Systematic analysis of a wasp parasitism arsenal

Abstract

Parasitoid wasps are among the most diverse insects on earth with many species causing major mortality in host populations. Parasitoids introduce a variety of factors into hosts to promote parasitism, including symbiotic viruses, venom, teratocytes and wasp larvae. Polydnavirus-carrying wasps use viruses to globally suppress host immunity and prevent rejection of developing parasites. Although prior results provide detailed insights into the genes viruses deliver to hosts, little is known about other products. RNAseq and proteomics were used to characterize the proteins secreted by venom glands, teratocytes and larvae from Microplitis demolitor, which carries M. demolitor bracovirus (MdBV). These data revealed that venom glands and teratocytes secrete large amounts of a small number of products relative to ovaries and larvae. Venom and teratocyte products exhibited almost no overlap with one another or MdBV genes, which suggested that M. demolitor effector molecules are functionally partitioned according to their source. This finding was well illustrated in the case of MdBV and teratocytes. Many viral proteins have immunosuppressive functions that include disruption of antimicrobial peptide production, yet this study showed that teratocytes express high levels of the antimicrobial peptide hymenoptaecin, which likely compensates for MdBV-mediated immunosuppression. A second key finding was the prevalence of duplications among genes encoding venom and teratocyte molecules. Several of these gene families share similarities with proteins from other species, while also showing specificity of expression in venom glands or teratocytes. Overall, these results provide the first comprehensive analysis of the proteins a polydnavirus-carrying wasp introduces into its host.

Keywords: genomics/proteomics; host; insects; microbial biology; molecular evolution; parasite interactions; transcriptomics.

Figure 1

Distribution and statistics describing locus abundance in RPKM values for teratocytes, venom glands, larvae and ovaries of wasps. Statistics used describe the distribution of RPKM values for all 3894 loci. These include the mean, one standard deviation, the median and the maximum RPKM value obtained for each locus. Note that venom and teratocyte RPKM values are depicted on a different scale from ovaries and larvae.

Figure 2

Pairwise comparisons of gene expression in venom glands (A), teratocytes (B) or larvae (C) relative to ovaries. Loci classified as differentially expressed in venom glands (red), teratocytes (blue) or larvae (green) are indicated in the upper left of each graph while loci classified as similarly expressed in both the query and ovaries are indicated in the upper right (light grey). All other loci are indicated as dark grey. Overlap between (D) loci differentially expressed between venom glands, teratocytes, and larvae, (E) loci similarly expressed between venom glands, teratocytes and larvae, and (F) differentially expressed loci whose corresponding proteins were detected in venom gland and teratocyte secretions. Colouration in D-E is the same as in A-C. Two loci from venom glands, a plancitoxin (a deoxyribonuclease) and an uncharacterized member of the conotoxin superfamily, met the criteria (see results) for both the differential and similar expression categories (Shiomi et al. 2004).

Figure 3

Phylogenetic relationships of multi-gene families expressed in parasitism-specific tissues. Relatedness of (A) Ci-48a-like proteins and (B) reprolysin-like metalloprotease domain-containing proteins and their expression in M. demolitor tissues. Groups of teratocyte-specialized loci are indicated in grey shaded boxes, and venom-specialized groups are in white. Venom or teratocyte data sets are indicated in bold type, asterisks indicate detection in both. The plus sign indicates a gene product detected in both venom and teratocytes but does not belong to the venom set due to not meeting expression requirements. Support values for 100 bootstrap replicates are shown at nodes in the tree for clades supported by >80 replicates. Expression values (RPKM) for each locus in ovaries, larvae, teratocytes or venom glands are shown using a heat-map. Other species in the phylogeny are hymenopteran species Am: Apis mellifera, Ci: Chelonus inanitus, Ep: Eulophus pennicornis, Md: Microctonus hyperodae, Nv: Nasonia vitripennis, and Ph: Pimpla hypochondriaca, and dipteran species Dm: Drosophila melanogaster, Gm: Glossina moristans, and Sn: Simulium nigrimanum. Not enough sequence was available for H. didymator Hd-Ven1 and Hd-Ven2 metalloproteases or Microctonus aethipoides Ci-48a to be included. Microctonus and Venturia canescens peptidases also belong to a different family of proteases to those featured here (Crawford et al. 2003; Asgari et al. 2002).