Effect of nuclear polyhedrosis virus infection on the development and pupation of gypsy moth larvae

An Empirical Test of the Role of Small-Scale Transmission in Large-Scale Disease Dynamics

A key assumption of epidemiological models is that population-scale disease spread is driven by close contact between hosts and pathogens. At larger scales, however, mechanisms such as spatial structure in host and pathogen populations and environmental heterogeneity could alter disease spread. The assumption that small-scale transmission mechanisms are sufficient to explain large-scale infection rates, however, is rarely tested. Here, we provide a rigorous test using an insect-baculovirus system. We fit a mathematical model to data from forest-wide epizootics while constraining the model parameters with data from branch-scale experiments, a difference in spatial scale of four orders of magnitude. This experimentally constrained model fits the epizootic data well, supporting the role of small-scale transmission, but variability is high. We then compare this model's performance to an unconstrained model that ignores the experimental data, which serves as a proxy for models with additional mechanisms. The unconstrained model has a superior fit, revealing a higher transmission rate across forests compared with branch-scale estimates. Our study suggests that small-scale transmission is insufficient to explain baculovirus epizootics. Further research is needed to identify the mechanisms that contribute to disease spread across large spatial scales, and synthesizing models and multiscale data are key to understanding these dynamics.

Effects of a Covert Infection with Phthorimaea operculella granulovirus in Insect Populations of Phthorimaea operculella

Virus infections of insects can easily stay undetected, neither showing typical signs of a disease, nor being lethal. Such a stable and most of the time covert infection with Phthorimaea operculella granulovirus (PhopGV) was detected in a Phthorimaea operculella laboratory colony, which originated from Italy (Phop-IT). This covert virus (named PhopGV-R) was isolated, purified and characterized at the genetic level by full genome sequencing. Furthermore, the insect colony Phop-IT was used to study the crowding effect, double infection with other PhopGV isolates (CR3 and GR1), and co-infection exclusion. An infection with a second homologous virus (PhopGV-CR3) activated the covert virus, while a co-infection with another virus isolate (PhopGV-GR1) led to its suppression. This study shows that stable virus infections can be common for insect populations and have an impact on population dynamics because they can suppress or enable co-infection with another virus isolate of the same species.

Physiological and metabolic consequences of viral infection in Drosophila melanogaster

An extensively used model system for investigating anti-pathogen defence and innate immunity involves Drosophila C virus (DCV) and Drosophila melanogaster. While there has been a significant effort to understand infection consequences at molecular and genetic levels, an understanding of fundamental higher-level physiology of this system is lacking. Here, we investigate the metabolic rate, locomotory activity, dry mass and water content of adult male flies injected with DCV, measured over the 4 days prior to virus-induced mortality. DCV infection resulted in multiple pathologies, notably the depression of metabolic rate beginning 2 days post-infection as a response to physiological stress. Even in this depressed metabolic state, infected flies did not decrease their activity until 1 day prior to mortality, which further suggests that cellular processes and synthesis are disrupted because of viral infection. Growth rate was also reduced, indicating that energy partitioning is altered as infection progresses. Microbial infection in insects typically results in an increase in excretion; however, water appeared to be retained in DCV-infected flies. We hypothesise that this is due to a fluid intake-output imbalance due to disrupted transport signalling and a reduced rate of metabolic processing. Furthermore, infected flies had a reduced rate of respiration as a consequence of metabolic depression, which minimised water loss, and the excess mass as a result of water retention is concurrent with impaired locomotory ability. These findings contribute to developing a mechanistic understanding of how pathologies accumulate and lead to mortality in infected flies.

Proteomic analysis of Frankliniella occidentalis and differentially expressed proteins in response to tomato spotted wilt virus infection

Tomato spotted wilt virus (TSWV) is transmitted by Frankliniella occidentalis in a persistent propagative manner. Despite the extensive replication of TSWV in midgut and salivary glands, there is little to no pathogenic effect on F. occidentalis. We hypothesize that the first-instar larva (L1) of F. occidentalis mounts a response to TSWV that protects it from pathogenic effects caused by virus infection and replication in various insect tissues. A partial thrips transcriptome was generated using 454-Titanium sequencing of cDNA generated from F. occidentalis exposed to TSWV. Using these sequences, the L1 thrips proteome that resolved on a two-dimensional gel was characterized. Forty-seven percent of the resolved protein spots were identified using the thrips transcriptome. Real-time quantitative reverse transcriptase PCR (RT-PCR) analysis of virus titer in L1 thrips revealed a significant increase in the normalized abundance of TSWV nucleocapsid RNA from 2 to 21 h after a 3-h acquisition access period on virus-infected plant tissue, indicative of infection and accumulation of virus. We compared the proteomes of infected and noninfected L1s to identify proteins that display differential abundances in response to virus. Using four biological replicates, 26 spots containing 37 proteins were significantly altered in response to TSWV. Gene ontology assignments for 32 of these proteins revealed biological roles associated with the infection cycle of other plant- and animal-infecting viruses and antiviral defense responses. Our findings support the hypothesis that L1 thrips display a complex reaction to TSWV infection and provide new insights toward unraveling the molecular basis of this interaction.

