Symbiont-mediated fly survival is independent of defensive symbiont genotype in the Drosophila melanogaster-Spiroplasma-wasp interaction

Preliminary Characterization of Phage-Like Particles from the Male-Killing Mollicute Spiroplasma poulsonii (an Endosymbiont of Drosophila)

Bacteriophages are vastly abundant, diverse, and influential, but with few exceptions (e.g. the Proteobacteria genera Wolbachia and Hamiltonella), the role of phages in heritable bacteria-arthropod interactions, which are ubiquitous and diverse, remains largely unexplored. Despite prior studies documenting phage-like particles in the mollicute Spiroplasma associated with Drosophila flies, genomic sequences of such phage are lacking, and their effects on the Spiroplasma-Drosophila interaction have not been comprehensively characterized. We used a density step gradient to isolate phage-like particles from the male-killing bacterium Spiroplasma poulsonii (strains NSRO and MSRO-Br) harbored by Drosophila melanogaster. Isolated particles were subjected to DNA sequencing, assembly, and annotation. Several lines of evidence suggest that we recovered phage-like particles of similar features (shape, size, DNA content) to those previously reported in Drosophila-associated Spiroplasma strains. We recovered three ~ 19 kb phage-like contigs (two in NSRO and one in MSRO-Br) containing 21-24 open reading frames, a read-alignment pattern consistent with circular permutation, and terminal redundancy (at least in NSRO). Although our results do not allow us to distinguish whether these phage-like contigs represent infective phage-like particles capable of transmitting their DNA to new hosts, their encoding of several typical phage genes suggests that they are at least remnants of functional phage. We also recovered two smaller non-phage-like contigs encoding a known Spiroplasma toxin (Ribosome Inactivating Protein; RIP), and an insertion element, suggesting that they are packaged into particles. Substantial homology of our particle-derived contigs was found in the genome assemblies of members of the Spiroplasma poulsonii clade.

Defensive Symbionts and the Evolution of Parasitoid Host Specialization

Insect host-parasitoid interactions abound in nature and are characterized by a high degree of host specialization. In addition to their behavioral and immune defenses, many host species rely on heritable bacterial endosymbionts for defense against parasitoids. Studies on aphids and flies show that resistance conferred by symbionts can be very strong and highly specific, possibly as a result of variation in symbiont-produced toxins. I argue that defensive symbionts are therefore an important source of diversifying selection, promoting the evolution of host specialization by parasitoids. This is likely to affect the structure of host-parasitoid food webs. I consider potential changes in terms of food web complexity, although the nature of these effects will also be influenced by whether maternally transmitted symbionts have some capacity for lateral transfer. This is discussed in the light of available evidence for horizontal transmission routes. Finally, I propose that defensive mutualisms other than microbial endosymbionts may also exert diversifying selection on insect parasitoids.

Endosymbiotic male-killing Spiroplasma affect the physiological and behavioural ecology of Macrocheles-Drosophila interactions

