Fig. 1

Sex-biased gene expression in the proPO signalling cascade. In a, schematic representation of key proteins in the proPO activating cascade, based on previous studies of insects and other invertebrates (reviewed in [54, 55, 67, 73, 78]). In b, sex-bias and effects of mating status on gene expression in the abdomen for C. maculatus orthologs from published data [79] mapped to the sequences of the functionally annotated proteins. Full results in Additional file 1: Table S1. Spätzle processing enzyme (SPE: pink) initiates cleavage of proPO (yellow) into active PO, which ultimately leads to wound healing as well as encapsulation and killing of foreign pathogens. However, SPE also regulates the production of Spätzle protein (SPZ) from proSPZ (blue), which ultimately leads to increased production of antimicrobial peptides (AMPs) via the TOLL pathway, which offers inducible immunity against pathogens, thus setting the stage for an allocation trade-off between PO activity and AMP production. Overactivation of the proPO cascade has toxic side-effects via the production of secondary metabolites, suggesting that overproduction of SPE may come at a cost to overall health. Here, production of serine protease inhibitors (serpins: grey) in the TOLL pathway exerts negative feedback and control over the cascade. b C. maculatus females show higher expression of SPE (pink) and proPO (yellow) as virgins. Males show higher expression of proSPZ (blue) and serpins (grey). Shown are individual samples and means ± 1 SD. These patterns in gene expression suggest a mechanistic basis for sex-specific immunity trade-offs between different components in the proPO activating cascade, where females are predicted to invest more in PO activity (wound healing and potentially encapsulation of pathogens transferred at mating) in their reproductive tract in response to mating, at the potential cost of reduced inducible immunity via AMP production and/or toxic side-effects of overactivation of the proPO cascade