Chelonus sp.: suppression of host ecdysteroids and developmentally stationary pseudoparasitized prepupae

Nonreproductive Effects of Insect Parasitoids on Their Hosts

The main modes of action of insect parasitoids are considered to be killing their hosts with egg laying followed by offspring development (reproductive mortality), and adults feeding on hosts directly (host feeding). However, parasitoids can also negatively affect their hosts in ways that do not contribute to current or future parasitoid reproduction (nonreproductive effects). Outcomes of nonreproductive effects for hosts can include death, altered behavior, altered reproduction, and altered development. On the basis of these outcomes and the variety of associated mechanisms, we categorize nonreproductive effects into ( a) nonconsumptive effects, ( b) mutilation, ( c) pseudoparasitism, ( d) immune defense costs, and ( e) aborted parasitism. These effects are widespread and can cause greater impacts on host populations than successful parasitism or host feeding. Nonreproductive effects constitute a hidden dimension of host-parasitoid trophic networks, with theoretical implications for community ecology as well as applied importance for the evaluation of ecosystem services provided by parasitoid biological control agents.

A literature-based review of HymenopteraParasitica and Chrysidoidea from Reunion Island

A review of the genera and species of Hymenoptera Parasitica and Chrysidoidea reported so far from Reunion Island is provided with host information. Data presented here is based on a review of the existing literature by the authors. The list includes: (1) 156 species of Ichneumonoidea belonging to 65 genera and 25 subfamilies (Braconidae: Agathidinae, Alysiinae, Aphidiinae, Braconinae, Charmontinae, Cheloninae, Doryctinae, Euphorinae, Gnamptodontinae, Microgastrinae, Opiinae; Ichneumonidae: Banchinae, Campopleginae, Cremastinae, Cryptinae, Diplazontinae, Ichneumoninae, Mesochorinae, Metopiinae, Ophioninae, Orthocentrinae, Pimplinae, Tersilochinae, Tryphoninae); (2) 121 species of Chalcidoidea belonging to 56 genera and 8 families (Agaonidae, Aphelinidae, Chalcididae, Encyrtidae, Eulophidae, Eupelmidae, Eurytomidae, Ormyridae, Pteromalidae, Signophoridae, Torymidae and Trichogrammatidae); (3) seven species of Cynipoidea (family Figitidae); (4) six species of Chrysidoidea in three families (Bethylidae, Chrysididae, Dryinidae); (5) five species of Platygastroidea (families Platygastridae and Scelionidae); (6) five species of Diaprioidea (family Diapriidae); (7) four species of Ceraphronoidae (families Ceraphronidae and Megaspilidae); and (8) two species of Evanioidea (family Evaniidae). This review records a total of 306 species.

Wasp parasitoid disruption of host development: implications for new biologically based strategies for insect control

Wasp parasitoids use a variety of methods to commandeer their insect hosts in order to create an environment that will support and promote their own development, usually to the detriment of the host insect. Parasitized insects typically undergo developmental arrest and die sometime after the parasitoid has become independent of its host. Parasitoids can deactivate their host's immune system and effect changes in host hormone titers and behavior. Often, host tissues or organs become refractory to stimulation by tropic hormones. Here we present an overview of the manipulative capabilities of wasp-injected calyx fluid containing polydnaviruses and venom, as well as the parasitoid larva and the teratocytes that originate from the serosal membrane that surrounds the developing embryo of the parasitoid. Possibilities for using regulatory molecules produced by the parasitoid or its products that would be potentially useful in developing new, environmentally safe insect control agents are discussed.

