A putative protein translation inhibitory factor encoded by Cotesia plutellae bracovirus suppresses host hemocyte-spreading behavior

An endoparasitoid, Cotesia plutellae (Hymenoptera: Braconidae), possesses a mutualistic bracovirus (CpBV), which plays significant roles in the parasitized host, Plutella xylostella (Lepidoptera: Plutellidae). CpBV15beta, a viral gene encoded by CpBV, is expressed at early and late parasitization periods, suggesting that it functions to manipulate the physiology of the parasitized host. This paper reports a physiological function of CpBV15beta as an immunosuppressive agent. The effect of CpBV15beta on cellular immunity was analyzed by assessing hemocyte-spreading behavior. Parasitization by C. plutellae caused altered behavior of hemocytes of P. xylostella, in which the hemocytes were not able to attach and spread on glass slides. CpBV15beta was expressed in Sf9 cells using a baculovirus expression system and purified from the culture media. When hemocytes of nonparasitized P. xylostella were incubated with purified CpBV15beta protein, spreading behavior was impaired in a dose-dependent manner at low micro-molar range. This inhibitory effect of CpBV15beta could also be demonstrated on hemocytes of a non-natural host, Spodoptera exigua. CpBV15beta protein significantly inhibited F-actin growth of hemocytes in response to an insect cytokine. Similarly, cycloheximide, a eukaryotic translation inhibitor, strongly inhibited the spreading behavior and F-actin growth of P. xylostella hemocytes. Under in vitro condition, hemocytes of nonparasitized P. xylostella released proteins into the surrounding medium. Upon incubation of hemocytes with either CpBV15beta or cycloheximide, their ability to release protein molecules was markedly inhibited. This study suggests that CpBV15beta suppresses hemocyte behavior by inhibiting protein translation.     

A novel polydnaviral gene family, BEN, and its immunosuppressive function in larvae of Plutella xylostella parasitized by Cotesia plutellae

A full genome sequence of the episomal form of Cotesia plutellae bracovirus (CpBV) suggests 11 BEN family genes. This study analyzed their expression and physiological function in the viral host, Plutella xylostella. All 11 BEN family genes were expressed during entire parasitization period of P. xylostella larvae. In addition, these BEN family genes were expressed in fat body, gut, epidermis, and hemocytes in final larval instar of parasitized P. xylostella. The 11 BEN family genes were transiently expressed in nonparasitized larvae by injection of each viral segment containing its corresponding BEN family gene. The transient expression of BEN family genes significantly suppressed hemocyte nodule formation in response to bacterial challenge. Subsequent injection of double-stranded RNA specific to each BEN family gene suppressed the expression of the BEN family gene and rescued the immunosuppression. These results indicate that 11 BEN family genes are expressed in larvae parasitized by C. plutellae and play crucial role in inducing immunosuppression. Homologous BEN family genes were found in other bracoviral genomes. We propose BEN domain-containing genes as a new functional gene family in polydnaviruses.     

Transient expression of a viral histone H4, CpBV-H4, suppresses immune-associated genes of Plutella xylostella and Spodoptera exigua

A viral histone H4, CpBV-H4, is encoded in the Cotesia plutellae bracovirus (CpBV) genome. This polydnavirus is symbiotic with C. plutellae, an endoparasitoid wasp. When the wasp parasitizes its host, Plutella xylostella, the symbiotic CpBV is delivered to host hemocoel and infects different internal tissues. CpBV-H4 encoded in the virus exhibits high sequence similarity to host histone H4, except for an extended N-terminal tail (38 amino acids long). When the CpBV-H4 cloned in a eukaryotic expression vector was transiently expressed in P. xylostella and a nonhost, Spodoptera exigua, it clearly inhibited several immune-associated genes, including cecropin, gloverin, serpin, apolipophorin III, and transferrin. However, its truncated construct, prepared by deleting 38 amino acids at the N-terminal tail, lost its inhibitory activity against immune-associated genes of the both species. This study has verified an inhibitory activity of CpBV-H4 against host immune-associated genes and has provided a possibility to expand its activity spectrum to the genes of other insect species.     

