Characterization of three ascovirus isolates from cotton insects

Enhanced virulence of genetically engineered Autographa californica nucleopolyhedrovirus owing to accelerated viral DNA replication aided by inserted ascovirus genes

Genetic engineering technology is an ideal method to improve insecticidal efficiency by combining the advantages of different pathogenic microorganisms. Thus, six ascovirus genes were introduced into the genomic DNA of Autographa californica nucleopolyhedrovirus (AcMNPV) to possibly transfer the intrinsically valuable insecticidal properties from ascovirus to baculovirus. The viral budded virus (BV) production and viral DNA replication ability of AcMNPV-111 and AcMNPV-165 were significantly stronger than that of AcMNPV-Egfp (used as the wild-type virus in this study), whereas AcMNPV-33 had reduced ones. AcMNPV-111 and AcMNPV-165 also exhibited excellent insecticidal efficiency in the in vivo bioassays: AcMNPV-111 showed a 24.1% decrease in the LT₅₀ value and AcMNPV-165 exhibited a 56.3% decrease in the LD₅₀ value compared with AcMNPV-Egfp against the 3rd instar of Spodoptera exigua larvae, respectively. Furthermore, the size of the occlusion bodies (OBs) of AcMNPV-33, AcMNPV-111, and AcMNPV-165 were significantly increased compared to that of AcMNPV-Egfp. AcMNPV-111 and AcMNPV-165 had stable virulence against the 2nd to 4th instars tested larvae and higher OB yield than AcMNPV-Egfp in the 3rd and 4th instar larvae. Correlation and regression analyses indicated that it is better to use 5 OBs/larva virus to infect the 2nd instar larvae to produce AcMNPV-111 and 50 OBs/larva virus to infect the 3rd instar larvae to produce AcMNPV-165. The results of this study obtained recombinant viruses with enhanced virulence and exhibited a diversity of ascovirus gene function based on the baculovirus platform, which provided a novel strategy for the improvement of baculovirus as a biological insecticide.

Towards Understanding the Function of Aegerolysins

Aegerolysins are remarkable proteins. They are distributed over the tree of life, being relatively widespread in bacteria and fungi, but also present in some insects, plants, protozoa, and viruses. Despite their abundance in cells of certain developmental stages and their presence in secretomes, only a few aegerolysins have been studied in detail. Their function, in particular, is intriguing. Here, we summarize previously published findings on the distribution, molecular interactions, and function of these versatile aegerolysins. They have very diverse protein sequences but a common fold. The machine learning approach of the AlphaFold2 algorithm, which incorporates physical and biological knowledge of protein structures and multisequence alignments, provides us new insights into the aegerolysins and their pore-forming partners, complemented by additional genomic support. We hypothesize that aegerolysins are involved in the mechanisms of competitive exclusion in the niche.

Extended in vivo transcriptomes of two ascoviruses with different tissue tropisms reveal alternative mechanisms for enhancing virus reproduction in hemolymph

Ascoviruses are large dsDNA viruses characterized by the extraordinary changes they induce in cellular pathogenesis and architecture whereby after nuclear lysis and extensive hypertrophy, each cell is cleaved into numerous vesicles for virion reproduction. However, the level of viral replication and transcription in vesicles compared to other host tissues remains uncertain. Therefore, we applied RNA-Sequencing to compare the temporal transcriptome of Spodoptera frugiperda ascovirus (SfAV) and Trichoplusia ni ascovirus (TnAV) at 7, 14, and 21 days post-infection (dpi). We found most transcription occurred in viral vesicles, not in initial tissues infected, a remarkably novel reproduction mechanism compared to all other viruses and most other intracellular pathogens. Specifically, the highest level of viral gene expression occurred in hemolymph, for TnAV at 7 dpi, and SfAV at 14 dpi. Moreover, we found that host immune genes were partially down-regulated in hemolymph, where most viral replication occurred in highly dense accumulations of vesicles.

Comparative Analysis of Transcriptional Regulation Patterns: Understanding the Gene Expression Profile in Nucleocytoviricota

The nucleocytoplasmic large DNA viruses (NCLDV) possess unique characteristics that have drawn the attention of the scientific community, and they are now classified in the phylum Nucleocytoviricota. They are characterized by sharing many genes and have their own transcriptional apparatus, which provides certain independence from their host's machinery. Thus, the presence of a robust transcriptional apparatus has raised much discussion about the evolutionary aspects of these viruses and their genomes. Understanding the transcriptional process in NCLDV would provide information regarding their evolutionary history and a better comprehension of the biology of these viruses and their interaction with hosts. In this work, we reviewed NCLDV transcription and performed a comparative functional analysis of the groups of genes expressed at different times of infection of representatives of six different viral families of giant viruses. With this analysis, it was possible to observe a temporal profile of their gene expression and set of genes activated in specific phases throughout the multiplication cycle as a common characteristic of this group. Due to the lack of information regarding the transcriptional regulation process of this group of pathogens, we sought to provide information that contributes to and opens up the field for transcriptional studies of other viruses belonging to Nucleocytoviricota.

