A characterization of structural proteins expressed by Bombyx mori bidensovirus

Reassortments in single-stranded DNA multipartite viruses: Confronting expectations based on molecular constraints with field observations

Single-stranded DNA multipartite viruses, which mostly consist of members of the genus Begomovirus, family Geminiviridae, and all members of the family Nanoviridae, partly resolve the cost of genomic integrity maintenance through two remarkable capacities. They are able to systemically infect a host even when their genomic segments are not together in the same host cell, and these segments can be separately transmitted by insect vectors from host to host. These capacities potentially allow such viruses to reassort at a much larger spatial scale, since reassortants could arise from parental genotypes that do not co-infect the same cell or even the same host. To assess the limitations affecting reassortment and their implications in genome integrity maintenance, the objective of this review is to identify putative molecular constraints influencing reassorted segments throughout the infection cycle and to confront expectations based on these constraints with empirical observations. Trans-replication of the reassorted segments emerges as the major constraint, while encapsidation, viral movement, and transmission compatibilities appear more permissive. Confronting the available molecular data and the resulting predictions on reassortments to field population surveys reveals notable discrepancies, particularly a surprising rarity of interspecific natural reassortments within the Nanoviridae family. These apparent discrepancies unveil important knowledge gaps in the biology of ssDNA multipartite viruses and call for further investigation on the role of reassortment in their biology.

Mechanisms Mediating Nuclear Trafficking Involved in Viral Propagation by DNA Viruses

Typical viral propagation involves sequential viral entry, uncoating, replication, gene transcription and protein synthesis, and virion assembly and release. Some viral proteins must be transported into host nucleus to facilitate viral propagation, which is essential for the production of mature virions. During the transport process, nuclear localization signals (NLSs) play an important role in guiding target proteins into nucleus through the nuclear pore. To date, some classical nuclear localization signals (cNLSs) and non-classical NLSs (ncNLSs) have been identified in a number of viral proteins. These proteins are involved in viral replication, expression regulation of viral genes and virion assembly. Moreover, other proteins are transported into nucleus with unknown mechanisms. This review highlights our current knowledge about the nuclear trafficking of cellular proteins associated with viral propagation.

Characterization of the RNA Transcription Profile of Bombyx mori Bidensovirus

Bombyx mori bidensovirus (BmBDV) is a single-stranded DNA (ssDNA) virus from the genus Bidensovirus of the Bidnaviridae family, which, thus far, solely infects insects. It has a unique genome that contains bipartite DNA molecules (VD1 and VD2). In this study, we explored the detailed transcription mapping of the complete BmBDV genome (VD1 and VD2) by rapid amplification of cDNA ends (RACE), reverse transcription quantitative real-time PCR (RT-qPCR), and luciferase assays. For the first time, we report the transcription map of VD2. Our mapping of the transcriptional start sites reveals that the NS genes in VD1 have separate transcripts that are derived from overlapping promoters, P5 and P5.5. Thus, our study provides a strategy for alternative promoter usage in the expression of BmBDV genes.

Rapid detection of Bombyx mori bidensovirus by loop-mediated isothermal amplification based lateral flow dipstick assay for field applications

Bombyx mori bidensovirus (BmBDV), is the only bipartite single-stranded DNA (ssDNA) insect virus reported to date. BmBDV causes fatal flacherie disease in silkworm, resulting in large economic losses to sericulture. We developed a loop-mediated isothermal amplification with lateral flow dipstick (LAMP-LFD) method that can successfully and rapidly detect BmBDV DNA with a low limit of detection (5 fg, 400 copies of the BmBDV genome). The method was evaluated and improved for direct field diagnosis using silkworm faeces. In the field, the actual limit of detection was ∼50 fg (4000 copies of the BmBDV genome). The results demonstrated that, compared with traditional methods for BmBDV detection, our new LAMP-LFD method was much more rapid, sensitive and cost-effective, with less dependence on equipment, making it easier to use. The method has potential to be translated into a new diagnostic product for field applications in the sericulture industry.

Identification and Characterization of BmVta1, a Bombyx mori (Lepidoptera: Bombycidae) Homologue for Vta1 That is Up-Regulated in Development

Vps20-associated 1 (Vta1) positively regulates Vacuolar protein sorting 4 (Vps4) to disassemble endosomal sorting complex required for transport III (ESCRT-III) for repeated uses in multivesicular body (MVB) pathway, virus budding and other processes. Currently, these proteins have mainly been studied in yeast and mammalian cells, while identities of them in insects remain largely unknown. We previously identified BmVps4, a Vps4 homologue from Bombyx mori. Here, we report the identification of a homologue for Vta1, designated as BmVta1. The BmVta1 cDNA contains an open reading frame of 933 bp and encodes a protein of 311 amino acid residues. We cloned BmVta1, expressed it in Escherichia coli, and prepared mouse polyclonal antibodies. Like BmVps4, BmVta1 is well conserved as shown by sequence analysis. Both proteins are localized in cytoplasm as revealed by subcellular location analysis. Interestingly, as revealed by semi-quantitative reverse transcription polymerase chain reaction (sqRT-PCR), transcriptions of BmVta1 and BmVps4 are highly up-regulated during silkworm metamorphosis and embryogenesis but down-regulated during larva stages, and are of higher levels in head, silk gland and testis than in Malpighian tube, fat body and ganglion, indicating important and similar roles of them in silkworm development and in silkworm tissues and organs. However, compared to BmVps4, the transcription of BmVta1 changes less drastically during development and is of much higher levels in midgut, ovary and hemolymph, suggesting the existence of distinct requirements of them in silkworm development and in certain tissues and organs.

Viruses of insects reared for food and feed

The use of insects as food for humans or as feed for animals is an alternative for the increasing high demand for meat and has various environmental and social advantages over the traditional intensive production of livestock. Mass rearing of insects, under insect farming conditions or even in industrial settings, can be the key for a change in the way natural resources are utilized in order to produce meat, animal protein and a list of other valuable animal products. However, because insect mass rearing technology is relatively new, little is known about the different factors that determine the quality and yield of the production process. Obtaining such knowledge is crucial for the success of insect-based product development. One of the issues that is likely to compromise the success of insect rearing is the outbreak of insect diseases. In particular, viral diseases can be devastating for the productivity and the quality of mass rearing systems. Prevention and management of viral diseases imply the understanding of the different factors that interact in insect mass rearing. This publication provides an overview of the known viruses in insects most commonly reared for food and feed. Nowadays with large-scale sequencing techniques, new viruses are rapidly being discovered. We discuss factors affecting the emergence of viruses in mass rearing systems, along with virus transmission routes. Finally we provide an overview of the wide range of measures available to prevent and manage virus outbreaks in mass rearing systems, ranging from simple sanitation methods to highly sophisticated methods including RNAi and transgenics.