An Evidence for Host Translation Inhibitory Factor Encoded in a Polydnavirus,Cotesia glomerata Bracovirus,Genome and Its Expression in Parasitized Cabbage White Butterfly,Pieris rapae

Polydnavirus is a DNA virus symbiotic to some endoparasitic wasps and plays a critical role in accomplishing successful parasitic life cycle of host wasps. Host translation inhibitory factor (HTIF) has been found in some polydnaviral genomes and performs parasitic functions leading to host immunosuppression and redirecting host nutrient usage to wasp development. The cabbage white butterfly, Pieris rapae, parasitized by a gregarious endoparasitoid, Cotesia glomerata, undergoes several physiological alterations including immune malfunctioning and failure of pupal metamorphosis. C. glomerata possesses its own symbiotic polydnavirus, C. glomerata bracovirus (CgBV). Its genome consisted of at least 12 segments in unequal amounts. Parasitized P. rapae hemolymph contained HTIF-like protein, which was determined through an immunoblotting assay using HTIF antibody of C. plutellae bracovirus (CpBV). RT-PCR using HTIF primers of CpBV produced an HTIF-like gene in P. rapae larvae parasitized by C. glomerata. Also, this HTIF-like gene was encoded in CgBV genome and its partial sequence of CgBV showed highly homology (98.5%) to amino acid sequence of an HTIF of CpBV, called CpBV15a. These results suggest that a common HTIF-like moiety may be shared among Cotesia-associated bracovirus.     

Parasitism by Cotesia plutellae alters the hemocyte population and immunological function of the diamondback moth, Plutella xylostella

Cotesia plutellae, a solitary endoparasitoid wasp, parasitizes the diamondback moth, Plutella xylostella, and induces host immunosuppression and lethality in the late larval stage. This study focused on changes of cellular immunity in the parasitized P. xylostella in terms of hemocyte composition and cellular functions. In third and fourth instar larvae of nonparasitized P. xylostella, granular cells represented the main hemocyte type (60-70%) and plasmatocytes were also present at around 15% among the total hemocytes. Following parasitization by C. plutellae, the relative proportions of these two major hemocytes changed very little, but the total hemocyte counts exhibited a significant reduction. Functionally, the granular cells played a significant role in phagocytosis based on a fluorescence assay using fluorecein isothiocyanate-labeled bacteria. The phagocytic activity of the granular cells occurred as early as 5 min after incubation with the bacteria, and increased during the first 40 min of incubation. The parasitism by C. plutellae significantly inhibited phagocytosis of the granular cells. Plasmatocytes also exhibited minor phagocytic activity. Moreover, plasmatocyte phagocytosis was not inhibited by parasitism. On the other hand, hemocyte-spreading behavior in response to pathogen infection was significant only for plasmatocytes, which exhibited a characteristic spindle shape upon infection. A significant spreading of the plasmatocytes was found as early as 5 min after pathogen incubation and their ratio increased during the first 40 min.An insect cytokine, plasmatocyte-spreading peptide 1 (PSP1) from Pseudoplusia includens, was highly active in inducing plasmatocyte-spreading behavior of P. xylostella in a dose-dependent manner. P. xylostella parasitized by C. plutella was significantly inhibited in plasmatocyte-spreading in response to an active dose of PSP1. An in vivo encapsulation assay showed that the parasitized P. xylostella could not effectively form the hemocyte capsules around injected agarose beads. This research demonstrates that the parasitism of C. plutellae adversely affects the total hemocyte populations in number and function, which would contribute to host immunosuppression.     

Immunoevasive property of a polydnaviral product, CpBV-lectin, protects the parasitoid egg from hemocytic encapsulation of Plutella xylostella (Lepidoptera: Yponomeutidae)

