Detailed investigation of the sequential pathological changes in silkworm larvae infected with Bombyx densovirus type 1

Resistance mechanism of Nid-1, a dominant non-susceptibility gene, against Bombyx mori densovirus 1 infection

Bombyx mori densovirus 1 (BmDV1) is a pathogen that causes flacherie disease in mulberry silkworms (B. mori). The absolute resistance (non-susceptibility) to BmDV1 of certain silkworm strains is determined independently by two genes, nsd-1 and Nid-1. Previously, we investigated the expression of viral transcript in virus-inoculated silkworms carrying different nsd-1 and Nid-1 genotypes, and observed that nsd-1 and Nid-1 expression blocked the early and late steps of BmDV1 infection, respectively. In addition, we found that nsd-1 encoded a Bombyx-specific mucin-like membrane protein only present on the surface of the midgut, where BmDV1 could infect. In this study, we dissected the resistance mechanism by Nid-1 against BmDV1 infection by investigating the sequential changes in the accumulation of viral DNA, transcripts, and proteins derived from BmDV1 in susceptible strain (pxj) and Nid-1-carrying resistant strain (No. 908) after inoculation with BmDV1. Genomic PCR results showed that the BmDV1 DNA was detected immediately after the infection in both strains but rapidly decreased in the Nid-1-carrying strain No. 908 compared with the susceptible strain pxj. RT-PCR results also showed that the BmDV1 transcripts of Nid-1-carrying strain No. 908 were rapidly decreased after the infection. Moreover, BmDV1-derived proteins were not detected in No. 908 throughout the infection. These results suggest that Nid-1 expression might inhibit the accumulation of viral DNA and transcripts. As Nid-1 has not been molecularly characterized, its identification will contribute to the elucidation of the interactions between the silkworm and BmDV1.

An investigation into the effects of infection and ORF expression patterns of the Indian bidensovirus isolate (BmBDV) infecting the silkworm Bombyx mori

The Indian isolate of Bombyx mori bidensovirus (BmBDV) is a bipartite virus that comprises of a segmented, non-homologous, two linear single-strands of DNA molecules (VD1 and VD2). It is one of the causative agents of the fatal silkworm disease 'Flacherie' that causes severe crop loss for the sericulture farmers. Genome analyses of the Indian isolate of BmBDV revealed that it consists of 6 putative ORFs similar to the Japanese and Chinese isolates. VD1 consists of 4 ORFs while VD2 has 2 ORFs that code for 4 non- structural (NS) and 2 structural (VP) proteins, in total. In this study, we investigated, in detail, the impact of BmBDV pathogenesis on growth and development of the silkworm Bombyx mori, at different developmental stages. Mortality rate and weight uptake analyses were also performed on newly ecdysed 4th instar larvae. BmBDV infection was not found to be developmental stage specific and it occurred at all stages. Onset of mortality took place 8 days post infection (dpi) and 100% mortality occurred at 11 dpi. The infected larvae showed a significant difference in weight uptake wherein from 7 dpi the larvae stopped gaining weight and from 8th dpi started demonstrating the typical symptoms of flacherie. Further, the expression pattern of the 6 viral ORFs were also investigated in the newly ecdysed 4th instar BmBDV infected silkworms. Among all the six ORFs, VD2 ORF 1 and 2 revealed the highest transcript numbers, which was followed by VD1 ORF 4 that encodes for the viral DNA polymerase enzyme. This was the first ever attempt to understand the pathogenesis and the expression pattern of all the six ORF transcripts of the Indian isolate of BmBDV.

