Pea aphid clonal resistance to the endophagous parasitoid Aphidius ervi

The physiological mechanism of resistance to the endophagous braconid Aphidius ervi Haliday (Hymenoptera, Braconidae) by a pink clone (PC) of Acyrthosiphon pisum (Harris) (Homoptera, Aphididae) has been investigated. Comparative data on parasitoid development and associated host biochemical changes in the resistant PC aphids and in a susceptible green clone (GC) of A. pisum are reported. When the PC aphids were attacked as early 4th instars, the developing parasitoid larvae showed a strongly reduced increase in size, compared to those synchronously developing in GC aphids, and were unable to produce a regular mummy. In contrast, parasitism of 2nd instar PC aphids, allowed completion of parasitoid development, but adults had a prolonged developmental time, due to a longer duration of parasitoid’s final (3rd) instar. In all cases, teratocytes, cells deriving from the A. ervi serosal membrane, and the proteins abundantly synthesised by them, were never found in the haemolymph of parasitised PC aphids. Host castration, as demonstrated by total protein incorporation into reproductive tissues, was total in the majority of early (2nd instar) parasitised host aphids, while it was limited when later instars (4th) of PC aphids were parasitised. This is partly due to the absence of the cytolytic activity of teratocytes on host embryos, which, through their persistence, may compete for nutritional resources with the developing parasitoid larvae. In parasitised PC aphids, this competitive effect is further aggravated for the parasitoid by the absence of the regulated amino acid titre increase in the host haemolymph, which is regularly observed in GC aphids. Failure of teratocyte development in the PC clone of the pea aphid is, then, the major functional constraint accounting for the reduction/inhibition of A. ervi larval growth. The reported results allow to assess in vivo the role of teratocytes in the host physiological redirection and nutritional exploitation by the parasitoid, and to integrate and validate the proposed physiological model of host-parasitoid interactions in the system A. pisum-A.ervi.     

Development and morphology of teratocytes in Encarsia berlesei and Encarsia citrina: first record for Chalcidoidea

In several species of hymenopteran parasitoids of the superfamilies of Ichneumonoidea and Platygastroidea, the membrane enveloping the parasitoid embryo dissociates at hatching into a number of cells, called teratocytes, which autonomously develop in the host haemolymph. In this work we report for Encarsia berlesei and Encarsia citrina (Hymenoptera: Chalcidoidea), the dissociation of the extraembryonic membrane into cells whose morphological and embryological features correspond to those of teratocytes. In E. berlesei the membrane dissociated at hatching into 4-9 larger cells (100 microm diameter) and about 10 smaller cells (60 microm), which scarcely doubled their size during maturation. In E. citrina the membrane dissociated into five large cells (250 microm) which did not grow appreciably. Ultrastructural investigation of the dissociated cells in E. berlesei revealed that their surface was covered by microvilli, whose density and length increased from the egg stage to the 12 h following hatching. During the same period, rough endoplasmic reticulum evolved from a parallel profile to that of the cisternal type, while abundant vesicles represented the dominant cytological feature. The ploidy level of these cells ranged between 8c and 140c at hatching, but increased to 40c-350c at maturation. These findings provide the first clear evidences for the presence of teratocytes in the superfamily Chalcidoidea.     

Inhibition of juvenile hormone esterase activity in Lymantria dispar (Lepidoptera, Lymantriidae) larvae parasitized by Glyptapanteles liparidis (Hymenoptera, Braconidae)

Glyptapanteles liparidis is a gregarious, polydnavirus (PDV)-carrying braconid wasp that parasitizes larval stages of Lymantria dispar. In previous studies we showed that parasitized hosts dramatically increase juvenile hormone (JH) titers, whereas JH degradation is significantly inhibited in the hemolymph. Here we (i) quantified the effects of parasitism on JH esterase (JHE) activity in hemolymph and fat body of penultimate and final instars of L. dispar hosts and (ii) assessed the relative contribution of individual and combined wasp factors (PDV/venom, teratocytes, and wasp larvae) to the inhibition of host JHE activity. The effects of PDV/venom was investigated through the use of gamma-irradiated wasps, which lay non-viable eggs (leading to pseudoparasitization), while the effects of teratocytes and wasp larvae were examined by injection or insertion of these two components in either control or pseudoparasitized L. dispar larvae. Parasitism strongly suppressed host JHE activity in both hemolymph and fat body irrespective of whether the host was parasitized early (premolt-third instar) or late (mid-fourth instar). Down-regulation of JHE activity is primarily due to the injection of PDV/venom at the time of oviposition, with only very small additive effects of teratocytes and wasp larvae under certain experimental conditions. We compare the results with those reported earlier for L. dispar larvae parasitized by G. liparidis and discuss the possible role of JH alterations in host development disruption.     