While many arthropod endosymbionts are vertically transmitted, phylogenetic studies reveal repeated introductions of hemolymph-dwelling Spiroplasma into Drosophila. Introductions are often attributed to horizontal transmission via ectoparasite vectors. Here, we test if mites prefer to infect Spiroplasma poulsonii MSRO infected flies, and if MSRO infection impairs fly resistance against secondary mite (Macrocheles subbadius) attack. First we tested if mites prefer MSRO+ or MSRO- flies using pair-wise-choice tests across fly ages. We then tested whether mite preferences are explained by changes in fly physiology, specifically increased metabolic rate (measured as CO₂ production). We hypothesize that this preference is due in part to MSRO+ flies expressing higher metabolic rates. However, our results showed mite preference depended on an interaction between fly age and MSRO status: mites avoided 14-days old MSRO+ flies relative to MSRO- flies (31% infection), but prefered MSRO+ flies (64% infection) among 26-day old flies. Using flow-through respirometry, we found 14 day-old MSRO+ flies had higher CO₂ emissions than MSRO- flies (32% greater), whereas at 26 days old the CO₂ production among MSRO+ flies was 20% lower than MSRO- flies. Thus, mite preferences for high metabolic rate hosts did not explain the infection biases in this study. To assess changes in susceptibility to infection, we measured fly endurance using geotaxis assays. Older flies had lower endurance consistent with fly senescence, and this effect was magnified among MSRO+ flies. Given the biological importance of male-killing Spiroplasma, potential changes in the interactions of hosts and potential vectors could impact the ecology and evolution of host species. Importance Male-killing endosymbionts are transmitted mother to daughter and kill male offspring. Despite these major ecological effects, how these endosymbionts colonize new host species is not always clear. Mites are sometimes hypothesized to transfer these bacteria between hosts/host species. Here we test if 1) if mites prefer to infect flies that harbour Spiroplasma poulisoni MSRO and 2) if flies infected with MSRO are less able to resist mite infection. Our results show that flies infected with MSRO have weaker anti-mite resistance but the mite preference/aversion for MSRO+ flies varied with fly age. Given the fitness and population impacts of male-killing Spiroplasma, changes in fly-mite interactions have implications for the ecology and evolution of these symbioses.

Differential gene expression in a tripartite interaction: Drosophila, Spiroplasma and parasitic wasps

BACKGROUND

Several facultative bacterial symbionts of insects protect their hosts against natural enemies. Spiroplasma poulsonii strain sMel (hereafter Spiroplasma), a male-killing heritable symbiont of Drosophila melanogaster, confers protection against some species of parasitic wasps. Several lines of evidence suggest that Spiroplasma-encoded ribosome inactivating proteins (RIPs) are involved in the protection mechanism, but the potential contribution of the fly-encoded functions (e.g., immune response), has not been deeply explored.

METHODS

Here we used RNA-seq to evaluate the response of D. melanogaster to infection by Spiroplasma and parasitism by the Spiroplasma-susceptible wasp Leptopilina heterotoma, and the Spiroplasma-resistant wasp Ganaspis sp. In addition, we used quantitative (q)PCR to evaluate the transcript levels of the Spiroplasma-encoded Ribosomal inactivation protein (RIP) genes.

RESULTS

In the absence of Spiroplasma infection, we found evidence of Drosophila immune activation by Ganaspis sp., but not by L. heterotoma, which in turn negatively influenced functions associated with male gonad development. As expected for a symbiont that kills males, we detected extensive downregulation in the Spiroplasma-infected treatments of genes known to have male-biased expression. We detected very few genes whose expression patterns appeared to be influenced by the Spiroplasma-L. heterotoma interaction, and these genes are not known to be associated with immune response. For most of these genes, parasitism by L. heterotoma (in the absence of Spiroplasma) caused an expression change that was at least partly reversed when both L. heterotoma and Spiroplasma were present. It is unclear whether such genes are involved in the Spiroplasma-mediated mechanism that leads to wasp death and/or fly rescue. Nonetheless, the expression pattern of some of these genes, which reportedly undergo expression shifts during the larva-to-pupa transition, is suggestive of an influence of Spiroplasma on the development time of L. heterotoma-parasitized flies. One of the five RIP genes (RIP2) was consistently highly expressed independently of wasp parasitism, in two substrains of sMel. Finally, the RNAseq data revealed evidence consistent with RIP-induced damage in the ribosomal (r)RNA of the Spiroplasma-susceptible, but not the Spiroplasma-resistant, wasp. Acknowledging the caveat that we lacked adequate power to detect the majority of DE genes with fold-changes lower than 3, we conclude that immune priming is unlikely to contribute to the Spiroplasma-mediated protection against wasps, and that the mechanism by which Ganaspis sp. resists/tolerates Spiroplasma does not involve inhibition of RIP transcription.