Immunosuppression is the main pathological symptom of the diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae), parasitized by an endoparasitoid wasp, Cotesia plutellae (vestalis, Hymenoptera: Braconidae). C. plutellae bracovirus (CpBV), which is a symbiotic virus of C. plutellae, has been known to be the main parasitic factor in the host-parasitoid interaction. CpBV-lectin, encoded in the viral genome and expressed in P. xylostella during early parasitization stage, was suspected to play a role in immunoevasion of defense response. Here we expressed CpBV-lectin in Sf9 cells using a recombinant baculovirus for subsequent functional assays. The recombinant CpBV-lectin exhibited hemagglutination against vertebrate erythrocytes. Its hemagglutinating activity increased with calcium, but inhibited by adding EDTA, indicating its C-type lectin property. CpBV-lectin showed specific carbohydrate-binding affinity against N-acetyl glucosamine and N-acetyl neuraminic acid. The role of this CpBV-lectin in immunosuppression was analyzed by exposing hemocytes of nonparasitized P. xylostella to rat erythrocytes or FITC-labeled bacteria pretreated with recombinant CpBV-lectin, which resulted in significant reduction in adhesion or phagocytosis, respectively. The immunosuppressive activity of CpBV-lectin was further analyzed under in vitro encapsulation response of hemocytes against parasitoid eggs collected at 1- or 24-h post-parasitization. Hemocytic encapsulation was observed against 1-h eggs but not against 24-h eggs. When the 1-h eggs were pretreated with the recombinant CpBV-lectin, encapsulation response was completely inhibited, where CpBV-lectin bound to the parasitoid eggs, but not to hemocytes. These results suggest that CpBV-lectin interferes with hemocyte recognition by masking hemocyte-binding sites on the parasitoid eggs.     

Transient expression of specific Cotesia plutellae bracoviral segments induces prolonged larval development of the diamondback moth, Plutella xylostella

A polydnavirus, Cotesia plutellae bracovirus (CpBV), possesses a segmented and dispersed genome that is located on chromosome(s) of its symbiotic endoparasitic wasp, C. plutellae. When the host wasp parasitizes larvae of the diamondback moth, Plutella xylostella, at least 27 viral genome segments are delivered to the parasitized host along with the wasp egg. The parasitized P. xylostella exhibits significant immunosuppression and a prolonged larval development. Parasitized larvae take about 2 days longer than nonparasitized larvae to develop until the wandering stage of the final larval instar, and die after egress of the full grown wasp larvae. Developmental analysis using juvenile hormone and ecdysteroid analogs suggests that altering endocrine signals could induce the retardation of larval developmental rate in P. xylostella. In this study we used a transient expression technique to micro-inject individual CpBV genome segments, and tested their ability to induce delayed larval development of P. xylostella. We demonstrated that a CpBV segment was able to express its own encoded genes when it was injected into nonparasitized larvae, in which the expression patterns of the segment genes were similar to those in the larvae parasitized by C. plutellae. Twenty three CpBV genome segments were individually cloned and injected into the second instar larvae of P. xylostella and their effects assessed by measuring the time taken for host development to the cocooning stage. Three CpBV genome segments markedly interfered with the host larval development. When the putative genes of these segments were analyzed, it was found that they did not share any common genes. Among these segments able to delay host development, segment S27 was predicted to encode seven protein tyrosine phosphatases (CpBV-PTPs), some of which were mutated by insertional inactivation with transposons, while other encoded gene expressions were unaffected. The mutant segments were unable to induce prolonged larval development of P. xylostella. These results suggest that CpBV can induce prolonged larval development of P. xylostella, and that at least some CpBV-PTPs may contribute to the parasitic role probably by altering titers of developmental hormones.     

Parasitism of Cotesia spp. Enhances Susceptibility of Plutella xylostella to Other Pathogens

Two endoparasitoids, Cotesia plutellae and C. glomerata, parasitize the diamondback moth, Plutella xylostella, and induce significant host immunosuppression. This study analyzed the susceptibility changes of the parasitized P. xylostella against other pathogens using an entomopathogenic bacterium, Xenorhabdus nematophila (Xn), and a viral pathogen, Autographa californica nucleopolyhedrosis virus (AcNPV). The P. xylostella parasitized by either C. plutellae or C. glomerata exhibited higher susceptibilities to both microbial pathogens than the nonpara-sitized. To determine the parasitism factors inducing the enhanced susceptibility, three polydnaviral genes so far successfully cloned were selected from C. plutellae bracovirus (CpBV). CpBV-lectin and CpBV15 α/β were inserted into AcNPV under a CpBV promote and analyzed in their pathogenicities against P. xylostella larvae. Two AcNPVs recombined with CpBV15α/β were more potent than the control AcNPV recombined with an enhanced green fluorescent protein gene or the AcNPV recombined with CpBV-lectin. These results suggest that the wasp parasitization enhances other pathogen susceptibilities by inducing host immunosuppression, in which the symbiotic polydnavirus can play significant role in the enhanced susceptibility.     

