Tenebrio molitor (Coleoptera): effect of metacestodes of Hymenolepis diminuta (Cestoda) on haemolymph amino acids

Is there life after parasitism? Survival, longevity, and oogenesis in Acheta domesticus (Orthoptera: Gryllidae) infected with the hairworm, Paragordius varius (Phylum: Nematomorpha)

The costs parasites impose on hosts can lead to reductions in survival and fecundity, but few studies have evaluated the impacts after infection. Hairworms are parasites of terrestrial arthropods that are free-living in aquatic systems as adults. As parasitic juveniles, hairworms acquire nutrients from their definitive hosts, shifting resources away from host development to parasite growth. However, until now, only one study has examined survivorship of naturally infected hosts with hairworms. Using a different hairworm and host system, we conducted experimental infections to examine growth, survivorship, and egg production in virgin female Acheta domesticus infected with the hairworm, Paragordius varius. We found that infected crickets grew significantly less during hairworm development compared to sham-infected control crickets. After releasing their worms, infected crickets survived for 73 ± 32 days but had significantly shorter life spans by an average of 13 days compared to sham-infected control crickets. However, we found that 50% of previously infected crickets produced eggs after releasing their worms. Taken together, these observations suggest that female crickets infected with hairworms may experience less mortality than previous anecdotal evidence suggests. Finally, we discuss the definition of parasitoid and how it relates to nematomorphs, and we suggest that more field and laboratory research is required before suggesting hairworms are parasitoids.

Hymenolepis diminuta infections in tenebrionid beetles as a model system for ecological interactions between helminth parasites and terrestrial intermediate hosts: a review and meta-analysis

The cestode Hymenolepis diminuta (Cyclophyllidea) uses a variety of insects as its intermediate host, where ingestion of eggs results in development in the hemocoel of a cysticercoid that is infective to a rat definitive host. Species in 2 genera, Tenebrio and Tribolium (Coleoptera: Tenebrionidae) have been used extensively as laboratory intermediate hosts. This review examines experimental studies on ecological aspects of the relationship between H. diminuta and tenebrionid beetles, including the acquisition and establishment of the parasite, host effects on the parasite, and parasite effects on the host. A meta-analysis of infection results from the literature revealed strong relationships across host species and strains between (1) prevalence and intensity of infection, (2) efficiency of cysticercoid production and exposure conditions, and (3) variance in abundance or intensity of infection relative to their respective means. The underlying mechanisms producing these patterns remain elusive. Comparative studies are infrequent, and the use of divergent methodologies hampers comparisons among studies. In spite of these problems, there is much to recommend this as a terrestrial host-parasite model system. It represents those relationships in which mostly minor, but occasionally major, responses to parasitic infection occur, and in which host genetics and environmental conditions can serve as modifying factors. Moreover, this is a tractable experimental system, and is backed by an extensive literature on host biology.

Pathophysiology ofHymenolepis diminutainfections inTenebrio molitor:effect of parasitism on haemolymph proteins

The effects of metacestodes ofHymenolepis diminutaon haemolymph proteins ofTenebrio molitorare restricted to female hosts. In beetles aged 15 days post-emergence, and harbouring 12-day-old metacestodes, haemolymph concentration is 46·7% higher than in non-infected animals and similar effects are found in longer standing infections. Electrophoresis of haemolymph revealed the presence of 13 bands. Densitometric analysis showed that only band 2/3 was significantly elevated in infected hosts although band 7/8 also showed an increase. These bands were also present in egg homogenates and are thought to be vitellogenins. It is therefore proposed that the excess protein found in infected beetles is a female-specific protein resulting from an interaction between the parasite and the host endocrine system.

