Abstract
Inherited symbionts are ubiquitous in insects and can have important consequences for the fitness of their hosts. Many inherited symbionts defend their hosts against parasites or other natural enemies; however, the means by which most symbionts confer protection is virtually unknown. We examine the mechanisms of defence in a recently discovered case of symbiont‐mediated protection, where the bacterial symbiont S piroplasma defends the fly D rosophila neotestacea from a virulent nematode parasite, H owardula aoronymphium . Using quantitative PCR of S piroplasma infection intensities and whole transcriptome sequencing, we attempt to distinguish between the following modes of defence: symbiont–parasite competition, host immune priming and the production of toxic factors by S piroplasma . Our findings do not support a model of exploitative competition between H owardula and S piroplasma to mediate defence, nor do we find strong support for host immune priming during S piroplasma infection. Interestingly, we recovered sequence for putative toxins encoded by S piroplasma , including a novel putative ribosome‐inactivating protein, transcripts of which are up‐regulated in response to nematode exposure. Protection via the production of toxins may be a widely used and important mechanism in heritable defensive symbioses in insects.