Host refractoriness of the tobacco hornworm, Manduca sexta, to the braconid endoparasitoid Cotesia flavipes

Regulation of the Larval Transcriptome of Diatraea saccharalis (Lepidoptera: Crambidae) by Maternal and Other Factors of the Parasitoid Cotesia flavipes (Hymenoptera: Braconidae)

Koinobiont endoparasitoid wasps regulate the host's physiology to their own benefit during their growth and development, using maternal, immature and/or derived-tissue weaponry. The tools used to subdue the wasps' hosts interfere directly with host transcription activity. The broad range of host tissues and pathways affected impedes our overall understanding of the host-regulation process during parasitoid development. Next-generation sequencing and de novo transcriptomes are helpful approaches to broad questions, including in non-model organisms. In the present study, we used Illumina sequencing to assemble a de novo reference transcriptome of the sugarcane borer Diatraea saccharalis, to investigate the regulation of host gene expression by the larval endoparasitoid Cotesia flavipes. We obtained 174,809,358 reads and assembled 144,116 transcripts, of which 44,325 were putatively identified as lepidopteran genes and represented a substantial number of pathways that are well described in other lepidopteran species. Comparative transcriptome analyses of unparasitized versus parasitized larvae identified 1,432 transcripts of D. saccharalis that were up-regulated under parasitization by C. flavipes, while 1,027 transcripts were down-regulated. Comparison of the transcriptomes of unparasitized and pseudoparasitized D. saccharalis larvae led to the identification of 1,253 up-regulated transcripts and 972 down-regulated transcripts in the pseudoparasitized larvae. Analysis of the differentially expressed transcripts showed that C. flavipes regulated several pathways, including the Ca+2 transduction signaling pathway, glycolysis/gluconeogenesis, chitin metabolism, and hormone biosynthesis and degradation, as well as the immune system, allowing us to identify key target genes involved in the metabolism and development of D. saccharalis.

Cotesia flavipes (CAM) (Hymenoptera: Braconidae) Supresses Immune Responses In Diatraea flavipennella (BOX) (Lepidoptera: Crambidae)

The present research aimed to elucidate which aspects of immune responses in Diatraea flavipennella are suppressed by the parasitoid Cotesia flavipes, thus, ensuring parasitism success. We investigated the presence of apoptosis in fat body cells through the TUNEL technique. According to the results, reduced levels of nitric oxide and phenoloxidase activity were observed in larvae parasitized for three days, and reduced total number of hemocytes, after three and seven days. An increase in plasmatocytes and decrease in spherulocytes numbers were observed in the differential count on the third day of parasitism. The number of melanized microspheres in parasitized larvae was low and indicated less intense melanization. The ultrastructural analysis confirmed the immunosuppressive effect of C. flavipes on the encapsulation response of D. flavipennella because only the formation of hemocytes capsules, adhered to the microspheres' surface, was evidenced in non-parasitized caterpillars. The effect of parasitism was also recorded on the third day with the presence of hemocytes and apoptosis in fat body cells, including aspects of degeneration in the latter. We concluded that C. flavipes suppresses cellular and humoral immunological responses in D. flavipennella and drastically affects the host's fat tissue.

Identification of parasite-responsive cysteine proteases inManduca sexta

Parasites have evolved different virulence strategies to manipulate host physiological functions. The parasitoid waspCotesia congregatainduces developmental arrest and immune suppression of its Lepidopteran hostManduca sexta. In this interaction, a symbiotic virus (C. congregataBracovirus, CcBV) associated with the wasp is essential for parasitism success. The virus is injected into the host with wasp eggs and virus genes are expressed in host tissues. Among potential CcBV virulence genes, cystatins, which are tight binding inhibitors of C1A cysteine proteases, are suspected to play an important role in the interaction owing to their high level of expression. So far, however, potentialin vivotargets inM. sextaare unknown. Here, we characterized for the first time fourM. sextaC1A cysteine proteases corresponding to cathepsin L and cathepsin B and two different ‘26–29 kDa’ cysteine proteases (MsCath1 and MsCath2). Our analyses revealed that MsCath1 and MsCath2 are transcriptionally downregulated in the course of parasitism. Moreover, viral Cystatin1 and MsCath1 co-localize in the plasma following parasitism, strongly suggesting that they interact. We also show that parasitism induces a general increase of cysteine protease activity which is later controlled. The potential involvement of cysteine proteases in defense against parasitoids is discussed.