Altering of host larval (Spodoptera exigua) calcineurin activity in response to ascovirus infection

BACKGROUND

Calcineurin (CaN) is involved in numerous cellular processes and Ca²⁺ -dependent signal transduction pathways. According to our previous transcriptome studies, thousands of host larval (Spodoptera exigua) transcripts were downregulated after the infection of Heliothis virescent ascovirus 3h (HvAV-3h), while the Spodoptera exigua calcineurin genes (SeCaNs) were significantly upregulated. To understand the regulation of SeCaNs in S. exigua larvae during the infection of HvAV-3h, the functions of CaN subunit A (SeCaN-SubA) and CaN binding protein (SeCaN-BP) were analysed.

RESULTS

The in vitro assays indicated that the bacterial expressed SeCaN-SubA is an acid phosphatase, but no phosphatase activity was detected with the purified SeCaN-BP. The transcription level of SeCaN-SubA was upregulated after HvAV-3h infection and the CaN activity was significantly increased after HvAV-3h infection in S. exigua larvae. Interestingly, the SeCaN-BP transcripts were only detectable in the HvAV-3h infected larvae. Further immunoblotting results consistently agree with those obtained by qPCR, indicating that the infection of HvAV-3h causes the upregulated expression of SeCaN-SubA and the appearance of SeCaN-BP. An interaction between the cleaved SeCaN-SubA and SeCaN-BP was detected by co-immunoprecipitation assays, and the expression of SeCaN-BP in Spodoptera frugiperda-9 (Sf9) cells can help to increase the CaN activity of SeCaN-SubA. Further investigations with CaN inhibitors suggested that HvAV-3h. Further investigations with CaN inhibitors suggested that the inhibition on host larval CaN activity can also inhibit the viral replication of HvAV-3h.

CONCLUSION

The increase in CaN activity caused by HvAV-3h infection might be due to the upregulation of SeCaN-SubA and the induced expression of SeCaN-BP, and increased CaN activity is essential for ascoviral replication. © 2019 Society of Chemical Industry.

In Vitro Infectious Risk Assessment of Heliothis virescens ascovirus 3j (HvAV-3j) toward Non-target Vertebrate Cells

As specific pathogens of noctuid pests, including Spodoptera exigua, S. litura, Helicoverpa armigera, and Mythimna separata, ascoviruses are suitable for the development of bioinsecticides. In this study, the infectivity of Heliothis virescens ascovirus 3j (HvAV-3j) on insect and mammalian cells was evaluated. HvAV-3j infection induced drastic morphological changes in Sf9, HzAM1, SeFB, and HaFB cells, including swelling and detachment. Notably, the latter phenomena did not occur in HvAV-3j-inoculated mammalian cells (HEK293, 7402, HePG2, PK15, ST, and TM3). MTT assays indicated that HvAV-3j inhibited the growth of host insect cells from the 6th hpi, but no effects were detected in the HvAV-3j-inoculated mammalian cells. Furthermore, viral DNA replication, gene transcription, and protein expression were investigated, and the results consistently suggested that HvAV-3j viruses were not able to replicate their genomic DNA, transcribe, or express their proteins in the non-target vertebrate cells. The HvAV-3j genes were only transcribed and expressed in the four insect cell lines. These results indicated that HvAV-3j was infectious to cells derived from S. frugiperda, S. exigua, H. armigera, and H. zea but not to cells derived from human, pig, and mouse, suggesting that ascoviruses are safe to non-target vertebrate cells.

Complete Genome Sequence of a Renamed Isolate, Trichoplusia ni Ascovirus 6b, from the United States

The complete genome of Trichoplusia ni ascovirus 6b (TnAV-6b) was sequenced for the first time. The TnAV-6b isolate, which has its closest phylogenetic relationship with the TnAV-6a isolate, has a circular genome of 185,664 bp, with a G+C content of 46.0% and 178 predicted open reading frames.

