Transcriptome profiling of intra-snail stages of the liver fluke Fasciola hepatica reveals key mediators underlying parasite development and interaction with the host

While mammalian host invasion by the liver fluke Fasciola hepatica has been extensively studied, reports on the interaction with its intermediate host at the molecular level remain scarce. These developmental stages are particularly interesting since at least two cycles of asexual amplification occur within lymnaeid snails, leading to the production of hundreds of infective metacercariae. We analyzed transcriptomic data from miracidia and intra-molluscan stages seeking clues of these interaction and developmental processes. We identified 1744 novel transcripts and several isoforms of already annotated genes. Analysis of expression across the whole life cycle resulted in five distinct gene expression clusters (egg, miracidia, intra-snail stages, invading stages and juvenile-adults). Few genes showed strict stage-specific expression, but notably most of those corresponding to miracidial and intra-snail stages were novel unannotated genes. Genes upregulated in the miracidium include enzymes involved in neurotransmitter synthesis, energy metabolism and calcium mediated signaling, consistent with the physiology of a short-lived free-living stage. Several genes associated with development and morphogenesis were characterized in early (15 days post-infection) intra-snail stages. Purine salvage pathway genes were upregulated in this time-point, consistent with a high biosynthetic demand and the absence of a complete purine synthesis pathway in F. hepatica. Mucins, glycan biosynthesis genes and aquaporins upregulated within late (30 dpi) intra-snail stages are interesting, considering their putative role in the next host transition. Different members of well-known protein families involved in host-parasite interaction such as cathepsin proteases, legumains, protease inhibitors and lipid transporters were detected with stage specific expression in early and late intra-snail stages. These findings suggest duplication and tunning during evolution of the same set of molecular mediators for the interactions with the intermediate and definitive hosts.