Timely trigger of caterpillar zombie behaviour: temporal requirements for light in baculovirus-induced tree-top disease

Mechanisms Underlying Ophiocordyceps Infection and Behavioral Manipulation of Ants: Unique or Ubiquitous?

Parasite manipulation of host behavior, as an effective strategy to establish transmission, has evolved multiple times across taxa, including fungi. Major strides have been made to propose molecular mechanisms that underlie manipulative parasite-host interactions including the manipulation of carpenter ant behavior by Ophiocordyceps. This research suggests that the secretion of parasite proteins and light-driven biological rhythms are likely involved in the infection and manipulation biology of Ophiocordyceps and other manipulating parasites. Here, we discuss research on Ophiocordyceps considering findings from other (fungal) parasites that either are relatively closely related (e.g., other insect- and plant-infecting Hypocreales) or also manipulate insect behavior (e.g., Entomophthorales). As such, this review aims to put forward this question: Are the mechanisms behind Ophiocordyceps manipulation and infection unique, or did they convergently evolve? From this discussion, we pose functional hypotheses about the infection biology of Ophiocordyceps that will need to be addressed in future studies.

The complete genome sequence of an alphabaculovirus from the brown tussock moth, Olene mendosa Hübner, expands our knowledge of lymantriine baculovirus diversity and evolution

The complete genome sequence was determined for an apparent alphabaculovirus isolated from larval cadavers of the brown tussock moth, Olene mendosa Hübner, collected during an epizootic in Coimbatore, India. The genome was determined to be a circular 142,291 bp molecule, and 147 ORFs and nine homologous regions were annotated for the sequence. Analysis of the sequence confirmed that this virus, Olene mendosa nucleopolyhedrovirus (OlmeNPV), was a member of genus Alphabaculovirus in family Baculoviridae. Phylogenies inferred from nucleotide and amino acid alignments indicated that OlmeNPV was part of a group of viruses that infect moths of genus Lymantria, suggesting that OlmeNPV may have shifted hosts from a Lymantria species to an ancestral Olene species at some point during its evolutionary history. OlmeNPV was most closely related to Lymantria xylina multiple nucleopolyhedrovirus isolate 5 (LyxyMNPV-5). The genomes of OlmeNPV and LyxyMNPV-5 were distinguished not only by differences in ORF content, but by a 27 kbp region of the genome that is inverted in LyxyMNPV-5 relative to OlmeNPV. Pairwise nucleotide distances between OlmeNPV and other Lymantria spp. alphabaculoviruses indicate that OlmeNPV represents a new baculovirus species.

Baculoviruses hijack the visual perception of their caterpillar hosts to induce climbing behavior, thus promoting virus dispersal

Baculoviruses can induce climbing behavior in their caterpillar hosts to ensure they die at elevated positions to enhance virus transmission, providing an excellent model to study parasitic manipulation of host behavior. Here, we demonstrate that climbing behavior occurred mostly during daylight hours, and that the height at death of Helicoverpa armigera single nucleopolyhedrovirus (HearNPV)-infected larvae increases with the height of the light source. Phototaxic and electroretinogram (ERG) responses were enhanced after HearNPV-infection in host larvae, and ablation of stemmata in infected larvae prevented both phototaxis and climbing behavior. Through transcriptome and quantitative PCR, we confirmed that two opsin genes (a blue light-sensitive gene, HaBL; and a long wave-sensitive gene, HaLW) as well as the TRPL (transient receptor potential-like channel protein) gene, all integral to the host's visual perception pathway, were significantly up-regulated after HearNPV infection. Knockout of HaBL, HaLW, or TRPL genes using the CRISPR/Cas9 system resulted in significantly reduced ERG responses, phototaxis, and climbing behavior in HearNPV-infected larvae. These results reveal that HearNPV alters the expression of specific genes to hijack host visual perception at fundamental levels - photoreception and phototransduction - in order to induce climbing behavior in host larvae.