Manipulation of host food availability and use of multiple exposures to assess the crowding effect onHymenolepis diminutainTribolium confusum

We studied the ‘crowding effect’ inTribolium confusuminfected withHymenolepis diminuta. Manipulations included age and number of parasites, and diet, sex, age and number of exposures of hosts. Volume per parasite was unaffected until an intensity of at least 5–10 parasites per host, then declined approximately inversely as intensities increased. Parasite size was affected by host sex but not age or reproductive status. Host diet affected parasite size and the impact of crowding. Daily gain in parasite volume peaked partway through the developmental period and preceded the first evidence of a crowding effect. Parasites that established during a second exposure had a transient developmental delay but eventually grew as large or larger than parasites from a single exposure with the same total intensity. Parasites responded to crowding by differential allocation of resources. Cercomer volume decreased even with slight crowding, the capsule surrounding the scolex was not reduced until crowding became more severe, and scolex width was reduced only in the most extreme conditions. The data support the hypothesis that the crowding effect in this system is driven primarily by nutrient, rather than space limitations.

The effect of Hymenolepis diminuta (Cestoda) cysticercoids on the weight change, frass production, and food intake of the intermediate host, Tenebrio molitor (Coleoptera)

Parasitism results in nutritionally related changes in hosts, often leading to altered feeding behavior. Infected hosts that increase their feeding also increase their probability of reinfection. To study this, I used a beetle (Tenebrio molitor)-tapeworm (Hymenolepis diminuta) system. Infected and uninfected male and female beetles were individually housed in vials with food. Each beetle's weight change, food intake, and frass production were measured over 24-h periods at 3, 7, 12, and 16 days postinfection. Treatment (infection) had no effect on weight change, but males lost more weight and produced more frass than females. Additionally, treatment had no effect on food consumption, but males had a higher food intake than females. These results suggest that infection status will not alter the probability of reinfection, but males will be more susceptible to infection than females. However, despite the male's greater food intake during the experimental infection period, parasite loads did not differ between males and females.

Apoptosis in the fat body tissue of the beetle Tenebrio molitor parasitised by Hymenolepis diminuta

Many insects experience a decrease in reproductive output when parasitised. We are investigating mechanisms underlying this fecundity reduction using the rat tapeworm, Hymenolepis diminuta infection of Tenebrio molitor beetles. These include an increase in the resorption of developing ovarian follicles and a decrease in fat body synthesis of vitellogenin. The latter is the direct effect of a molecule produced by the parasite. Here we report a study to determine whether vitellogenin synthesis and follicle resorption are the result of parasite-induced apoptosis in the respective tissues and whether the parasite molecule acts directly on the fat body by inducing apoptosis. In vivo, the number of fat body cell nuclei with chromatin condensation are significantly elevated in parasitised females at all days examined and peaked at day 7 post-infection. A TUNEL assay to detect DNA fragmentation confirmed these observations of apoptosis. However, when fat body from uninfected females was co-cultured with live metacestodes they did not cause cells to die by apoptosis, showing that the induction signal does not come directly from the parasite. The follicle resorption observed in the ovaries of infected beetles was not associated with apoptosis of the epithelial cells. The possibility of several mechanisms underlying fecundity reduction is discussed.

Juvenile hormone titre and egg production in Tenebrio molitor infected by Hymenolepis diminuta: effect of male and/or female infection, male age and mating

Infection of Tenebrio molitor with Hymenolepis diminuta induces curtailment of female fertility. We examined ovulation and oviposition, and associated titres of juvenile hormone (JH), in relation to parasitism and mating. Oviposition was significantly increased in infected mated and virgin beetles by days 6 and 9 post-emergence. Ovulation was not changed by infection; by the end of the 18-day experiment, the total number of laid eggs was not significantly altered. On day 6, JH levels were significantly higher in virgin infected insects, compared to non-infected controls (236+/-37.7 and 107+/-9.62 pg/g wet weight). Oviposition increased after mating, but total eggs ovulated remained the same. JH levels were higher in mated females on days 12 and 18 post-emergence, for infected and control insects. Previous studies suggested that male reproductive potential might rise following infection, because uninfected females lay more eggs when mated to infected males. We tested whether this caused an increase in female JH. Males were mated on days 5 or 12, when significant changes in their reproductive physiology begin to be observed, and are maximal, respectively. However, male age was of greater significance in promoting JH levels in females (p=0.001), than infection status of either partner (p=0.33).