Promoter Motifs in NCLDVs: An Evolutionary Perspective

For many years, gene expression in the three cellular domains has been studied in an attempt to discover sequences associated with the regulation of the transcription process. Some specific transcriptional features were described in viruses, although few studies have been devoted to understanding the evolutionary aspects related to the spread of promoter motifs through related viral families. The discovery of giant viruses and the proposition of the new viral order Megavirales that comprise a monophyletic group, named nucleo-cytoplasmic large DNA viruses (NCLDV), raised new questions in the field. Some putative promoter sequences have already been described for some NCLDV members, bringing new insights into the evolutionary history of these complex microorganisms. In this review, we summarize the main aspects of the transcription regulation process in the three domains of life, followed by a systematic description of what is currently known about promoter regions in several NCLDVs. We also discuss how the analysis of the promoter sequences could bring new ideas about the giant viruses’ evolution. Finally, considering a possible common ancestor for the NCLDV group, we discussed possible promoters’ evolutionary scenarios and propose the term “MEGA-box” to designate an ancestor promoter motif (‘TATATAAAATTGA’) that could be evolved gradually by nucleotides’ gain and loss and point mutations.

Genome sequence and organization analysis of Heliothis virescens ascovirus 3f isolated from a Helicoverpa zea larva

The complete genome sequence of Heliothis virescens ascovirus 3f (HvAV-3f) was obtained. The HvAV-3f genome has a circular genome of 198,157bp with a G+C content of 46.0%, and encodes 190 open reading frames (ORFs) longer than 69 amino acids. Two major homologous regions (hrs) and 29 'baculovirus repeat ORFs' (bro) were found in the genome. BLAST analyses revealed that three HvAV-3f genes were homologous to that of lepidopteran insects. Nine ORFs were unique to HvAV-3f, in which two ORFs showed significant levels of similarity to genes that have not been previously described for ascoviruses in the Genbank database.

A comparison of growth and development of three major agricultural insect pests infected with Heliothis virescens ascovirus 3h (HvAV-3h)

Ascoviruses are double-stranded DNA viruses that are pathogenic to lepidopteran hosts, particularly noctuid larvae. Infection of a larva is characterized by retarded growth, reduced feeding and yellowish body color. In this paper, we reported the growth and development of three major agricultural noctuid insect pests, Helicoverpa armigera (Hübner), Spodoptera exigua (Hübner) and Spodoptera litura (Fabricius), infected with Heliothis virescens ascovirus 3h (HvAV-3h). Using 10-fold serial dilutions (0 to 7) of HvAV-3h-containing hemolymph to infect S. litura larvae, we found no significant difference in larval mortalities from 0 to 10(3)-fold dilutions; however, significant differences were observed at 10(4)-fold dilution and above. Using a 10-fold dilution of HvAV-3h-containing hemolymph to infect H. armigera, S. exigua and S. litura larvae, we found that the growth and development were significantly affected. All infected larvae could not pupate; the survival times of treated H. armigera, S. litura and S. exigua larvae were significantly longer than untreated control larvae. Body weight showed significant difference between treated and untreated control group from day 1 after inoculation in H. armigera and S. exigua, but day 2 in S. litura. Additionally, food intake also showed significant difference between treated and untreated control group from day 2 after inoculation in H. armigera and S. litura, but day 3 in S. exigua.

Genomic sequence of Heliothis virescens ascovirus 3g isolated from Spodoptera exigua

Heliothis virescens ascovirus 3a (HvAV-3a), a member of the family Ascoviridae, has the highest diversity among ascovirus species that have been reported in Australia, Indonesia, China, and the United States. To understand the diversity and origin of this important ascovirus, the complete genome of the HvAV Indonesia strain (HvAV-3g), isolated from Spodoptera exigua, was determined to be 199,721 bp, with a G+C content of 45.9%. Therefore, HvAV-3g has the largest genome among the reported ascovirus genomes to date. There are 194 predicted open reading frames (ORFs) encoding proteins of 50 or more amino acid residues. In comparison to HvAV-3e reported from Australia, HvAV-3g has all the ORFs in HvAV-3e with 6 additional ORFs unique to HvAV-3g, including 1 peptidase C26 gene with the highest identity to Drosophila spp. and 2 gas vesicle protein U (GvpU) genes with identities to Bacillus megaterium. The five unique homologous regions (hrs) and 25 baculovirus repeat ORFs (bro) of HvAV-3g are highly variable.

