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Evidence for Covert Baculovirus Infections in the Field-Collected Spodoptera Litura Larvae

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Evidence for Covert Baculovirus Infections in the Field-Collected Spodoptera Litura Larvae Journal of Advanced Agricultural Technologies Vol. 7, No. 2, December 2020 Evidence for Covert Baculovirus Infections in the Field-Collected Spodoptera litura Larvae Sudawan Chaeychomsri Central Laboratory and Greenhouse Complex, Faculty of Agriculture at Kamphaeng Saen, Kasetsart University, Kamphaeng Saen, Nakhon Pathom, Thailand Email: [email protected] Win Chaeychomsri and Jindawan Siruntawineti Department of Zoology, Faculty of Science, Kasetsart University, Bangkok, Thailand Email: {fsciwcc, fscijws}@ku.ac.th Abstract—A total of 55 Spodoptera litura larvae collected whereas Betabaculovirus is classified into three types from marigold fields were reared on an artificial diet in the based on the host tissue tropism [4]. Typically, the laboratory and determined for the presence of covert baculovirus virions (enveloped nucleocapsids) are present baculovirus infections. Polymerase Chain Reaction (PCR) as two types, Occlusion-Derived Virions (ODVs) and analysis by using primers specific to the highly conserved Budded Virions (BVs). ODVs are responsible for oral baculovirus genes, granulin and polyhedrin, combined with transmission electron microscopy confirmed the presence of infection of insect hosts and horizontal transmission, granulovirus (GV) and nucleopolyhedrovirus (NPV). Eight while BVs are responsible for the systemic cell to cell out of 55 larvae (14.5%) were PCR positive for S. litura infection within infected insects. The ODVs of NPV and granulovirus (SpltGV), while one larva (1.8%) was PCR GV are embedded in a crystalline protein matrix to form positive for S. litura multicapsid nucleopolyhedrovirus Occlusion Bodies (OBs), which protect the virions (SpltNPV). Electron microscopic observations of the PCR- against adverse environmental conditions. NPV produces positive samples revealed the presence of a single many virions embedded in large polyhedral OBs called baculovirus infection, either GV or NPV. The identification polyhedra or Polyhedral Inclusion Bodies (PIBs), of SpltGV and SpltNPV in the field-collected S. litura larvae whereas GV produces a single virion embedded in a indicated that the viruses were found to be present in this population. The findings in the present study suggest that smaller ovocylindrical OBs called granules or capsules. the persistent baculoviruses in the field populations of S. The crystalline protein matrix in either form is composed litura may be activated by some stressors rather than by of a single polypeptide which is known as polyhedrin or exposure to another baculovirus. It can be speculated that granulin in NPV or GV, respectively [5]. such covert baculovirus infections may lead to the decline in As baculoviruses are obligate pathogens, these viruses insect populations and also can lead to a collapse of can only be propagated in living cells, so it is necessary laboratory-reared insect colonies for in vivo production of to propagate them either in larvae or cultured cells of bioinsecticides. susceptible insects. For this reason, the commercial Index Terms—Spodoptera litura, nucleopolyhedrovirus, baculovirus products used in insect pest control are granulovirus, PCR, electron microscopy currently produced using in vivo production systems. The process for baculovirus production involves mass rearing and infecting of host larvae, and then harvesting virus I. INTRODUCTION from the dead larvae [6]. It is labour-intensive and has to be managed carefully to avoid contamination with other The members of the family Baculoviridae are unwanted organisms. Insect pathogens, especially insect classified as a group of arthropod-specific viruses with pathogenic viruses, can contaminate host-insect cultures, rod-shaped nucleocapsids [1]. Based on morphological, destroy the rearing colonies, and cause serious production biological characteristics, and phylogenetic features, the problems [7]. Epizootics caused by baculoviruses are members of this family are classified into four genera, as generally lethal and can reduce dramatically their host follows: Alphabaculovirus (lepidopteran-specific population. However, baculoviruses have also been found nucleopolyhedrovirus, NPV), Betabaculovirus to cause covert infections in the field and laboratory host (lepidopteran-specific granulovirus, GV), populations [8]-[12]. Covert infections (also known as Gammabaculovirus (hymenopteran-specific NPV) and inapparent, sublethal, silent or occult infections) are Deltabaculovirus (dipteran-specific NPV) [2]. characterized by the absence of visible signs of disease Alphabaculovirus can be further subdivided into groups I and II according to the phylogenetic analysis of the