A parasite that increases host lifespan

Tenebrio molitor is an intermediate host for the rat tapeworm, Hymenolepis diminuta. Parasite oncospheres hatch in the beetle midgut and burrow through into the haemocoel, where they rapidly grow and mature into metacestodes. Repair of damage incurred during invasion and the nutritional demands of the parasites are likely to impose costs on the host. Despite these costs, there is an overall very highly significant difference in survival time (p < 0.001) between infected and control populations of beetles, with a hazard ratio of 2.35 (control versus infected). Infected females showed a 40% increase in survival time to 50% mortality and males showed a 25% increase in survival time to 50% mortality. This parasite-induced increase in host longevity is discussed in the light of changes in resource allocation that may occur in infected beetles. Previous findings have demonstrated that reproductive success is significantly reduced in infected females. The outcome of changes in the reproductive effort made by male beetles is less clear. We suggest that the optimum trade-off between reproduction and longevity may be altered to favour longer host survivorship, which is likely to enhance parasite transmission.

Development and intensity dependence of Ornithodiplostomum ptychocheilus metacercariae in fathead minnows (Pimephales promelas)

Intensity-dependent development of Ornithodiplostomum ptychocheilus metacercariae was studied in fathead minnows (Pimephales promelas) exposed to 0, 20, or 120 cercariae. Subsamples of hosts were necropsied at 2-wk intervals to monitor parasite recruitment, growth, and time to encystment. The complex development of metacercariae within the cranium of minnows involved growth, encystment, and consolidation phases, each of which were affected by intensity. At the end of the growth phase, metacercariae from low-dose fish were 20% longer than those from high-dose fish and the latter took 2-4 wk longer to encyst. At the end of the postencystment consolidation phase (6-8 wk postinfection), the size of metacercariae decreased by approximately 50%. The rate of consolidation was slower in high-dose fish. Our results show that time, intensity, and temperature affect development of O. ptychocheilus. Because metacercariae development and differentiation are linked to infectivity, events occurring in intermediate hosts can potentially impact the structure and size of trematode suprapopulations.

Parasite manipulation of insect reproduction: who benefits?

Host fertility is often curtailed as a result of parasitic infection. The hypothesis that this may confer an adaptive advantage upon the symbionts if nutrients are directed from reproduction and made available for host/parasite maintenance is explored. The suggestion is made that an understanding of the mechanisms underlying the pathophysiology of fecundity reduction may shed light upon the evolutionary implications of this strategy for both parasite and host. To illustrate this the down-regulation of egg production is explored with reference to a particular model system, the association between metacestodes of the rat tapeworm, Hymenolepis diminuta and the mealworm beetle, Tenebrio molitor. Several aspects of host reproductive behavior and physiology are affected by infection in this association, including vitellogenesis. Metacestodes directly inhibit the fat body synthesis of vitellogenin in a stage-specific, density-dependent manner. This inhibition is likely to be orchestrated by a modulator molecule, produced by the parasite. In the ovarian follicles, juvenile hormone III binding to a specific follicular membrane-binding protein is inhibited in infected beetles, resulting in the down-regulation of a cascade of events which enables vitellogenin to pass into the developing oocyte. Data to support the proposed existence of a parasite-induced antigonadotrophin, of host origin, are discussed. Evidence that similar mechanisms operate in Plasmodium-infected anopheline mosquitoes and Onchocerca-infected blackflies is presented in support of the possibility that a parasite-induced reduction in host reproductive fitness is an adaptive strategy and an assessment of who is manipulating whom is made.