Additive effect of teratocyte and calyx fluid from Cotesia plutellae on immunosuppression of Plutella xylostella

Abstract.  Teratocytes are cells that originate from the extra-embryonic tissues of some hymenopteran parasitoids, typically dissociate upon hatching, and develop in the host haemolymph. They are considered to be involved in parasitoid larval nutrient uptake, host immunosuppression and/or repression of competing parasitoid development. Teratocytes of the parasitoid, Cotesia plutellae (Kurdjumov) (Hymenoptera: Braconidae) are found in its natural host, Plutella xylostella (Linnaeus) (Lepidoptera: Yponomeutidae) and can be cultured in vitro . The present study demonstrates that teratocytes of C. plutellae possess a significantly depressive effect on host cellular immunity. When the hosts are preinjected with 200 cultured teratocytes (corresponding to the normal number of teratocytes released during wasp hatching), haemocyte nodulation is inhibited by approximately 40%, with younger teratocytes being more potent than older ones. Similarly, the medium in which teratocytes are cultured has similar immunosuppressive properties. In comparison, calyx fluid extracted from the C. plutellae ovary also has an immunosuppressive effect on P. xylostella . These two maternal (calyx fluid) and embryonic (teratocytes) factors are additive and result in a reduced level of nodule formation equivalent to that induced by natural parasitization. However, the immunosuppression of the parasitized P. xylostella does not appear to be due to inhibition of phospholipase A₂, an immune mediator, because injection of arachidonic acid failed to restore haemocyte nodulation capability.     

Transient expression of a polydnaviral gene, CpBV15beta, induces immune and developmental alterations of the diamondback moth, Plutella xylostella

The diamondback moth, Plutella xylostella, parasitized by its endoparasitoid wasp, Cotesia plutellae, undergoes various physiological alterations which include immunosuppression and an extended larval development. Its symbiotic virus, C. plutellae bracovirus (CpBV), is essential for their successful parasitization with more than 136 putative genes encoded in the viral genome. CpBV15β, a CpBV gene, has been known to play significant role in altering host physiological processes including hemocyte-spreading behavior through inhibition of protein synthesis under in vitro conditions. In the current study, we investigated its specific involvement in physiological processes of the host by transient expression and RNA interference techniques. The open reading frame of CpBV15β was cloned into a eukaryotic expression vector and this recombinant CpBV15β was transfected into nonparasitized 3rd instar P. xylostella by microinjection. CpBV15β was expressed as early as 24 h and was consistent up to 72 h. Due to the expression of this gene, plasma protein levels were significantly reduced and the ability of the hemocytes to adhere and spread on extracellular matrix was inhibited, wherein CpBV15β was detectable in the cytoplasm of hemocytes based on an indirect immunofluorescence assay. To confirm the role of CpBV15β, its double stranded RNA could efficiently recover the hemocyte-spreading behavior and synthesis of plasma proteins suppressed by the transient expression of CpBV15β. In addition, the larvae transfected with CpBV15β significantly suffered poor adult development probably due to lack of storage proteins. Thus these results demonstrate the role of CpBV15β in altering the host physiological processes involving cellular immune response and metamorphic development, which are usually induced by wasp parasitization.     

<Emphasis Type="Italic">Cotesia plutellae</Emphasis> bracovirus suppresses expression of an antimicrobial peptide,cecropin, in the diamondback Moth, <Emphasis Type="Italic">Plutella xylostella</Emphasis>, challenged by bacteria

An endoparasitoid wasp, Cotesia plutellae, induces significant immunosuppression of host insect, Plutella xylostella. This study was focused on suppression in humoral immune response of P. xylostella parasitized by C. plutellae. An EST database of P. xylostella provided a putative cecropin gene (PxCec) which is 627 bp long and encodes 66 amino acids. A signal peptide (22 amino acids) is predicted and two putative O-glycosylation sites in threonine are located at positions 58 and 64. Without bacterial infection, PxCec was expressed in pupa and adult stages but not in the egg and larval stages. Upon bacterial challenge, however, the larvae expressed PxCec as early as 3 h post infection (PI) and maintained high expression levels at 12–24 h PI. By 48 h PI, its expression noticeably diminished. All tested tissues of bacteria-infected P. xylostella showed PxCec expression. However, other microbes, such as virus and fungus, did not induce the PxCec expression. Parasitization by C. plutellae suppressed the expression of PxCec in response to bacterial challenge. Among the parasitic factors of C. plutellae, its symbiotic virus (C. plutellae bracovirus: CpBV) alone was able to inhibit the expression of PxCec of P. xylostella challenged by bacteria. These results indicate that PxCec expression is regulated by both immune and developmental processes in P. xylostella. The parasitization by C. plutellae inhibited the expression of PxCec by the wasp’s symbiotic virus.     

