Transcriptomic response during wasp parasitism in the Drosophila-Spiroplasma interaction

Several facultative bacterial symbionts of insects protect their hosts against natural enemies. Spiroplasma poulsonii strain sMel, 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 has not been deeply explored. 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 hookeri. In the absence of Spiroplasma infection, we found evidence of Drosophila immune activation by G. hookeri, 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 was influenced by the Spiroplasma-L. heterotoma interaction, and they do not appear to be related to immune response. For most of them, parasitism by L. heterotoma (in the absence of Spiroplasma) caused an expression change that was at least partly reversed when Spiroplasma was also present. It is unclear whether such genes are involved in the Spiroplasma-mediated mechanism that leads to wasp death 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. In addition, we used the RNAseq data and quantitative (q)PCR to evaluate the transcript levels of the Spiroplasma-encoded RIP genes. 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. We conclude that immune priming is unlikely to contribute to the Spiroplasma-mediated protection against wasps, and that the mechanism by which G. hookeri resists/tolerates Spiroplasma does not involve inhibition of RIP transcription.