Survival and transcriptomic responses to different Perkinsus marinus exposure methods in an Eastern oyster family

The parasite Perkinsus marinus, which causes Dermo disease, affects Eastern oyster (Crassostrea virginica) survival and is responsible for substantial losses to aquaculture production. Controlled laboratory challenges enable definition and measurement of disease response phenotypes as well as characterization of gene expression patterns during the infection process. The method of parasite exposure and severity of infection likely affect response measures and exploring variation among them can enhance our understanding of this host-parasite interaction. Here we compared response traits and global expression patterns over time for a selectively bred Eastern oyster family exposed to P. marinus via either suspension feeding or direct injection to elucidate mechanisms determining survival. All exposed oysters (n = 56 per group) received a dose of 5 x 106 cultured P. marinus cells g-1 wet tissue weight but oysters in the injected group had higher initial parasite load and significantly reduced survival probability compared to the fed group. Using RNAseq, we identified differentially expressed transcripts between fed and control animals 6 h after exposure and between fed and injected groups at 36 h, 7 d, and 28 d post exposure. Immediately after infection, caspase-8 and transcripts associated with mediating cell damage from reactive oxygen species were upregulated in the fed group compared to unchallenged controls. The comparison between fed and injected groups early in the infection process revealed increased innate immune activity and overexpression of transcripts involved in proinflammatory cytokine pathways, including toll-like receptor 4 and TNF receptor-associated factor 6-like protein, in the injected group. Later in the infection process, the fed group expressed transcripts associated with growth, specifically microtubule-based processes, at a higher level than the injected group. This study demonstrates that dosing method influences initial parasite load, survival, and transcriptomic response to Dermo disease and highlights the importance of considering exposure method when interpreting interactions between P. marinus and the Eastern oyster.