Naldaviricetes is a class of large double-stranded DNA viruses that infectarthropods. These viruses share a set of homologous genes that encodeper os infectivity factors (PIFs), which are essential for oral infection of insect hosts. The class currently includes four recognized families —Baculoviridae,Nudiviridae,Hytrosaviridae, andNimaviridae — three of which (Baculoviridae,Nudiviridae, andHytrosaviridae) are grouped within the orderLefavirales. Naldaviricetes was formally established by theInternational Committee on Taxonomy of Viruses (ICTV) in 2021 to unify these families under a higher-rank taxon based on conserved genomic and structural features[1]. The nameNaldaviricetes derives from "Nuclear Arthropod Large DNA Viruses" (NALDVs), a term historically used to describe these four virus families.[1][2]
Concatenated alignments of five PIF amino acid sequences (pif-0/p74, pif-1, pif-2, pif-3, and pif-5/odv-e56), DNA polymerase (dnapol), and sulfhydryl oxidase (p33) of the members of the class Naldaviricetes
Viruses in this class areenveloped, rod-shaped, large dsDNA viruses that replicate in the nuclei of infected cells. All members ofNaldaviricetes share several diagnostic features:
Circular dsDNA genomes (≈ 80 – 300 kb) that replicate in the host cell nucleus.
Enveloped, rod-shaped or ovoid nucleocapsids.
Absence of vertical double-jelly-roll capsid proteins typical ofVaridnaviria, indicating an independent evolutionary origin.
These shared traits, especially the PIF gene complement, distinguishNaldaviricetes from other arthropod-infecting large-DNA viruses such asAscoviridae,Entomopoxvirinae, andBetairidovirinae, which lackpif homologs[1].
Model of the ODV entry complex showing Pifs associated with the baculoviruses.
Thepif genes encode a complex of envelope proteins that enable oral infectivity in insect midgut cells. They were first discovered in baculoviruses and later found in nudiviruses, hytrosaviruses, and nimaviruses.The conservation ofpif genes across these families is considered the molecular signature ofNaldaviricetes[1][3]. Homologs are also present in endogenous nudivirus-derived elements of braconid wasps (familyPolydnaviriformidae), reflecting ancient gene capture events[4].
Enveloped, rod-shaped viruses of insects with circular dsDNA genomes of 80–180 kb. They producetwo virion phenotypes—budded virus (BV) for systemic spread and occlusion-derived virus (ODV) for oral transmission[5][6]. Baculoviruses are divided into four genera:Alphabaculovirus,Betabaculovirus,Gammabaculovirus, andDeltabaculovirus[6], and are transmittedorally andvertically from parent to offspring through infected eggs[7][8][9].
Viruses with ellipsoidal to rod-shaped virions, typically non-occluded but some produce occlusion bodies[10]. Their genomes (96–232 kb) encode ≈ 100–150 genes, including ~28 core genes shared withbaculoviruses[11]. Most nudiviruses like members of Alpha-, Delta- and Gammanudiviruses are transmitted orally[12][13]; withHelicoverpa zeaNudivirus2, as the only exception that is known to be transmittedsexually andvertically[14][15].
Hytrosaviruses areenveloped, non-occluded, non-icosahedral rod-shaped particles with rounded and/ or conical ends[16]. The members with genomes sequenced areGlossina pallidipes salivary gland hypertrophy virus (GpSGHV, infecting thetsetse fly) andMusca domestica salivary gland hypertrophy virus (MdSGHV, infectinghousefly) while the sequence of the hytrosavirus infectingnarcissus bulb fly is not yet published and remains unclassified. They are transmittedorally,vertically,mechanically, andtransovarially[17][18][19][20][21].
Theorder Lefavirales was created withinNaldaviricetes to include the familiesBaculoviridae,Nudiviridae, andHytrosaviridae. The name derives from"late expression factors (lef)", a group of baculoviral genes encoding the subunits of aDNA-directed RNA polymerase responsible for late-phase transcription. These genes—lef-4,lef-5,lef-8,lef-9,p47, andvlf-1—areconserved among lefaviral families but absent fromNimaviridae.[1][25]
Phylogenetic analyses of concatenated PIF,DNA polymerase, and p33 (sulfhydryl oxidase) sequences indicate thatNaldaviricetes form amonophyletic group distinct from theNucleocytoviricota (formerly NCLDVs).[1][26] Network analyses suggest they may represent an ancient branch of theVaridnaviria-related dsDNA virus network, albeit lacking the canonical double-jelly-roll capsid protein.[1][27] Evidence of nudivirus-derived elements integrated into insect genomes points to deep co-evolution between these viruses and their arthropod hosts.[28]
In 2023, the ICTV adopted abinomial species-naming format for all viruses in the orderLefavirales. Each species name consists of the genus name followed by an epithet derived from the host species (e.g.,Alphabaculovirus aucalifornicae,Betanudivirus hezeae,Glossinavirus glopallidipedis)[1]. This reform standardized naming while retaining traditional virus abbreviations and common names.
WhileNaldaviricetes share certain ancestral traits with members ofVaridnaviria, they lack hallmark major-capsid-protein genes of that realm[1][27]. Within arthropod-infecting dsDNA viruses, they are distinguished fromAscoviridae,Entomopoxvirinae, andBetairidovirinae by nuclear replication and possession ofpif genes. Members of the genusBracoviriform (familyPolydnaviriformidae) also retain ancient nudivirus-derivedpif homologs.[4]
