Hymenolepis diminuta-induced fecundity reduction may be caused by changes in hormone binding to Tenebrio molitor ovaries

Evolutionary drivers of parasite-induced changes in insect life-history traits from theory to underlying mechanisms

Many hosts are able to tolerate infection by altering life-history traits that are traded-off one against another. Here the reproductive fitness of insect hosts and vectors is reviewed in the context of theories concerning evolutionary mechanisms driving such alterations. These include the concepts that changes in host reproductive fitness are by-products of infection, parasite manipulations, host adaptations, mafia-like strategies or host compensatory responses. Two models are examined in depth, a tapeworm/beetle association, Hymenolepis diminuta/Tenebrio molitor and malaria infections in anopheline mosquitoes. Parasite-induced impairment of vitellogenesis ultimately leads to a decrease in female reproductive success in both cases, though by different means. Evidence is put forwards for both a manipulator molecule of parasite origin and for host-initiated regulation. These models are backed by other examples in which mechanisms underlying fecundity reduction or fecundity compensation are explored. It is concluded that evolutionary theories must be supported by empirical evidence gained from studying molecular, biochemical and physiological mechanisms underlying changes in host life-history traits, ideally using organisms that have evolved together and that are in their natural environment.

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).

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.

The Interplay between Patency, Microsomal Na(+)/K(+) ATPase Activity and Juvenile Hormone, in Tenebrio molitor Parasitized by Hymenolepsis diminuta

Beetles infected with metacestodes of the rat tapeworm, Hymenolepis diminuta, exhibit reduced fecundity, due to alterations in vitellogenesis. Follicle cell patency is retarded and inefficient vitellogenin uptake ensues. Here, we have reassessed patency and its stimulation by JH III at day 3 post-infection, when the most detrimental changes are observed in other ovarian processes. In Rhodnius prolixus, patency is believed to be brought about by the action of a JH-dependent membrane-bound Na(+)/K(+) ATPase (EC 3.6.1.3); however, this had not been established in Tenebrio molitor. Therefore, the properties of the enzyme, with respect to optimal assay conditions and juvenile hormone dependency, are reported. Maximal stimulation occurred between 50 and 500 nM JH III, a range over which greatest increases in patency were also observed. In infected insects, a 35% reduction in Na(+)/K(+) ATPase activity was noted, but exposure to 50 nM JH III is sufficient for stimulation to a specific activity 89% that of JH-treated controls. In a similar fashion, patency in infected insects is reduced, but can be 'rescued' by 50 nM JH III. Moreover, in the absence of exogenous hormone, patency in infected beetles can be elevated to control levels after in vitro culture (6 h), with exchange of medium every 2 h. The possibility that such reversible decreases in enzyme activity and patency are caused by a JH binding inhibitor molecule is discussed. Copyright 1997 Elsevier Science Ltd. All rights reserved

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.

Plasmodium yoelii nigeriensis: the effect of high and low intensity of infection upon the egg production and bloodmeal size of Anopheles stephensi during three gonotrophic cycles

Anopheles stephensi mosquitoes showed a reduction in fecundity over 3 successive gonotrophic cycles, after becoming infected with Plasmodium yoelii nigeriensis. This effect could be observed at high oocyst burdens (> 75) or at low oocyst burdens (mean of 4.36). Mean bloodmeal size of the infected mosquitoes was significantly reduced only when feeding upon a mouse with a high gametocytaemia and the conversion of the bloodmeal into eggs by the infected mosquitoes was disrupted. Patterns of infected mosquito mortality, over the 3 gonotrophic cycles, varied with severity of infection. Although in 1 case increased mortality and decreased bloodmeal size may have affected fecundity, this could not have accounted for all of the observed fecundity reduction. We propose that other, unknown parasite related factors, are involved.

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.