Uncoupling of sequential heteromorphic developmental programs

The regulatory basis for differences among species in the developmental rate at which successive life stages are reached ("heterochrony") is a subject of much controversy among vertebrate and invertebrate developmental biologists. The heterochrony in development of different insect species is characterized in part by the intercalcation between the embryo and adult of a (varied) number of heteromorphic larval instars. These heteromorphic larval instars exhibit changes of body form from one larval instar to the next, prior to the final metamorphic molt to the pupal form. The intractability of larval heteromorphosis to experimental dissection is due in part to the lack of suitable experimental probes that can test the nature of the coupling of each heterochronically expressed instar-specific program. The epistatic basis of expression of heteromorphic developmental programs was assessed by two-dimensional electrophoretic analysis of hemolymph proteins during the normal and experimentally manipulated feeding stages of the 3rd, 4th, and 5th (final) instar larvae, and during the prepupal stage, of Trichoplusia ni. Of the several hundred protein spots tracked, some were identified that were uniquely detected during a single stage, while others were observed during combinations of certain stages. The nature of coupling of sequential heterochronic expression of these proteins during successive instars or stages was tested by use of a parasite (Chelonus sp.) that injects regulatory material into the host embryo that later causes the subsequent precocious expression of the final instar larval program. Following the expression of a normal 3rd instar pattern, such larvae were observed to omit expression of the 4th instar program, including omission of the proteins heteromorphically specific to that instar, and instead then express an essentially normal final instar pattern. Thus, normal expression of the final instar feeding stage pattern was not invariantly coupled to prior expression of the penultimate instar-specific proteins or pattern. Also, expression of the full program of the final instar feeding stage was epistatic to teh penultimate instar program, i.e., the protein pattern unique to the penultimate larval instar was not co-expressed with the precociously expressed final instar pattern. Larvae developmentally redirected in this manner failed to fully express the final instar prepupal stage pattern of protein expression, due at least in part to failed expression of prepupal ecdysteroids, but this was shown not to arise from omission of any of the first 4 larval instars per se. The nature of the redirections in host development caused by this parasite finally provides means of probing the coupling of successive expression on heteromorphic programs during larval development.

Preultimate 4th/5th instar Trichoplusia ni naturally-injected with venom/calyx fluid from Chelonus curvimaculatus precociously metamorphose, rather than obey the metamorphic size threshold that would normally compel molting to a 5th/6th instar

In normally regulated larval metamorphosis of Trichoplusia ni, a 4th, 5th or other numbered instar is a 'preultimate' instar, and will normally continue larval molting, if the larva has not yet surpassed the critical (minimal) size threshold corresponding to attainment of the 'ultimate' (metamorphic) instar. Natural injection of T. ni embryos with venom/calyx fluid of female Chelonus sp. near curvimaculatus caused 'penultimate' 4th or 5th instar larvae that would normally molt at least once more, to a 5th/6th instar, to instead precociously metamorphose without another larval molt. These effects were observed in naturally-injected insects that never contained either a parasite larva, a viable parasite embryo, or a parasite egg. These data demonstrate that this effect of venom/calyx fluid of this wasp to induce precocious metamorphosis, at an instar earlier than would otherwise have typically occurred under normal growth conditions, does not require the presence of a parasite larva. Other data did indicate the parasite larva contributes an additional effect that causes a 4th instar host (that from its size would normally require not just one, but at least two more larval molts to reach the metamorphic instar) to not grow to the size metamorphic threshold, but to instead, precociously metamorphose at an even smaller size than occurs with the venom/calyx fluid alone. Additionally, arylphorin was precociously highly expressed in parasitized hosts in a manner independent of a decline in the host JH titer. Therefore, the main target of the venom/calyx fluid activity to induce precocious metamorphosis appears to be an event upstream of the decline in JH production by the corpora allata. Pseudoparasitized hosts become developmentally arrested as precocious prepupae and express a 2.7kb polydnavirus transcript. The larger (but still subthreshold size) larvae showed less suppressed prepupal ecdysteroid titers, less developmental suppression, and a much weaker expression of that transcript. A general model for mechanisms of action of chelonine venom/calyx fluid, and larvae, to cause precocious host metamorphosis and suppressed prepupal development is presented that is based on the current 'size threshold' model of normal lepidopteran development, rather than the older, displaced 'instar count' model. By basing the model for chelonine regulation of host development on the current 'size threshold' model for normal development, the proposed model for chelonine action both accounts for observations reported on various species of that subfamily and makes useful, testable predictions.