Host range limitation caused by incomplete host regulation in an aphid parasitoid

Defining host ranges in parasitoid insects is important both from a theoretical and an applied point of view. Based on the literature, some species seem able to use a wide range of hosts, while field studies indicate possible local host specialization. In koinobiont endoparasitoid species, such specialization could involve physiological processes. We tested the ability of two strains of the cosmopolitan and polyphagous parasitoid Diaeretiella rapae, to develop in three of its recorded aphid host species. Both strains produced high parasitism rates on the cabbage aphid Brevicoryne brassicae and the green peach aphid Myzus persicae but almost no progeny on the cherry-oat aphid Rhopalosiphum padi. This last species was less attacked by female parasitoids. Moreover, parasitoid eggs and larvae were smaller than in the two other host aphid species and their development was delayed. This abnormal development appeared to be due to an incomplete host regulation process, probably related to the low number and the size of teratocytes produced by D. rapae in R. padi individuals. Such a failure as far as gaining control of the host's metabolism is concerned could play an important role in shaping the host range of parasitoid insects, leading to local variation of the host spectrum in populations from various geographical areas.     

一种中红侧沟茧蜂畸形细胞分泌的寄生特异蛋白

SDS-PAGE电泳表明,黏虫Pseudaletia separa-ta、棉铃虫 Helicoverpa armigera、小地老虎 Agrotisypsilon的幼虫受中红侧沟茧蜂Microplitis mediator寄生后,血淋巴中都出现一个98.6 kDa的寄生特异蛋白(p98.6)。畸形细胞(teratocytes)的体外培养发现,p98.6是由来自中红侧沟茧蜂胚胎浆膜层的畸形细胞分泌的。这一结果将为研究寄生蜂的寄生生理和畸形细胞在协调寄生蜂和寄主关系中的作用打下基础。     

畸形细胞生物学意义的探讨

中红侧沟茧蜂Microplitis mediator不同个体的畸形细胞在开始形成时,数目和大小比较恒定,为930±78个,直径13.6±1.8μm。到寄生后期,不同个体间的数目和大小变化很大,其变异系数分别为41.77和21.92(半径),但单位质量的寄主所含细胞的体积却相对比较恒定,变异系数为8.95。畸形细胞注射黏虫的实验表明,畸形细胞通过降低寄主的体重来抑制寄主的发育,但寄主可以克服这种抑制作用。这些结果暗示了寄主和畸形细胞两者之间存在一种营养上的关系。     

Extended in vitro culture of Microplitis croceipes teratocytes and secretion of TSP14 protein

Teratocytes, cells which originate from the serosal membrane of some Braconidae and Scelionidae, can be found in the hemocoel of permissive hosts during part or all of the developmental time of the parasitoid larva. Teratocytes from Microplitis croceipes are known to secrete biologically active proteins, which contribute to developmental arrest and failure to pupate of Heliothis virescens larvae. One such protein, which has a molecular weight of approximately 14 kDa is called TSP14. The presence of parasitoid larvae is essential to maintain teratocytes under in vitro conditions with protein-free EX-CELL 400. The teratocyte viability was maintained in vitro for at least 12 days in the presence of larvae when medium was exchanged every three days. Western blots show that TSP14 was secreted during the entire period of exchanges. In the absence of parasitoid larvae, teratocyte viability was only 30% by day 6 and no TSP14 had been secreted. In the absence of parasitoid larvae, teratocytes maintained in vitro in EX-CELL 400 medium supplemented with 10% FBS remained viable for at least nine days and secreted TSP14 for at least six days. This suggests that parasitoid larval secretions are sufficient but not uniquely essential to maintain teratocyte viability. Parasitoid larvae maintained in the absence of teratocytes did not secrete TSP14 and their secretory products did not inhibit pupation of H. virescens larvae.