Cotesia plutellae Bracovirus Genome and Its Function in Altering Insect Physiology

Polydnavirus is a group of animal DNA virus mutually associated with some ichneumonoid wasp. Its relatively large size of genome has been considered as a major source of the parasitoid function to manipulate developmental and immunological processes of target parasitized insects. Cotesia plutellae bracovirus (CpBV) is a polydnavirus derived from C. plutellae, which parasitizes the diamondback moth, Plutella xylostella. Parasitized P. xylostella exhibits altered physiological symptoms in development and immune reactions. Though several other parasitic factors such as ovarian proteins, venom, and teratocytes are identified, CpBV has been more focused on elucidating various host physiological alterations occurring due to the parasitism, which has driven the CpBV genome project. CpBV attains a typical bracovirus structure by its single unit membrane envelope, in which multiple nucleocapsids are enclosed. Its genome DNAs are segmented and located on the genome of C. plutellae. Its replication begins at adult tissue development during pupal stage. An apparent genome size is 471 kb estimated from 27 segments separated on 5% agarose gel. A current work on the genome has been completely sequenced 24 genomic segments and analyzed their genomic structure. The aggregated genome size is 351, 299 bp long and exhibits an average GC content of approximately 34.6%. Average coding density is about 32.3% and 125 putative open reading frames are predicted. Though more than half (52.5%) of predicted genes are annotated as hypothetical, the annotated CpBV genes share amino acid sequence homologies with those of other bracoviral genomes. The annotated genes are classified into the known bracoviral families, in which a family of protein tyrosine phosphatase is the largest including 36 ORFs, suggesting a significant role during parasitization. In addition, 8 and 7 ORFs encode Iκβ-like and EP1-like, respectively. Some predicted genes are known only in Cotesia-associated bracoviral genomes. Finally, two homologous genes, CpBV15α/β, are unique in CpBV genome, which are not matched to any other known polydnaviral genes. Their homology with malarian circumsporozoite toxin and eukaryotic translation inhibition factors suggests their function in host translation inhibitory factor. This review discusses CpBV genes on their putative physiological functions based on the molecular interactions between the host-parasite.     

Characteristics Of Parasitism Of Plutella Xylostella (Lep., Plutellidae) By Oomyzus Sokolowskii (Hym., Eulophidae)

Laboratory and greenhouse studies were conducted on Oomyzus sokolowskii Kurdjumov, a parasite of the crucifer pest Plutella xylostella ( L.). O. sokolowskii preferred the 3^rd and 4^th instar P. xylostella larvae over fresh pupae for parasitization. It is thus a larval parasite. Within the range of 10?C to 35?C, parasitism was positively correlated with temperature. High parasitism at temperatures of 30?C and 35?C indicates that this insect is suitable for the control of P. xylostella in the tropical lowlands. In a no- choice test where only fresh pupae of Cotesia plutellae Kurdjumov, another potentially competing larval parasite of P. xylostella, were offered, O. sokolowskii failed to parasitize pupae of C. plutellae. In a choice test where the 4^th instar P. xylostella larvae and fresh C. plutellae pupae were offered, O. sokolowskii parasitized only P. xylostella larvae. This parasite, therefore, is not a hyperparasite of P. xylostella. When C. plutellae-ovvposited P. xylostella larvae were offered at intervals, O. sokolowskii, parasitized only freshly oviposited host larvae. The longer the period that elapsed after C. plutellae oviposition of P. xylostella larvae, the lesser was the parasitism of these larvae by O. sokolowski.     