Hijacking time: How Ophiocordyceps fungi could be using ant host clocks to manipulate behavior

Ophiocordyceps fungi manipulate ant behaviour as a transmission strategy. Conspicuous changes in the daily timing of disease phenotypes suggest that Ophiocordyceps and other manipulators could be hijacking the host clock. We discuss the available data that support the notion that Ophiocordyceps fungi could be hijacking ant host clocks and consider how altering daily behavioural rhythms could benefit the fungal infection cycle. By reviewing time‐course transcriptomics data for the parasite and the host, we argue that Ophiocordyceps has a light‐entrainable clock that might drive daily expression of candidate manipulation genes. Moreover, ant rhythms are seemingly highly plastic and involved in behavioural division of labour, which could make them susceptible to parasite hijacking. To provisionally test whether the expression of ant behavioural plasticity and rhythmicity genes could be affected by fungal manipulation, we performed a gene co‐expression network analysis on ant time‐course data and linked it to available behavioural manipulation data. We found that behavioural plasticity genes reside in the same modules as those affected during fungal manipulation. These modules showed significant connectivity with rhythmic gene modules, suggesting that Ophiocordyceps could be indirectly affecting the expression of those genes as well.

A proctolin-like peptide is regulated after baculovirus infection and mediates in caterpillar locomotion and digestion

Baculoviruses constitute a large group of invertebrate DNA viruses, predominantly infecting larvae of the insect order Lepidoptera. During a baculovirus infection, the virus spreads throughout the insect body producing a systemic infection in multiple larval tissues, included the central nervous system (CNS). As a main component of the CNS, neuropeptides are small protein-like molecules functioning as neurohormones, neurotransmitters, or neuromodulators. These peptides are involved in regulating animal physiology and behavior and could be altered after baculovirus infection. In this study, we have investigated the effect of Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) infection on expression of Spodoptera exigua neuropeptides and neuropeptide-like genes. Expression of the gene encoding a polypeptide that resembles the well-known insect neuropeptide proctolin and named as proctolin-like peptide (PLP), was downregulated in the larval brain following infection and was chosen for further analysis. A recombinant Autographa californica multiple nucleopolyhedrovirus (AcMNPV) overexpressing the C-terminal part of the PLP was generated and used in bioassays using S. exigua larvae to study its influence on the viral infection and insect behavior. AcMNPV-PLP-infected larvae showed less locomotion activity and a reduction in growth compared to larvae infected with wild type AcMNPV or mock-infected larvae. These results are indicative of this new peptide as a neuromodulator that regulates visceral and skeletal muscle contractions and offers a novel effector involved in the behavioral changes during baculovirus infection.

High-resolution analysis of baculovirus-induced host manipulation in the domestic silkworm, Bombyx mori

Many parasites manipulate host behaviour to enhance their transmission. Baculoviruses induce enhanced locomotory activity (ELA) combined with subsequent climbing behaviour in lepidopteran larvae, which facilitates viral dispersal. However, the mechanisms underlying host manipulation system are largely unknown. Previously, larval locomotion during ELA was summarized as the distance travelled for a few minutes at several time points, which are unlikely to characterize ELA precisely, as ELA typically persists for several hours. In this study, we modified a recently developed method using time-lapse recording to characterize locomotion of Bombyx mori larvae infected with B. mori nucleopolyhedrovirus (BmNPV) for 24 h at 3 s resolution. Our data showed that the locomotion of the mock-infected larvae was restricted to a small area, whereas the BmNPV-infected larvae exhibited a large locomotory area. These results indicate that BmNPV dysregulates the locomotory pattern of host larvae. Furthermore, both the mock- and BmNPV-infected larvae showed periodic cycles of movement and stationary behaviour with a similar frequency, suggesting the physiological mechanisms that induce locomotion are unaffected by BmNPV infection. In contrast, the BmNPV-infected larvae exhibited fast and long-lasting locomotion compared with mock-infected larvae, which indicates that locomotory speed and duration are manipulated by BmNPV.

Genetic Underpinnings of Host Manipulation by Ophiocordyceps as Revealed by Comparative Transcriptomics

Ant-infecting Ophiocordyceps fungi are globally distributed, host manipulating, specialist parasites that drive aberrant behaviors in infected ants, at a lethal cost to the host. An apparent increase in activity and wandering behaviors precedes a final summiting and biting behavior onto vegetation, which positions the manipulated ant in a site beneficial for fungal growth and transmission. We investigated the genetic underpinnings of host manipulation by: (i) producing a high-quality hybrid assembly and annotation of the Ophiocordyceps camponoti-floridani genome, (ii) conducting laboratory infections coupled with RNAseq of O. camponoti-floridani and its host, Camponotus floridanus, and (iii) comparing these data to RNAseq data of Ophiocordyceps kimflemingiae and Camponotus castaneus as a powerful method to identify gene expression patterns that suggest shared behavioral manipulation mechanisms across Ophiocordyceps-ant species interactions. We propose differentially expressed genes tied to ant neurobiology, odor response, circadian rhythms, and foraging behavior may result by activity of putative fungal effectors such as enterotoxins, aflatrem, and mechanisms disrupting feeding behaviors in the ant.

