JH biosynthesis and degradation in parasitized host larvae

Our previous investigations on the endocrine interaction between gypsy moth larvae (Lymantria dispar) as host and the endoparasitic wasp Glyptapanteles liparidis showed that parasitism increased the juvenile hormone (JH) titre of the host, but also induced a change in the predominant JH homologue. Due to the high content of JHs, the development of the host larva is arrested before parasitoid emergence, apparently in order to prevent further energy consuming moults and the pupation of the host. In host hemolymph, this rise in JH is at least partly explained by a low activity of the hormone specific degrading enzyme, the juvenile hormone esterase (JHE). However, it remains unclear why parasitism leads to this drastic change in the composition of the JH homologues, too. Thus, the aim of the proposed project is to determine the origin of the elevated JH haemolymph titre in parasitized host larvae and to study its metabolism in host tissues. We will check in vitro a potential release of JH III by the parasitoid larvae and a possible influence of parasitism on the hormone secreting activity of the corpora allata from parasitized and unparasitized gypsy moth larvae. To support the results of the in vitro studies and to investigate the role of the parasitoid and its associated factors (venom and polydnavirus, teratocytes) on the JH composition in the host haemolymph, we will implant or inject these components in unparasitized gypsy moth larvae which will be immune-suppressed by pseudoparasitism or by chemicals. Changes in JH titre caused by the single factors will be recorded in vivo by a highly sensitive physicochemical method with GC-MS. Although parasitism reduces the activity of JHE in the host haemolymph, no data are available on JH degradation in other insect tissues. Therefore, our research will focus on the significance of cellular JHE and JH epoxide hydrolases (JH-EH) in JH cleavage of fat body, integument and midgut. This study will give new insights in JH metabolism of insect tissues and will extend our knowledge of endocrine regulation processes influenced by parasitism.

Glyptapanteles liparidis is a gregarious endoparasitoid that develops inside the hemocoel of its lepidopteran host, the gypsy moth larva (Lymantria dispar). Parasitized hosts are developmentally arrested before the parasitoid larvae emerge and die some days later. In this study we investigated the mechanisms by which the parasitoids regulate and/or interfere with the endocrine systems of their host. From previous studies we know that parasitization causes a prominent rise in the juvenile hormone (JH) levels of the host larvae. While unparasitized larvae contain low amounts of JH II and JH III, parasitized hosts exhibit a significant increase in both homologs, with extraordinarily high titers of JH III. With a highly sensitive gas chromatography/mass spectrometry method to quantify the different JH homologs we were able to demonstrate that second instar parasitoids produce and release JH III into the host hemocoel. The increase in hemolymph JH titers starts with the parasitoids` molt to second instars, peaks prior to parasitoid emergence and drops to negligible levels shortly after. The parasitoids` ability to secrete JH and to molt independently from their host`s molting cycles indicates that at least second instar parasitoids are hormonally self-reliant. In contrast, the secretory activity of the host`s corpora allata, the glands that synthesize and release juvenile hormones, was not enhanced by parasitization. Furthermore, we showed that parasitization not only affected JH biosynthesis but also its degradation by JH esterase and JH epoxide hydrolase in various host insect tissues. The activity of the JH clearing enzymes was significantly lower in the hemolymph and fat body of parasitized gypsy moth larvae than in untreated controls. Interestingly, reduced JH esterase levels were not accompanied by a reduction in JH esterase messenger RNA, suggesting that parasitization had no effect on host JH esterase gene transcription. However, JH degradation was also reduced in gypsy moth larvae that contained only venom, calyx and polydnavirus, but no viable eggs and/or parasitoid larvae (pseudoparasitized hosts). These factors are injected by the female wasps at oviposition and interfere with the host`s immune response to prevent encapsulation of parasitoid eggs and larvae. The observed reduction in JH esterase due to the associated factors alone suggests their supporting activity on hormonal disruption of the host larva. To elucidate the mechanisms by which the parasitoid larva and its associated factors (venom, calyx, polydnavirus, teratocytes) manipulate host physiology and endocrinology may contribute to develop new and ecologically sound control strategies for insect pests.