Toxins produced by arthropod parasites: salivary gland proteins of human body lice and venom proteins of chelonine wasps

A review is presented of our ongoing research projects on the protein components of the saliva of human body lice and of the non-paralyzing venom of wasps in the subfamily Cheloninae. Sodium dodecyl sulfate-polyacryamide gel electrophoretic analysis of lice salivary gland proteins showed a predominance of high and intermediate mol. wt proteins. Immunoblotting with a low titer polyclonal antiserum to lice salivary proteins indicated that some, but not all, of the predominant high mol. wt salivary gland proteins are injected into the host during feeding. The venom of a Chelonus sp. wasp contains a chitinase, and a 33,000 mol. wt protein with a primary structure composed mostly of a series of 12 tandem repeats of a 14-residue sequence. The N-terminus of this protein and its homologs in a related species of Ascogaster share a conserved adjacent pair of acidic residues. Epitope mapping/immunoprecipitation experiments now in progress will provide information on which linear motifs are on the surface of the protein, and will thereby provide information on the tertiary structure of the protein.

Altered hexamerin regulation in prepupal Trichoplusia ni pseudoparasitized by Chelonus sp. near curvimaculatus

Adult female wasps of species in the subfamily Cheloninae inject an egg, venom, polydnavirus and other materials into the host egg during oviposition. Hosts then exhibit precocious expression of the metamorphic developmental program, but then further development by the precocious prepupa is suppressed. These effects occur in truly parasitized hosts (those that contain a live endoparasite larva) as well as in pseudoparasitized hosts (that do not contain a live endoparasite). We report here that during the precocious prepupal stage, the hexamerins BJHSP1 and BJHSP2 persist in the hemolymph of pseudoparasitized hosts, whereas in normal larvae these proteins are cleared from the hemolymph in response to the normal surge in prepupal ecdysteroids. Northern blot analysis of poly(A) RNA showed that the basis for this persistence is not an abnormally high abundance of the transcripts on the day following wandering in pseudoparasitized larvae. Nor is the source of the hexamerins the parasite larva, for it is missing from the pseudoparasitized hosts. The hypothesis that the persistence is due to a suppressed titer of ecdysteroids in pseudoparasitized hosts (reported earlier: [jones et al., Arch Insect Biochem Physiol 21:155 (1992)] was tested by use of a large size variant of pseudoparasitized hosts in which the prepupal ecdysteroid titer is partially restored by endogenous ecdysteroid production. In such pseudoparasitized prepupae, the two hexamerins were cleared from the hemolymph on the day following host wandering behavior, as in normal larvae. Thus, the regulatory basis of the persistence of the hexamerins BJHSP1 and BJHSP2 in the hemolymph of pseudoparasitized hosts appears to be at the posttranslational level, with suppression of the prepupal ecdysteroid titer causing omission of the normal trigger for fat body uptake of the hexamerins.

Isomeric and quaternary properties of homogenous 33 kDa protein from the venom of Chelonus near curvimaculatus

The 33,000 Dalton venom protein of Chelonus near curvimaculatus was characterized for structural properties of charge, quaternary associations, and relationship to polydnavirus encoded proteins. Homogenous isoforms of the protein were isolated from the venom by sequential steps of 1) microdissection, 2) separation based on charge (Mono-Q column HPLC or narrow-range electrofocusing), and 3) centrifugal filtration based on molecular weight using Centricon microconcentrators. The purified protein dimerized under native conditions, and this quaternary association became denaturation resistant under certain conditions. Chemical modification of lysine epsilon amino groups did not disrupt such dimerization. The cDNA for the protein did not possess high similarity to any sequence encoded in the polydnavirus, as indicated by results of Southern blotting, but does possess similarity in its repeats to the repeats of the immunologically protective surface glycoprotein of Leishmania amazonensis.