Phylogenetic position and replication kinetics of Heliothis virescens ascovirus 3h (HvAV-3h) isolated from Spodoptera exigua

Insect-specific ascoviruses with a circular genome are distributed in the USA, France, Australia and Indonesia. Here, we report the first ascovirus isolation from Spodoptera exigua in Hunan, China. DNA-DNA hybridization to published ascoviruses demonstrated that the new China ascovirus isolate is a variant of Heliothis virescens ascovirus 3a (HvAV-3a), thus named HvAV-3h. We investigated the phylogenetic position, cell infection, vesicle production and viral DNA replication kinetics of HvAV-3h, as well as its host-ranges. The major capsid protein (MCP) gene and the delta DNA polymerase (DNA po1) gene of HvAV-3h were sequenced and compared with the available ascovirus isolates for phylogenetic analysis. This shows a close relationship with HvAV-3g, originally isolated from Indonesia, HvAV-3e from Australia and HvAV-3c from United States. HvAV-3h infection induced vesicle production in the SeE1 cells derived from S. exigua and Sf9 cells derived from S. frugiperda, resulting in more vesicles generated in Sf9 than SeE1. Viral DNA replication kinetics of HvAV-3h also demonstrated a difference between the two cell lines tested. HvAV-3h could readily infect three important insect pests Helicoverpa armigera (Hübner), Spodoptera exigua (Hübner) and Spodoptera litura (Fabricius) from two genera in different subfamilies with high mortalities.

Comparative analysis of a highly variable region within the genomes of Spodoptera frugiperda ascovirus 1d (SfAV-1d) and SfAV-1a

The recently discovered ascoviruses have a worldwide distribution. Here we report a new member of the family Ascoviridae, Spodoptera frugiperda ascovirus 1d (SfAV-1d) with a variable region in the genome. Restriction fragment length polymorphism, Southern hybridization and genome sequencing analyses confirmed that SfAV-1d and the earlier reported SfAV-1a are closely related but are not identical. The genome size of SfAV-1d is approximately 100 kbp, which is about 57 kbp smaller than SfAV-1a. The SfAV-1d genome has a major deletion of 14 kbp that corresponds to one of the inverted repeat (IR) regions of SfAV-1a. Cloning and sequencing revealed that the region flanking the deletion within the SfAV-1d genome is highly variable. In all the variants of this region, the whole IR region is missing, with 88.2 % of the variants missing part of or the whole adjacent SfAV-1a ORF71, 94.1 % missing part of or the whole of adjacent ORF72 and 64.6 % missing part of or the whole of ORF73.

The protein P23 identifies capsule-forming plasmatocytes in the moth Pseudoplusia includens

The moth Pseudoplusia includens produces four types of hemocytes named granulocytes, plasmatocytes, spherule cells and oenocytoids. Prior studies established that the main function of plasmatocytes in P. includens is encapsulation of parasitoids and other foreign entitites. P. includens plasmatocytes are also recognized by several monoclonal antibodies that bind unknown antigens. Of particular interest is the antibody 43E9A10 whose binding properties indicate that plasmatocytes consist of two subpopulations: cells that can spread on foreign surfaces and cells that cannot. Here we report 43E9A10 recognizes P23, which is a member of the aegerolysin protein family. Expression analyses confirmed that p23 is specifically expressed in plasmatocytes. Functional studies indicated that only P23-expressing plasmatocytes form capsules and spread in response to the cytokine plasmatocyte spreading peptide. In contrast, P23 showed no antibacterial or cytolytic activity toward bacteria and mammalian erythrocytes. Overall, our results suggest that P23 is a maturation marker that identifies capsule-forming plasmatocytes.

Ascoviruses: superb manipulators of apoptosis for viral replication and transmission

Ascoviruses are members of a recently described new family (Ascoviridae) of large double-stranded DNA viruses that attack immature stages of insects belonging to the order Lepidoptera, in which they cause a chronic, fatal disease. Ascoviruses have several unusual characteristics not found among other viruses, the most novel of which are their transmission by endoparasitic wasps and a unique cytopathology that resembles apoptosis. Cell infection induces apoptosis and in some species is associated with synthesis of a virus-encoded executioner caspase and several lipid-metabolizing enzymes. Rather than leading directly to cell death, synthesis of viral proteins results in the rescue of developing apoptotic bodies that are converted into large vesicles in which virions accumulate and continue to assemble. In infected larvae, millions of these virion-containing vesicles begin to disperse from infected tissues 48-72 h after infection into the blood, making it milky white, a major characteristic of the disease. Circulation of virions and vesicles in the blood facilitates mechanical transmission by parasitic wasps. Although ascoviruses appear to be very common, only five species are currently recognized, with the type species being the Spodoptera frugiperda ascovirus 1a. Ascovirus virions are large, enveloped, typically bacilliform or reniform in shape, and, depending on the species, have genomes that range from 119 to 186 kbp. Molecular phylogenetic evidence indicates that ascoviruses evolved from iridoviruses (family Iridoviridae) that attack lepidopteran larvae and are likely the evolutionary source of ichnoviruses (family Polydnaviridae), which assist endoparasitic hymenopterans in overcoming the defense responses of their insect hosts. Thus, as other molecular evidence suggests that iridoviruses evolved from phycodnaviruses (family Phycodnaviridae), an evolutionary pathway is apparent from phycodnaviruses via iridoviruses and ascoviruses to ichnoviruses.