Glycosyltransferases encoded by viruses

Studies of cellular biology in recent decades have highlighted the crucial roles of glycans in numerous important biological processes, raising the concept of glycomics that is now considered as important as genomics, transcriptomics and proteomics. For millions of years, viruses have been co-evolving with their hosts. Consequently, during this co-evolution process, viruses have acquired mechanisms to mimic, hijack or sabotage host processes that favour their replication, including mechanisms to modify the glycome. The importance of the glycome in the regulation of host–virus interactions has recently led to a new concept called ‘glycovirology’. One fascinating aspect of glycovirology is the study of how viruses affect the glycome. Viruses reach that goal either by regulating expression of host glycosyltransferases or by expressing their own glycosyltransferases. This review describes all virally encoded glycosyltransferases and discusses their established or putative functions. The description of these enzymes illustrates several intriguing aspects of virology and provides further support for the importance of glycomics in biological processes.

BmNPV alters NADP-dependent malate dehydrogenase activity and associated macromolecules and retards growth and development of the mulberry silkworm,Bombyx moriL., during the final instar

Changes in tissue-specific NADP-dependent malate dehydrogenase (MDH) activity and protein and cholesterol contents and retardation of growth and development in the mulberry silkworm, Bombyx mori L., induced by infection with a baculovirus, the B. mori nucleopolyhedrovirus (BmNPV), were investigated. The study revealed that the relative growth rate and development of the fifth-instar larva was significantly inhibited during the progression of nuclear polyhedrosis disease at various times post inoculation (p.i.). Percent pupation and reproductive output were also adversely affected. NADP-dependent MDH activity in the hemolymph peaked sharply at 6 h p.i. and then gradually decreased, reaching a minimum at 264 h p.i., with exceptions at 30 and 72 h p.i. These levels of MDH activity were different from those in non-infected controls. In fat-body tissues, NADP-dependent MDH activity was significantly higher in infected insects than in non-infected controls of the same age. These results indicate that baculovirus infection causes significant changes in intermediary metabolic pathways, causing a significant fall and rise in protein and cholesterol contents in tissues during development of fifth-instar larvae.

Effects of the Epap granulovirus on its host, Epinotia aporema (Lepidoptera: Tortricidae)

The bean shoot borer, Epinotia aporema, is a major pest of soybeans in Argentina. Larvae of this pest are attacked by a granulovirus (EpapGV) that is the most important cause of sporadic epizootics in E. aporema populations. We studied the pathology of this virus in last-instar larvae using light and electron microscopy, and evaluated the effect of the disease on larval growth and development. EpapGV caused a polyorganotropic infection. No nucleocapsids were observed in the nuclei of infected cells prior to nuclear membrane disruption. Nevertheless, granulin was detected in the nucleus by immuno-gold staining, indicating that late gene expression occurred prior to nuclear membrane disruption. Establishment of the virogenic stroma led to complexes of continuous parallel convoluted membranous sheets. Nucleocapsids were enveloped in these areas to form virions, which were then occluded. Apparently as part of the cell-to-cell spread of infection, nucleocapsids were observed enclosed in large numbers within membrane-bound vesicles located between the cells and basal lamina. Larvae infected by EpapGV suffered a retardation of development and typically failed to pupate, but exhibited a weight increase greater than that of healthy E. aporema.

Ecological considerations for the environmental impact evaluation of recombinant baculovirus insecticides

The history of baculoviruses in insect control and the current status of recombinant baculovirus (recBV) insecticides in the laboratory and the field are briefly outlined. A conceptual model for impact evaluation is described that distinguishes between scientific impact evaluation and regulatory risk assessment. Its components are identified and reviewed in the light of existing ecological theory and experimental study under the categories of impact identification, exposure identification, and impact evaluation. Impact identification aims to identify species and populations sensitive to direct or indirect impacts by a recBV. Exposure identification examines how susceptible populations may be exposed to a recBV. Impact evaluation combines these data to predict the potential for recBV impacts in the environment.

Nucleopolyhedrovirus interactions with their insect hosts

It is clear from this brief review that our understanding of the molecular cross-talk between insects and their baculovirus pathogens is still very limited. Studies in cell culture have taught us a great deal about the basic baculovirus molecular machinery and how it is regulated, and in many cases this information has been predictive of what occurs in infected insects. Frequently, however, studies in cell culture do not adequately predict the infection process in insect hosts, as demonstrated by viral mutants (some of which were discussed in this review) that behave identically to wild-type virus in cell culture but differ markedly in larvae. More baculovirus studies, therefore, need to be conducted in vivo if we are to improve our understanding of the complex interactions between baculoviruses and their hosts. Conducting baculovirus studies in insects (or at least in primary cell culture) also offers the opportunity to address questions that reach beyond the baculovirus community in significance. For example, almost all of our knowledge of viral fusion mechanisms comes from infection of cells in culture where the pH is neutral or acidic and the temperature is constant at 27 degrees or 37 degrees C. An answer to the question of how the ODV envelope fuses with the microvillar membrane of columnar epithelial cells in the highly alkaline midgut environment at low temperatures will not only be important for an improved understanding of baculovirus infection in the natural world, but will also constitute a new chapter on viral entry mechanisms. Similarly, the answer to the question of how baculovirus nucleocapsids move basally within microvilli promises to involve factors and/or a mechanism not yet described by cell biologists, and so will constitute a valuable contribution to both baculovirology and cell biology. There are many more such examples of biological mechanisms that can be uniquely explored within the context of baculoviruses and their insect hosts, some of which have been highlighted in this review. As more and more young investigators realize the importance of combining a knowledge of virology, molecular technology, and insect biology, however, many of the outstanding mysteries will be solved.