Abstract
Insect immune responses to multiple pathogen groups including viruses, bacteria, fungi, and entomopathogenic nematodes have traditionally been documented in model insects such as Drosophila melanogaster, or medically important insects such as Aedes aegypti. Despite their potential importance in understanding the efficacy of pathogens as biological control agents, these responses are infrequently studied in agriculturally important pests. Additionally, studies that investigate responses of a host species to different pathogen groups are uncommon, and typically focus on only a single time point during infection. As such, a robust understanding of immune system responses over the time of infection is often lacking in many pest species. This study was conducted to understand how 3rd instar larvae of the major insect pest Helicoverpa zea responded through the course of an infection by four different pathogenic groups: viruses, bacteria, fungi, and entomopathogenic nematodes; by sampling at three different times post-inoculation. Physiological immune responses were assessed at 4-, 24-, and 48-hours post-infection by measuring hemolymph phenoloxidase concentrations, hemolymph prophenoloxidase concentrations, hemocyte counts, and encapsulation ability. Transcriptional immune responses were measured at 24-, 48-, and 72-hours post-infection by quantifying the expression of PPO2, Argonaute-2, JNK, Dorsal, and Relish. This gene set covers the major known immune pathways: phenoloxidase cascade, siRNA, JNK pathway, Toll pathway, and IMD pathway. Our results indicate H. zea has an extreme immune response to Bacillus thuringiensis bacteria, a mild response to Helicoverpa armigera nucleopolyhedrovirus, and little-to-no detectable response to either the fungus Beauveria bassiana or Steinernema carpocapsae nematodes.
Highlights
The insect innate immune system is non-specific and assumed to be without memory, yet it can be vitally effective at attacking and overcoming challenges by pathogens or parasitoids [1,2,3]
Pathogens induce differential immune responses in non-model insects by means of pathogen recognition receptors (PRRs) that bind to lipid particles and pathogenassociated molecular patterns (PAMPs)
We demonstrate that Helicoverpa zea responds differently to each major pathogen group during different stages of infection at a physiological and transcriptional level
Summary
The insect innate immune system is non-specific and assumed to be without memory, yet it can be vitally effective at attacking and overcoming challenges by pathogens or parasitoids [1,2,3]. Insect immune systems are able to recognize non-self and altered-self molecular structures. Pathogens induce differential immune responses in non-model insects by means of pathogen recognition receptors (PRRs) that bind to lipid particles and pathogenassociated molecular patterns (PAMPs). Cytokines and signaling pathways such as Toll, IMD, JNK and JAK/ STAT are important in the identification of non-self/altered-self, and the initiation of an immune response [9,10,11,12,13,14,15]. The second primary component of an insect’s innate immune system is the cellular response to infection involving encapsulation, nodulation, phagocytosis, and apoptosis of pathogens mediated by the JNK signaling pathway, hemocyte signaling, and regulation by eicosanoids (Reviewed in: [20,21,22])
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