Transcriptional analysis of a major capsid protein gene from Spodoptera exigua ascovirus 5a

The major capsid protein (mcp) gene of Spodoptera exigua ascovirus 5a (SeAV-5a) was confirmed by aphidicolin viral DNA replication inhibition analysis to be a late gene. The 5' and 3' ends of mcp gene transcripts have been mapped. Primer extension analyses indicated that transcription of the mcp gene initiates from a cytosine 25 nucleotides (nt) upstream of the translation start codon. Two independent approaches by 3' rapid amplification of cDNA ends (3' RACE) and oligo (dT) cellulose binding assay suggested that SeAV-5a mcp mRNA is polyadenylated. Analyses by 3' RACE also revealed that mcp transcripts terminate at a U, either at 26 or 38 nt downstream of the translation stop codon. The putative 5' transcription control region of the SeAV-5a mcp gene shares similarities with other ascoviruses and Chilo iridescent virus (CIV), containing a conserved TATA-box-like motif (TAATTAAA) and an ATTTGATCTT motif upstream of it. The 3' downstream regions of the mcp gene of all the ascoviruses examined and CIV can form a stem-loop structure, and the ends of the mcp gene transcripts of SeAV-5a are within the predicted stem-loop region. This suggests that the stem-loop structure of the mcp gene might be involved in transcription termination.

Identification of Trichoplusia ni ascovirus 2c virion structural proteins

Ascoviruses are a family of insect viruses with circular, double-stranded DNA genomes. With the sequencing of the Trichoplusia ni ascovirus 2c (TnAV-2c) genome, the virion structural proteins were identified by using tandem mass spectrometry. From at least eight protein bands visible on a Coomassie blue-stained gel of TnAV-2c virion proteins, seven bands generated protein sequences that matched predicted open reading frames (ORFs) in the genome, i.e. ORFs 2, 43, 115, 141, 142, 147 and 153. Among these ORFs, only ORF153, encoding the major capsid protein, has been characterized previously.

Sequence and organization of the Heliothis virescens ascovirus genome

The nucleotide sequence of the Heliothis virescens ascovirus (HvAV-3e) DNA genome was determined and characterized in this study. The circular genome consists of 186,262 bp, has a G+C content of 45.8 mol% and encodes 180 potential open reading frames (ORFs). Five unique homologous regions (hrs), 23 'baculovirus repeat ORFs' (bro) and genes encoding a caspase homologue and several enzymes involved in nucleotide replication and metabolism were found in the genome. Several ascovirus (AV)-, iridovirus- and baculovirus-homologous genes were identified. The genome is significantly larger than the recently sequenced genomes of Trichoplusia ni AV (TnAV-2c) and Spodoptera frugiperda AV (SfAV-1a). Gene-parity plots and overall similarity of ORFs indicate that HvAV-3e is related more closely to SfAV-1a than to TnAV-2c.

Sequence and organization of the Trichoplusia ni ascovirus 2c (Ascoviridae) genome

The complete Trichoplusia ni ascovirus 2c (TnAV-2c) genome sequence was determined. The circular genome contains 174,059 bp with 165 open reading frames (ORFs) of greater than 180 bp and two major homologous regions (hrs). The genome is quite A+T rich at 64.6%. Fifty-four ORFs had homologues in other insect viruses, such as ascoviruses, iridoviruses, baculoviruses and entomopoxviruses; 30 ORFs showed low identities with those from different parasitic protozoa and 12 ORFs were unique to TnAV-2c. TnAV-2c has 15 ORFs that could be grouped into six gene families. Three major conserved repeating sequences were identified and were interspersed in two regions. BLAST analyses revealed that there were 16 enzymes involved in gene transcription, DNA replication, and nucleotide metabolism. TnAV-2c has 12 and 25 ORFs sharing high identities with ascovirus and iridovirus homologues, respectively. The codon usage bias appears to be more similar to Spodoptera frugiperda ascovirus 1a than to iridoviruses.