Conflicts between a fungal entomopathogen, Zoophthora radicans, and two larval parasitoids of the diamondback moth

Zoophthora radicans (Zygomycetes: Entomophthorales), Diadegma semiclausum (Hymenoptera: Ichneumonidae), and Cotesia plutellae (Hymenoptera: Braconidae) are all natural enemies of the diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae). Adult C. plutellae are not susceptible to Z. radicans infection but the pathogen can infect and kill adult D. semiclausum. Infection of adult D. semiclausum prior to exposure to P. xylostella host larvae significantly reduced the number of parasitoid cocoons subsequently developing from the host larvae. Although Z. radicans infection of P. xylostella larvae prior to parasitism by D. semiclausum or C. plutellae always resulted in the death of the immature parasitoids, neither species discriminated between healthy and Z. radicans-infected host larvae in an oviposition choice experiment. However, host larvae recently killed by Z. radicans were always rejected by D. semiclausum but sometimes accepted by C. plutellae. At 20 degrees C, egg to pupa development took 6.7 and 7.8 days for D. semiclausum and C. plutellae, respectively. C. plutellae parasitism significantly increased host instar duration but D. semiclausum parasitism did not. Cadavers of P. xylostella larvae parasitized 1 day prior to fungal infection showed no reduction in Z. radicans conidia yield. However, cadavers of larvae parasitized 3 days prior to fungal infection demonstrated a marked decrease in Z. radicans conidia yield. Z. radicans infection of P. xylostella larvae < or = 4 days after parasitism resulted in 100% parasitoid mortality; thereafter, the reduction in parasitoid cocoon yield decreased as the time between parasitism and initiation of fungal infection increased. The extended duration of the host larval stage induced by C. plutellae parasitism increased the availability of the parasitoid to the pathogen. Estimates of interspecific competition indicated a similar pattern for the interaction between Z. radicans and each species of parasitoid.     

Effects of specialist parasitoids on oviposition preference of phytophagous insects: encounter–dilution effects in a tritrophic interaction

Abstract.  1. Host plant preferences of the female diamondback moth Plutella xylostella were studied.
2. Female moths preferred conspecific-damaged cabbage plants over undamaged cabbage plants. The performance of P. xylostella larvae on conspecific-infested plants did not differ significantly from that of larvae on undamaged plants.
3.  Cotesia plutellae , the specialist parasitoid wasp of P. xylostella larvae, displayed equal preference for plants with differing levels of host-larvae damage, and the wasp attacked only one or two hosts on average before leaving an infested plant, irrespective of the number of hosts on the plant. It is hypothesised that the oviposition preferences of P. xylostella females for host plants already damaged by conspecific larvae demonstrate an encounter–dilution effect against C. plutellae .     

Proteomic analysis of parasitized Plutella xylostella larvae plasma

Insects use their innate immunity to defend themselves against foreign invaders, such as microorganisms, nematodes and parasites. Cotesia plutellae, an endoparasitoid wasp that parasitizes the diamondback moth Plutella xylostella, uses several strategies to attack the host immune system, such as injection of viruses, venom, and serosal membrane-derived cells denoted teratocytes. However, the proteome profiles related to these immune deficiency systems have yet to be clearly defined. In this study, we investigate differences in protein expression patterns in parasitized P. xylostella larvae, with a view to identifying parasitism-specific factors. Using 2D polyacrylamide gel electrophoresis, proteins in the host plasma were assessed every 48 h after parasitism by C. plutellae. A large number of protein spots (350 in total) were detected, and approximately 50 spots were differentially expressed in the parasitized P. xylostella larvae every 48 h. In total, 26 potential candidates, including P. xylostella Serpin 2 (pxSerpin 2), translationally controlled tumor protein, signal transduction histidine kinase, apolipophorin-III, and fatty-acid binding protein were identified through quadrupole time-of-flight tandem mass spectrometry and sequence homology analysis. These proteins were classified into the following functional groups: immunity, signaling, lipid metabolism, energy metabolism, amino acid/nucleotide metabolism, and others. The pxSerpin 2 gene was cloned, and its expression profile investigated during the course of parasitism. Real-time PCR analysis of pxSerpin 2 revealed a poor correlation between the mRNA level and protein abundance. Our results clearly suggest that parasitism-specific proteins participate in suppression of the host immune response.     

