Host Transcriptomics Reveal Reduction in Defence-Reproduction Trade-Offs During Coinfection.

During infection, hosts may shift resources away from reproduction towards immune defence. It is unclear to what degree these costly trade-offs can be alleviated during protective coinfections, whereby antagonism between parasites reduces disease severity. We used transcriptomics to assess the extent to which host gene expression reflected the effect of protection and whether reducing or increasing investment in immunity carried costs to reproduction. Virulent infections by Leucobacter musarum bacteria elicited greater trade-offs in nematode hosts compared to the naturally coinfecting 'protective parasite' Leucobacter celer. We further found that coinfection attenuated host investment in pro-immune trade-offs, without significantly changing which host genes were involved. We then sought to understand if this attenuated host response would be consistent with possible mechanisms of inter-parasite competition. Our chromosome length genome assemblies for both parasite species revealed that protective coinfection may operate by competition for public goods, such as siderophore-mediated uptake of metal ions (e.g., iron) or colonisation of the host cuticle. Ultimately, we show that competition between coinfecting parasites can complement endogenous host defences and ease the reproductive costs of fighting harmful infection.