Where the baculoviruses lead, the caterpillars follow: baculovirus-induced alterations in caterpillar behaviour

Baculoviruses are well-known for altering the behaviour of their caterpillar hosts by inducing hyperactivity (enhanced locomotion) and/or tree-top disease (climbing to elevated positions before death). These features, along with the genomic small size of baculoviruses compared to non-viral parasites and the at hand techniques for producing mutants, imply that baculoviruses are excellent tools for unravelling the molecular mechanisms underlying parasitic alteration of host behaviour. Baculoviruses can be easily mutated, allowing an optimal experimental setup in comparative studies, where for instance host gene expression can be compared between insects infected with wild-type viruses or with mutant viruses lacking genes involved in behavioural manipulation. Recent studies have revealed the first insight into the underlying molecular pathways that lead to the typical behaviour of baculovirus-infected caterpillars and into the role of light therein. Since host behaviour in general is mediated through the host's central nervous system (CNS), a promising future step will be to study how baculoviruses regulate the neuronal activity of the host.

Identification and expression analysis of the Spodoptera exigua neuropeptidome under different physiological conditions

Neuropeptides are small signalling molecules acting as neurohormones, neurotransmitters and neuromodulators. Being part of the chemical communication system between cells within an organism, they are involved in the regulation of different aspects of animal physiology and behaviour such as feeding, reproduction, development and locomotion. Transcriptomic data from larval and adult tissues have been obtained and mined to generate a comprehensive neuropeptidome for the polyphagous insect pest Spodoptera exigua. Sixty-three neuropeptides have been identified and described based on their tissue specificity and their regulation in response to different abiotic perturbations. Expression analyses have identified those neuropeptides involved in ingestive and digestive behaviour of S. exigua larvae and revealed a general pattern of upregulation in the midgut during larval starvation. Our results represent a comprehensive neuropeptidome of a lepidopteran species that will be highly relevant to future studies and provide novel information of the insect's perception of its environment.

Effect of Different Light Spectrum in Helicoverpa armigera Larvae during HearNPV Induced Tree-Top Disease

Lepidopteran larvae upon infection by baculovirus show positive photo-tactic movement during tree-top disease. In light of many insects exploiting specific spectral information for the different behavioral decision, each spectral wavelength of light is an individual parsimonious candidate for such behavior stimulation. Here, we investigated the responses of third instar Helicoverpa armigera larvae infected by Helicoverpa armigera nucleopolyhedrovirus (HearNPV) to white (broad-spectrum), blue (450–490 nm), UVA (320–400 nm), and UVB (290–320 nm) lights for the tree-top disease. Our findings suggest that tree-top phenomenon is induced only when the light is applied from above. Blue, white and UVA lights from above induced tree-top disease, causing infected larvae to die in an elevated position compared to those larvae living in the complete dark. In contrast, UVB from above did not induce tree-top disease. Blue light exerted the maximum photo-tactic response, significantly (p < 0.01) higher than white light. The magnitude of the response decreased with decreasing wavelength to UVA, and no response at UVB. Our results suggested that the spectral wavelength of the light has a significant effect on the induction of the tree-top disease in H. armigera third instar larvae infected with HearNPV.

Phototransduction and circadian entrainment are the key pathways in the signaling mechanism for the baculovirus induced tree-top disease in the lepidopteran larvae

The tree-top disease is an altered behavioral state, displayed by baculovirus-infected lepidopteran larvae, and characterized by climbing to an elevated position before death. The detailed molecular mechanism underlying this phenomenal behavior change has not been reported yet. Our study focused on the transcriptomic changes in the host larvae due to baculovirus infection from pre-symptomatic to tree-top disease stage. Enrichment map visualization of the gene sets grouped based on the functional annotation similarity revealed 34 enriched pathways in signaling mechanism cluster during LdMNPV induced tree-top disease in third instar Lymantria dispar asiatica larvae. Directed light bioassay demonstrated the positively phototactic larvae during tree-top disease and the gene expression analysis showed altered rhythmicity of the host’s core circadian genes (per and tim) during the course of infection emphasizing the role of Circadian entrainment and Phototransduction pathways in the process, which also showed maximum interactions (>50% shared genes with 24 and 23 pathways respectively) among other signaling pathways in the enrichment map. Our study provided valuable insights into different pathways and genes, their coordinated response and molecular regulation during baculovirus infection and also improved our understanding regarding signaling mechanisms in LdMNPV induced tree-top disease.