Diversity and function of arthropod endosymbiont toxins

Abstract

Bacterial endosymbionts induce dramatic phenotypes in their arthropod hosts, including cytoplasmic incompatibility, feminization, parthenogenesis, male killing, parasitoid defense, and pathogen blocking. The molecular mechanisms underlying these effects remain largely unknown but recent evidence suggests that protein toxins secreted by the endosymbionts play a role. Here, we describe the diversity and function of endosymbiont proteins with homology to known bacterial toxins. We focus on maternally transmitted endosymbionts belonging to the Wolbachia, Rickettsia, Arsenophonus, Hamiltonella, Spiroplasma, and Cardinium genera because of their ability to induce the above phenotypes. We identify at least 16 distinct toxin families with diverse enzymatic activities, including AMPylases, nucleases, proteases, and glycosyltransferases. Notably, several annotated toxins contain domains with homology to eukaryotic proteins, suggesting that arthropod endosymbionts mimic host biochemistry to manipulate host physiology, similar to bacterial pathogens.

Figure 1.. Diversity of arthropod endosymbiont infections and their consequences.

Maternally transmitted endosymbionts belonging to the Wolbachia, Rickettsia, Arsenophonus, Hamiltonella, Spiroplasma, and Cardinium genera (see Glossary) infect a diverse set of arthropod orders. The cladogram depicting relatedness among endosymbionts is based on alignment of 16S rRNA sequences described in [97]. Images of arthropods are shown for each endosymbiont where the presence of species belonging to these endosymbiont genera was confirmed by PCR [98]. Phenotypes in blue, purple, red, orange, green, and black summarize reproductive or resistance traits induced by Wolbachia, Rickettsia, Arsenophonus, Hamiltonella, Spiroplasma, and Cardinium species, respectively [83, 87, 96, 99, 100]. However, there is considerable phenotypic variation among different endosymbiont-host associations within a particular group. For example, not all Wolbachia species or strains induce cytoplasmic incompatibility in each arthropod order depicted, and different Wolbachia strains infecting the same host can produce different phenotypes.

Figure 2.. Diversity of protein toxin families identified in each endosymbiont genus.

To identify protein toxins in each endosymbiont genera, we used protein homolog queries (BLASTp; E < 1e-4) from known toxins in pathogenic bacteria as well as known homologs from toxins that cause specific toxic phenotypes in certain arthropod-endosymbiont interactions. HHPred was then used to identify domain architectures. Accession IDs for each protein toxin are referenced in Table 1. Black-filled circles denote the presence of the toxin in at least one endosymbiont species for each genus. Red bacteriophage icons denote toxins that are located within bacteriophage regions in the endosymbiont genome (Table 1). Open circles denote the absence of the toxin according to multiple failed BLASTp searches using protein queries of toxin homologs. Protein domain arrangements are shown below certain toxins to illustrate multidomain architectures.

Figure 3.. Models illustrating hypothesized modes of action for toxins capable of inducing arthropod phenotypes.

For each panel, broken arrows denote direct or indirect interactions, where current evidence is insufficient to distinguish. Solid arrows denote direct interactions. (A) Spaid (Spiroplasma poulsonii androcidin) over-expression in S. poulsonii-free Drosophila melanogaster hosts induces potent male killing [4]. Previous work found genes encoding proteins in the male-specific lethal (MSL) dosage compensation complex in D. melanogaster are required, in part, for S. poulsonii-induced male killing [101]. Spaid co-localizes with MSL1 in D. melanogaster cells, and ectopic expression of MSL2 with Spaid in females is sufficient to induce cell death [4]. Spaid thus appears to cause male killing by interfering with dosage compensation on the male X chromosome, leading to apparent DNA damage, chromatin bridge formation, and mis segregation during cell division—eventually causing death. (B) In D. melanogaster hosts, co-expression of (cytoplasmic incompatibility factor) cifA and cifB genes in males induces cytoplasmic incompatibility (CI) in embryos from matings with Wolbachia-free females [3, 5, 46, 47], Females infected with Wolbachia or over-expressing cifA can rescue embryos from CI, and biochemical assays show CifA binds directly with CifB. CifA and CifB co-expression in male sperm appears to recapitulate Wolbachia-induced CI effects, causing chromatin bridge formation, missegregation, and lethality in embryos from matings with CifA-free mothers. (C) D. melanogaster and D. neotestacea infected with Spiroplasma that express ribosomal inactivating toxins (RIPs) are resistant to wasp parasitoid attacks. This appears to be caused in part by RIPmediated cleavage of an adenosine residue in the sarcin-ricin loop (depurination) of wasp 28S rRNA [61]. Cleavage at this residue likely inhibits protein translation (causing death) in wasp larvae infecting the fly host.

Figure I.

Phenotypes induced by arthropod endosymbionts range from reproductive manipulations (A-D) to defense against parasites and pathogens (E,F).