A NMR study of parasitized Tenebrio molitor and Hymenolepis diminuta cysticercoids

In vivo NMR spectra of uninfected and Hymenolepis diminuta-infected Tenebrio molitor fed D-(1-13C)glucose showed that infected beetles of both sexes had a significantly higher ratio for (glycogen C1/lipid (CH2)n) than the corresponding controls. Quantitative metabolic profiles and the per cent 13C-label in metabolites, based on NMR of perchloric acid extracts, are presented for control and infected beetles fed D-(1-13C)glucose and for H. diminuta cysticercoids. Female beetles, both control and infected, contained more glycogen than their male counterparts and infected beetles of both sexes possessed less glycerophos-phocholine, but more glycogen and a higher percentage label in glucose and trehalose than their respective controls. Label was also incorporated into glycogen, succinate, acetate, alanine and lactate. Extracts of cysticercoids from beetles fed D-(1-13C)glucose contained the following labelled compounds, in order of decreasing per cent 13C label: glucose, trehalose, alanine, succinate, lactate, glycogen and acetate. In vitro cultivation experiments, employing D-(1-13C)glucose, revealed that trehalose found in cysticercoids was of parasite, and not beetle, origin.

Hymenolepis diminuta: metacestode-induced reduction in the synthesis of the yolk protein, vitellogenin, in the fat body of Tenebrio molitor

Vitellogenin synthesis by the fat body has been monitored using in vitro culture and immunoprecipitation. This system was found to be efficient for measuring vitellogenin production in both non-infected Tenebrio molitor and those infected with Hymenolepis diminuta. In fat bodies from infected beetles, vitellogenin production was decreased by up to 75% (day 24 post-infection) and, at all times investigated, vitellogenin synthesis was significantly below control levels (days 3-30 post-infection). Incubating fat bodies from control insects with isolated metacestodes indicated that this may be a direct effect by the parasite which is developmental stage-specific. Stage II, but not Stage III-IV, not heat-killed parasites could bring about this decrease in vitellogenin. In addition, these effects may be density dependent within the range of 2-20 parasites per fat body; only 2 metacestodes were necessary to cause a significant decrease. Since metacestodes do not take up vitellogenin, nor limit the amount of [14C]leucine available to the fat body for vitellogenin production, it is conceivable that the parasite produces a potent inhibitor of vitellogenin synthesis, or a molecule which induces cells within the fat body.

Microsomal juvenile hormone binding proteins in the follicle cells of Tenebrio molitor

The microsomal fraction of Tenebrio molitor follicle cells has been found to contain both high and low affinity binding sites for juvenile hormone (JH) III. Using Scatchard analysis, the equilibrium dissociation constants, Kd, were calculated as 1.0 x 10(-8) and 4.3 x 10(-7) M respectively. Kinetic data support a rapid binding of the hormone to the site(s), with rate constants of ka = 3.77 x 10(8) M-1 min-1 and kd = 0.0075 min-1. Affinity of the binding site(s) for JH III was higher than for either JH I or methoprene. The significance and possible function of such microsomal binding proteins are discussed, with reference to the perturbance of vitellogenesis found in beetles parasitized by Hymenolepis diminuta.

Effect of metacestodes of Hymenolepis diminuta on storage and circulating carbohydrates in the intermediate host, Tenebrio molitor

The metamorphosis of oncospheres of the rat tapeworm, Hymenolepis diminuta, to mature metacestodes induces several pathophysiological effects in the intermediate host, Tenebrio molitor (Coleoptera). Previous investigations have failed to elucidate the mechanism responsible for changes in the host reproductive physiology and behaviour. This work forms part of an assessment of the degree to which nutrient resource management may be involved in these interactions. We report that developing metacestodes exert a pronounced effect upon host carbohydrate reserves and also alter circulating carbohydrate titres at some stages post-infection. Biochemical studies of fat body glycogen demonstrated a significant depletion of reserves as early as 3 days post-infection in males and 5 days post-infection in females. Gas chromatography revealed trehalose to be the predominant haemolymph carbohydrate, titres being significantly elevated in male beetles 5 and 9 days post-infection and in females 12 days post-infection. Metacestodes had no effect on glucose concentrations in male or female beetles.