Interspecific competition between Diadegma semiclausum Hellen (Hym., Ichneumonidae) and Cotesia plutellae (Kurdjumov) (Hym., Braconidae) in parasitizing Plutella xylostella (L.) (Lep., Plutellidea)

Abstract:  Interspecific competition between Diadegma semiclausum and Cotesia plutellae was investigated at 25°C in the laboratory, by exposing the third instar larvae of the diamondback moth, Plutella xylostella to both species together, either species alone or by exposing the host larvae already parasitized by one species, at different intervals, to the other. When host larvae were exposed simultaneously to two species in one arena, parasitism rates of the host by each species were not reduced by the presence of the other species; joint parasitism rate by two species was not significantly higher than that by either parasitoid alone. Both parasitoids could lay eggs into the host larvae which had previously been parasitized by the other species, leading to the occurrence of multiparasitized hosts. When host larvae were parasitized first by D. semiclausum and then being followed within 1–2 h by exposing to C. plutellae , or vice versa, ensuing parasitoid cocoons from the multiparasitized host larvae were nearly all C. plutellae . When host larvae were parasitized initially by C. plutellae and then being followed by D. semiclausum two or more days later, all parasitoids ensued from the multiparasitized hosts were C. plutellae . In contrast, when host larvae were parasitized initially by D. semiclausum and then being followed by C. plutellae two or more days later, most host larvae could not survive to prepupae and most of the ensuing parasitoid adults from the surviving hosts were D. semiclausum . Dissections of host larvae at various time intervals after parasitization by the two parasitoids showed that development of both parasitoids in multiparasitized hosts were somewhat arrested, and that the first instar larvae of C. plutellae could initiate a physical attack on the larvae of D. semiclausum and remove the latter.     

Immunoanalysis of unique protein in Trichoplusia ni larvae parasitized by the braconid wasp Chelonus near curvimaculatus

Parasitization in insects brings about profound biochemical and physiological effects in the host which may include complete overriding of the normal endocrinological program, resulting in precocious metamorphosis and in blockage of pupal development. The subtle effects of parasitization include changes in the expression of hemolymph proteins and the appearance of proteins which are unique to parasitized hosts. One such protein has been identified in the hemolymph of Trichoplusia ni larvae parasitized by the braconid wasp Chelonus near curvimaculatus. In this study, purified preparations of the parasitism-specific protein were used to generate polyconal antibodies against the protein. Results from the immunocharacterization indicate the antibodies obtained are highly specific for the protein and are present in a high titer (1:8000 antiserum dilution yielded strong signals in analysis of the protein in 0.25 μl hemolymph). Subsequently, the expression of the parasitism-specific protein in the hemolymph and tissues was analyzed by immunoblotting during the entire course of development in normal and parasitized insects. The parasitism-specific protein was not detected in normal, unparasitized larvae. In parasitized insects, expression of the parasitism-specific protein appears to be stage-specific in that it is only detected during the last larval stadium of precociously metamorphosing larvae, but is absent from all earlier stages of development.     

Bottom-up cascading effects in a tritrophic system: interactions between plant quality and host-parasitoid immune responses

Abstract.  1. Little is known about underlying mechanisms by which plants indirectly affect parasitism success in hymenopteran endoparasitoids. The hypothesis that host-plant effects can challenge the innate immune system of an insect host was experimentally tested in this study using a model tritrophic, crucifer – lepidopteran [ Plutella xylostella (L.)] – parasitoid [ Cotesia plutellae (Kurdjumov)], system.
2. The effects of host-plant suitability on herbivore performance and parasitism were examined. The bottom-up effect of plant suitability on host-parasitoid immune responses was then evaluated using measures of cellular and humoral effectors.
3. Host-plant quality showed a significant effect on the encapsulation response of P. xylostella to first instar but not to second instar parasitoid larvae. Encapsulation was never sufficient to prevent parasitoid emergence.
4. Poor host-plant suitability suppressed phenoloxidase activity in the absence of the parasitoid. The suppressive effect of C. plutellae on phenoloxidase activity was much greater and no plant effects were detectable after insects had been parasitized.
5. Despite strong plant effects on parasitism, those on immune effectors of the host were transitory or overwhelmed by the effect of the parasitoid.
6. These results demonstrated that plant-mediated variation in parasitism success by C. plutellae were not as a result of plant nutritional status or other attributes affecting the immune function of P. xylostella , nor to host-plant effects on superparasitism.
7. In these experiments, P. xylostella was a fully permissive host to C. plutellae and host-plant-mediated effects on the innate immune response appeared to play no part in parasitoid survival within hosts.