Regulatory mediators in the venom of Chelonus sp.: their biosynthesis and subsequent processing in homologous and heterologous systems

Following titration of the contents of the venom gland reservoir, the rate of biosynthesis of venom proteins was sufficiently rapid over the next 6-24 hrs to restore their titer to the level initially synthesized during early adulthood. There was no evidence of processing of smaller molecular weight components from much larger forms. Although most proteins were stable in young host embryos, two specific processing products of a 32.5 kDa venom protein were found in such hosts. The natural injection of venom proteins into either very old embryos or young embryos subsequently held at 4 degrees C for six days resulted in rapid degradation to biologically inactive forms. These data are the first report of direct examination of the biosynthesis of wasp venom proteins and the first analysis of the processing of specific hymenopteran venom proteins in target tissues.

Comparative studies on the protein composition of hymenopteran venom reservoirs

The proteins of venom reservoirs from 25 hymenopteran species from 21 genera were investigated with regard to their protein composition and immunological similarities. It was found that low mol. wt proteins and polypeptides are typical for the venoms of ants, social wasps and bees. Six species of ichneumonoid parasitic wasps lack these low mol. wt proteins. Except for bee venoms none of the low mol. wt proteins contain mannose. The venoms of ichneumonoid parasites and ants contained primarily acidic proteins, whereas those of social wasps and bees contained abundant neutral and basic proteins. Immunological analysis showed that a number of venom proteins from parasitic braconid (Chelonus) wasps shared conserved immunoreactive determinants with the venom proteins of higher Hymenoptera, including some not of parasitic habit. Implications of phylogenetic relationships among hymenopteran species are discussed.

The parasitoid Apanteles kariyai inhibits pupation of its host, Pseudaletia separata, via disruption of prothoracicotropic hormone release

When the parasitoid Apanteles kariyai laid eggs into host Pseudaletia separata larvae, before prothoracicotropic hormone (PTTH) was released in the last instar preparatory to metamorphosis, the host did not pupate and the larvae of the wasps emerged. The ecdysteroid titer of unparasitized intact larvae increased up to 1 microgram/ml 1 day before pupation, whereas the titer of parasitized larvae was maintained at a low level without the surge. Isolated prothoracic glands from intact larvae synthesized much more ecdysone than those of parasitized larvae both in vivo and in vitro. Administration of exogenous PTTH caused the activation of the prothoracic glands seen during parasitization. Injection of 20-hydroxyecdysone (20-HE) into the parasitized larvae caused by host's pupation, but did not affect the development of the wasp larvae. However, the sensitivity of the integument to 20-HE was lower in parasitized than in unparasitized larvae. Injection of a mixture of adult wasp calyx and venom fluids into last instar unparasitized larvae delayed their pupation, suggesting that calyx and venom fluids are factors contributing to disturbance of the normal function of brain-prothoracic gland system. These results show that parasitization inhibits secretion and/or synthesis of PTTH and also delays the larval-pupal commitment of the integument by keeping the ecdysteroid level low.

Precocious expression of the final larval instar developmental pattern in larvae of Trichoplusia ni pseudoparasitized by Chelonus spp

The present study has used a number of electrophoretic approaches to analyze the proteins in normal Trichoplusia ni larvae and those pseudoparasitized by Chelonus spp. A number of feeding-stage, hemolymph proteins appear or increase dramatically only during the final larval stadium. Other proteins highly abundant only during the penultimate stadium disappear or decrease dramatically during the final stadium. The comparative protein profiles of penultimate instar, pseudoparasitized larvae are very similar to those of last instar larvae. These changes in hemolymph proteins are seen on gels resulting from electrophoresis, isoelectric focusing (wide range and narrow range Ampholine and very narrow range Immobiline gels) and SDS-disc electrophoresis. It is concluded that the entire last instar developmental pattern of protein gene products is occurring precociously in pseudoparasitized larvae.