Hymenolepis diminuta: an investigation of juvenile hormone titre, degradation and supplementation in the intermediate host, Tenebrio molitor

Metacestodes of Hymenolepis diminuta cause a perturbance of vitellogenesis in the intermediate host Tenebrio molitor. The reduction in host reproductive output associated with infection may be due to this pathophysiology. Many of these events are regulated by host juvenile hormone (JH). A comparison of the titre of JH and its rate of degradation in female control and parasitized 15-day-old insects has been made. Haemolymph from female beetles contained 1.27 pMol JH equivalents/100 microliters. No significant difference was associated with infection. Likewise, the activity of JH esterase in female haemolymph was not affected by infection. However, topical application of a JH analogue, methoprene, at the time of infection or 8 days post-infection reduced the significant accumulation of vitellogenin usually found in the haemolymph of females 12 days or more post-infection. These findings indicate that parasite-induced alteration of host vitellogenesis is not mediated via alteration in JH titres, although observations made after hormone supplementation suggest some form of interaction between the parasite and the host endocrine system.

Evidence against the hypothesis that metacestodes of Hymenolepis diminuta inhibit corpora allata functioning in the intermediate host, Tenebrio molitor

Several of the pathophysiological responses made by the beetle Tenebrio molitor, when infected with metacestodes of Hymenolepis diminuta, may be attributed to a parasite-induced reduction in host juvenile hormone titre. It has been suggested that production of this hormone by the corpora allata may be inhibited in parasitized insects. This hypothesis was tested using an in vitro radiochemical assay to compare the biosynthesis of juvenile hormone by single pairs of corpora allata taken from mated 12- and 15-day-old infected and control female insects. The results demonstrate, however, that there was no difference in hormone production associated with metacestode infection. Other possible mechanisms for parasite interference in this host endocrine system are discussed.

Hymenolepis diminuta: influence of metacestodes on synthesis and secretion of fat body protein and its ovarian sequestration in the intermediate host, Tenebrio molitor

Female Tenebrio molitor infected with metacestodes of Hymenolepis diminuta exhibit elevated concentrations of female-specific proteins in their haemolymph and the origin of these has been investigated. Following a 4 h in vitro incubation with [14C]leucine, fat bodies from non-infected females secreted 13 times more protein than those from females 12 days post-infection. A comparison of the uptake in vivo of radio-isotope labelled amino acids by ovaries from non-infected and infected beetles of various ages revealed no differences; however, a 51.5% decrease in protein sequestration was detected in females 12 days post-infection. Electrophoresis of homogenates of radio-isotope labelled ovaries demonstrated that the majority of label was associated with vitellin sub-units. It is suggested that the decrease in vitellogenin sequestration associated with infection results in an increase in the haemolymph concentration of these proteins despite a concomitant reduction in their secretion by fat bodies. Both fat body synthesis and ovarian sequestration are under juvenile hormone control and it is proposed that metacestodes of H. diminuta may cause a reduction in the concentration of this hormone in the intermediate host.

Experimental infection of Tribolium confusum (Coleoptera) by Hymenolepis diminuta (Cestoda): host fecundity during infection

Some effects of Hymenolepis diminuta on the fecundity of Tribolium confusum are described. Host fecundity is observed to be reduced exponentially with increasing parasite burden/host, although there are differences in the ability of individual hosts to respond to parasitism. Of particular interest is the finding that host fecundity is greatly reduced in young beetles on or by day 14 post-infection (p.i.). This age-related reduction in host fecundity is discussed in relation to the population dynamics of this host-parasite relationship.

Hymenolepis diminuta (Cestoda): uptake of cycloleucine by metacestodes

Cycloleucine uptake by metacestodes of H. diminuta of various ages was investigated. Absorption occurs by active mediated transport, mean Kt = 0.28 mM. Vmax values are age-related, and can be correlated to developmental changes. Cycloleucine uptake in the metacestode is very similar to that in the adult worm and the implications of this are discussed.