Oogenesis of Diadegma semiclausum (Hymenoptera: Ichneumonidae) and its associated polydnavirus

We present a detailed ultrastructural analysis of ovary and calyx cell differentiation of Diadegma semiclausum. Numerous gametangia in various developmental stages were examined with electron microscopy to characterize the ultrastructure features of oogenesis, the most important of which is the development of nurse cells. In the germarium, the undifferentiated germ cells diversify, and one of the central cells of the cluster differentiates into an oocyte while the remaining become nurse cells. Germ cells continue developing in the vitellarium until mature and then enter into the calyx region. The calyx epithelium contains typical ichneumonid polydnaviruses with the following characteristics: (1) the virus particles assemble in the nuclei of calyx cells where they acquire an initial (inner) membrane, then migrate through the cytoplasm and budd out into the lumen of the ovary, at which time they acquire a second envelope (outer membrane); (2) the particle has a genome comprising several DNA segments. However, this new polydnavirus (Diadegma semiclausum polydnavirus) in the genus ichnovirus was not attached to the surface of the egg chorion.

Comparison of three methods of parasitoid polydnavirus genomic DNA isolation to facilitate polydnavirus genomic sequencing

A major long-term goal of polydnavirus (PDV) genome research is to identify novel virally encoded molecules that may serve as biopesticides to target insect pests that threaten agriculture and human health. As PDV viral replication in cell culture in vitro has not yet been achieved, several thousands of wasps must be dissected to yield enough viral DNA from the adult ovaries to carry out PDV genomic sequencing. This study compares three methods of PDV genomic DNA isolation for the PDV of Cotesia flavipes, which parasitizes the sugarcane borer, Diatraea saccharalis, preparatory to sequencing the C. flavipes bracovirus genome. Two of these protocols incorporate phenol-chloroform DNA extraction steps in the procedure and the third protocol uses a modified Qiagen DNA kit method to extract viral DNA. The latter method proved significantly less time-consuming and more cost-effective. Efforts are currently underway to bioengineer insect pathogenic viruses with PDV genes, so that their gene products will enhance baculovirus virulence for agricultural insect pests, either via suppression of the immune system of the host or by PDV-mediated induction of its developmental arrest. Sequencing a growing number of complete PDV genomes will enhance those efforts, which will be facilitated by the study reported here.

Differential expression of the CrV1 haemocyte inactivation-associated polydnavirus gene in the African maize stem borer Busseola fusca (Fuller) parasitized by two biotypes of the endoparasitoid Cotesia sesamiae (Cameron)

Polydnaviruses are rarely studied for their natural variation in immune suppressive abilities. The polydnavirus harboring braconid Cotesia sesamiae, a widespread endoparasitoid of Busseola fusca and Sesamia calamistis in sub-Saharan Africa exists as two biotypes. In Kenya, the western biotype completes development in B. fusca larvae. However, eggs of the coastal C. sesamiae are encapsulated in this host and ultimately, no parasitoids emerge from parasitized B. fusca larvae. Both biotypes develop successfully in S. calamistis larvae. Encapsulation activity by B. fusca larvae towards eggs of the avirulent C. sesamiae was detectable six hours post-parasitization. The differences in encapsulation of virulent and avirulent strains were associated with differences in nucleotide sequences and expression of a CrV1 polydnavirus (PDV) gene, which is associated with haemocyte inactivation in the Cotesia rubecula/Pieris rapae system. CrV1 expression was faint or absent in fat body and haemolymph samples from B. fusca parasitized by the avirulent C. sesamiae, which exhibited encapsulation of eggs. Expression was high in fat body and haemolymph samples from both B. fusca and S. calamistis larvae parasitized by the virulent C. sesamiae, encapsulation in the former peaking at the same time points as CrV1 expression in the latter. Non synonymous difference in CrV1 gene sequences between virulent and avirulent wasp suggests that variations in B. fusca parasitism by C. sesamiae may be due to qualitative differences in CrV1-haemocyte interactions.