Host Response against Virus Infection in an Insect: Bidensovirus Infection Effect on Silkworm (Bombyx mori)

Silk cocoons obtained from silkworms are the primary source of commercial silk, making the silkworm an economically important insect. However, the silk industry suffers significant losses due to various virus infections. Bombyx mori bidensovirus (BmBDV) is one of the pathogens that cause flacherie disease in silkworms. Most silkworm strains die after BmBDV infection. However, certain silkworm strains show resistance to the virus, which is determined by a single recessive gene, nsd-2. The +^nsd-2 gene (allele of nsd-2; the susceptibility gene) encodes a putative amino acid transporter expressed only in the insect’s midgut, where BmBDV can infect, suggesting that this membrane protein may function as a receptor for BmBDV. Interestingly, the expression analysis revealed no changes in the +^nsd-2 gene expression levels in virus-uninfected silkworms, whereas the gene expression drastically decreased in the virus-infected silkworm. This condition indicates that the host factor’s expression, the putative virus receptor, is affected by BmBDV infection. It has recently been reported that the expression levels of some host genes encoding cuticle, antioxidant, and immune response-related proteins were significantly regulated by BmBDV infection. In this review, we discuss the host response against virus infection based on our knowledge and long-term research experience in this field.

Mass mortality in freshwater mussels (Actinonaias pectorosa) in the Clinch River, USA, linked to a novel densovirus

Freshwater mussels (order Unionida) are among the world's most biodiverse but imperiled taxa. Recent unionid mass mortality events around the world threaten ecosystem services such as water filtration, nutrient cycling, habitat stabilization, and food web enhancement, but causes have remained elusive. To examine potential infectious causes of these declines, we studied mussels in Clinch River, Virginia and Tennessee, USA, where the endemic and once-predominant pheasantshell (Actinonaias pectorosa) has suffered precipitous declines since approximately 2016. Using metagenomics, we identified 17 novel viruses in Clinch River pheasantshells. However, only one virus, a novel densovirus (Parvoviridae; Densovirinae), was epidemiologically linked to morbidity. Clinch densovirus 1 was 11.2 times more likely to be found in cases (moribund mussels) than controls (apparently healthy mussels from the same or matched sites), and cases had 2.7 (log₁₀) times higher viral loads than controls. Densoviruses cause lethal epidemic disease in invertebrates, including shrimp, cockroaches, crickets, moths, crayfish, and sea stars. Viral infection warrants consideration as a factor in unionid mass mortality events either as a direct cause, an indirect consequence of physiological compromise, or a factor interacting with other biological and ecological stressors to precipitate mortality.

Interaction of a Densovirus with Glycans of the Peritrophic Matrix Mediates Oral Infection of the Lepidopteran Pest Spodoptera frugiperda

The success of oral infection by viruses depends on their capacity to overcome the gut epithelial barrier of their host to crossing over apical, mucous extracellular matrices. As orally transmitted viruses, densoviruses, are also challenged by the complexity of the insect gut barriers, more specifically by the chitinous peritrophic matrix, that lines and protects the midgut epithelium; how capsids stick to and cross these barriers to reach their final cell destination where replication goes has been poorly studied in insects. Here, we analyzed the early interaction of the Junonia coenia densovirus (JcDV) with the midgut barriers of caterpillars from the pest Spodoptera frugiperda. Using combination of imaging, biochemical, proteomic and transcriptomic analyses, we examined in vitro, ex vivo and in vivo the early interaction of the capsids with the peritrophic matrix and the consequence of early oral infection on the overall gut function. We show that the JcDV particle rapidly adheres to the peritrophic matrix through interaction with different glycans including chitin and glycoproteins, and that these interactions are necessary for oral infection. Proteomic analyses of JcDV binding proteins of the peritrophic matrix revealed mucins and non-mucins proteins including enzymes already known to act as receptors for several insect pathogens. In addition, we show that JcDV early infection results in an arrest of N-Acetylglucosamine secretion and a disruption in the integrity of the peritrophic matrix, which may help viral particles to pass through. Finally, JcDV early infection induces changes in midgut genes expression favoring an increased metabolism including an increased translational activity. These dysregulations probably participate to the overall dysfunction of the gut barrier in the early steps of viral pathogenesis. A better understanding of early steps of densovirus infection process is crucial to build biocontrol strategies against major insect pests.