lef-8, [11]. The apparently healthy insects, collected in the field lef-9 and polyhedrin (polh)/granulin (gran) genes [3], and reared in the laboratory, may suddenly succumb to a virus infection affecting most, if not all, of the individuals Manuscript received April 3, 2020; revised October 28, 2020. in a colony. ©2020 Journal of Advanced Agricultural Technologies 20 doi: 10.18178/joaat.7.2.20-27 Journal of Advanced Agricultural Technologies Vol. 7, No. 2, December 2020 In Thailand, the baculovirus Spodoptera litura NPV discarded. A total volume of 1 mL PBS was added to the (SpltNPV) has been developed as bioinsecticide and used pellet and larval sample was homogenized using a to control the common cutworm, S. litura, in cut flower microtube pestle. The homogenate was centrifuged to and orchid [13]. The production process for SpltNPV is pellet tissue debris at 2,500 g for 5 min and the an in vivo process, that relies on the successful mass- supernatant was transferred into a new tube. The rearing of the host insects which are then infected with supernatant was passed through a sterile 100-m nylon the virus. The main limitations of the in vivo mass mesh filter into a sterile tube which was centrifuged at production systems in Thailand have been reported [13]. 10,000 g for 5 min. Then the supernatant was discarded. One of the main limitations of this process is the rearing Subsequently, 500 μL of 0.1% Sodium Dodecyl Sulfate of sufficient healthy host insects. (SDS) was added and the suspension was repeatedly In large-scale production systems both lethal and sub- washed with SDS followed by centrifugation at 10,000 g lethal insect disease effects may cause serious damage; for 5 min until the supernatant was clear. The final pellet thus monitoring, prevention and control of diseases are was resuspended in 200 μL TE buffer (10 mM Tris-HCl, essential [14]. Therefore, the objective of the present pH 8.0, 1 mM EDTA) and mixed with an equal volume study was to determine the incidence of covert infections of 2X alkaline buffer (0.2 M Na2CO3, 0.02 M EDTA, of baculoviruses in the field-collected S. litura larvae. 0.34 M NaCl) for 30 min on ice to dissolve baculovirus Primers specific to the SpltGV gran and SpltNPV polh OBs and release ODVs. The undissolved materials were genes were used for Polymerase Chain Reaction (PCR) to removed by centrifugation at 10,000 g for 1 min. The detect the presence of SpltGV and SpltNPV, respectively. supernatant was incubated with proteinase K in the Ultrastructural studies on virus replication and presence of SDS overnight at 37°C. After incubation with morphogenesis in the infected larvae of S. litura were RNase A at 37°C for 1 h, viral DNA was then purified also described. once with TE-saturated phenol and extracted once with phenol/chloroform/isoamyl alcohol (25:24:1). A final II. MATERIALS AND METHODS extraction with chloroform/isoamyl alcohol (24:1) was used to remove residual phenol, followed by ethanol A. Insects precipitation. DNA was pelleted by centrifugation and the precipitate was rinsed twice with 70% ethanol, dried The larvae of S. litura were originally collected from under vacuum, and finally suspended in 100 μL of sterile marigold fields in Kamphaeng Saen District, Nakhon MilliQ water (Millipore Corporation, Molsheim, France). Pathom, Thailand, and maintained in the insectary The extracted DNA was subsequently used for PCR facilities of the Central Laboratory and Greenhouse amplification of the SpltGV gran and SpltNPV polh Complex, Kasetsart University, Kamphaeng Saen genes. DNA was also extracted from the healthy-looking Campus, Thailand. In the laboratory the field-collected larvae as described above and used for comparative larvae were transferred individually to the sterile petri purposes. dishes (90 mm dia 15 mm deep) containing a piece of mung bean-based artificial diet [15]. The larvae were C. PCR Amplification and RFLP Analysis of Viral DNA reared at 27±1C, 60±10% Relative Humidity (RH), PCR was performed with specific primers designed 12L:12D photoperiod and observed daily for the signs of from nucleotide sequence of SpltGV gran and SpltNPV baculovirus infection. The larvae exhibiting signs of polh obtained from the sequences available in the public baculovirus infection were collected and subjected to databases. The sequence accession number of the SpltGV PCR analysis. The infection was also confirmed by gran region used in this study was NC_009503 [16], transmission electron microscopy. while accession number of the sequences of the SpltNPV polh gene used in this study was AF325155 [17]. Primers B. DNA Extraction from Insects SGF (5’-ATG GGA TAT AAT AGA TCA TT-3’) and In order to detect baculovirus infection, each larva was SGR (5’-TTA GTA CGC GGG TCC AGT-3’) were used dissected, just prior to death, in a sterile petri dish (90 to amplify the entire gran coding region, whereas primers
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