanemia.

In general, we have used the first five letters of the name of the host or the source family (e.g., "phoca" for Phocidae) followed by a number such that each epithet is unique. In the case of species in the genus Betatorquevirus, we have used the epithet "homini" followed by a number to reflect "mini" from the name torque teno mini virus, e.g., the previous species "Torque teno mini virus 1" has been renamed Betatorquevirus homini1. On the other hand, in the case of the species in the genus Gammatorquevirus, we have used the epithet "homidi" followed by a number to reflect "midi" from the name torque teno midi virus, e.g., the previous species name "Torque teno midi virus 1" has been changed to Gammatorquevirus homidi1.

A summary of the changes is provided in Table 1. In addition, we provide an ORF1/VP1-protein-sequence-based phylogeny in Figure 1 showing genera and species using representative virus sequences as a reference. The ORF1/VP1 protein of the representative anellovirus sequences detailed in the virus metadata resources (https://ictv.global/vmr) [8] were assembled and aligned using MAFFT [13], and the alignment was then trimmed using TrimAL [14] with a gap setting of 0.2. This trimmed alignment was used to infer a maximum-likelihood phylogenetic tree in PHYML 3 [15] with aLRT branch support and with VT+G+F as the best substitution model determined using ProtTest 3 [16]. The resulting phylogeny was visualized in iTOL 6 [17].

As more anellovirus genomes are identified from various hosts, coupled with more insights into their genomes and coding regions, we expect to see an increased number of new species and genera and a revision of species demarcation in the near future. Further, it is likely that the family Anelloviridae, which is currently not assigned to any phylum or higher ranks would be classified in a new phylum, "Commensaviricota", in the kingdom Shotokuvirae (realm Monodnaviria), as proposed by Butkovic et al. [7].