Infection by the lungworm Rhabdias pseudosphaerocephala affects the expression of immune-related microRNAs by its co-evolved host, the cane toad Rhinella marina.

Parasites may suppress the immune function of an infected host using microRNAs (miRNAs) to prevent protein production. Nonetheless, little is known about the diversity of miRNAs and their mode(s) of action. In this study, we investigated the effects of infection by a parasitic lungworm (Rhabdias pseudosphaerocephala) on miRNA and mRNA expression of its host, the invasive cane toad (Rhinella marina). We compared miRNA and mRNA expression in naive toads that had never been infected by lungworms to toads that were infected with lungworms for the first time in their lives, and to toads that were infected the second time in their lives (i.e., had two consecutive infections). In total, we identified 434 known miRNAs and 106 potential novel miRNAs. Compared to uninfected toads, infected animals upregulated five (single-infection treatment) or four (multiple-infection treatment) miRNAs. Seven of these differentially expressed miRNAs were associated with gene pathways related to the immune response, potentially reflecting immunosuppression of cane toads by their parasites. Infected hosts did not respond with substantial mRNA transcription, with only one differentially expressed gene between control and single-infection hosts. Our study suggests that miRNA-mediated interactions may play a role in mediating the interaction between the parasite and its host. Our findings clarify the role of miRNAs in host-parasite interactions, in a system in which an ongoing range expansion by the host has generated substantial divergence in host-parasite interactions.