Antioxidant Systems as a Response to Midgut Cellular of Bombyx mori Lineu, 1758 (Lepidoptera: Bombycidae) Infection for Baculoviruses

Bombyx mori nucleopolyhedrovirus (BmNPV) is a DNA virus that infects different tissues in Bombyx mori at immature stage. Caterpillars become infected after ingesting polyhedral occlusion bodies (POB) present in contaminated mulberry leaves and spread through the body after passing the epithelium of the midgut. As this organ is responsible for digestion, most absorption of nutrients requires an intact epithelium to maintain gastrointestinal physiology. Considering the importance of this organ in the feeding of caterpillars and in the production of quality silk threads, and because it is also the first barrier faced by the BmNPV, the study analyzed details of cytopathological events in the intestinal cells as well as evaluated the action of the antioxidant systems as a response to cellular infection. For this purpose, B. mori hybrid caterpillars of fifth instar were inoculated with a suspension of 7.8 × 107 POB ml-1 and, from the first to the eighth day post-inoculation (dpi), segments of the midgut were collected and processed for light and electronic microscopy. The nuclei of columnar cells showed polyhedric occlusion bodies in the seventh dpi and fragmentation of those cells, with peritrophic matrix disorganization. Analysis of antioxidant systems shows some moments of changes of the catalase enzymes and superoxide dismutase. Analysis of the cholinergic system revealed changes only at the beginning of the infection. Thus, the article acknowledges the antioxidant system as a barrier to stop viral infection, albeit it cannot stop infection from occurring, once a coevolutionary bond is maintained between virus and host.

Gut immunity in Lepidopteran insects

Lepidopteran insects constitute one of the largest fractions of animals on earth, but are considered pests in their relationship with man. Key to the success of this order of insects is its ability to digest food and absorb nutrition, which takes place in the midgut. Because environmental microorganisms can easily enter Lepidopteran guts during feeding, the innate immune response guards against pathogenic bacteria, virus and microsporidia that can be devoured with food. Gut immune responses are complicated by both resident gut microbiota and the surrounding peritrophic membrane and are distinct from immune responses in the body cavity, which depend on the function of the fat body and hemocytes. Due to their relevance to agricultural production, studies of Lepidopteran insect midgut and immunity are receiving more attention, and here we summarize gut structures and functions, and discuss how these confer immunity against different microorganisms. It is expected that increased knowledge of Lepidopteran gut immunity may be utilized for pest biological control in the future.

Identification and methods for prevention of Enterococcus mundtii infection in silkworm larvae, Bombyx mori, reared on artificial diet

Previously, it was reported that Enterococcus mundtii (E. mundtii) was associated with flacherie disease of silkworm larvae reared on artificial diet. In this study, we report that E. mundtii was isolated from diseased silkworm larvae, and validated as a pathogenic bacterium of the animal. When silkworm larva was infected with 1.04 × 10⁶ colony-forming units of E. mundtii via oral administration of diet, half population died within six days, indicating that the bacterium is pathogenic to silkworm. Less severe infection was found to cause anorexia and hamper the development of larvae. This pathogen was found to proliferate in both time- and dose-dependent manner in the gastrointestinal tract of the animal. The bacterium was isolated from powder of artificial diet made from mulberry leaves, and from mulberry leaves growing at a field. Minimum inhibitory concentration determination revealed that this bacterium was susceptible to tested antibiotics. Vancomycin treatment of diet significantly decreased the number of E. mundtii in intestine of silkworm larvae infected with the bacteria, compared to control. Furthermore, autoclaving or gamma ray irradiation of diet was also effective for exclusion of E. mundtii from the diet without the loss of its nutrient capacities. These results suggest that mulberry leaves used in making artificial diet for silkworm larvae is one of the sources of E. mundtii infection; and that antibiotic treatment, autoclaving or gamma ray irradiation of artificial diet can exclude the bacteria.