Generation of transcriptomic resources in Oolichan to assess marine diesel gene expression responses

Transcriptomic tools that quantify gene activity and expression offer a powerful approach to assess fish health response to environmental contaminants. Oolichan (Thaleichthys pacificus), a culturally and ecologically important and threatened anadromous fish, migrates through large river and estuarine waterways and may be exposed to oil products released from coastal industrial development and marine shipping activities. As part of the Conservation and Recovery Research on Oolichan in Haisla Territory (CAROOHT) program, we developed transcriptomic resources for Oolichan to assess gene expression response to low sulfur marine diesel (hereafter: marine diesel) exposures. We constructed a de novo transcriptome for Oolichan using RNA-Seq data from larvae exposed to a range of 100-1000 mg/L marine diesel water-accommodated fractions (WAFs), as well as from wild adult liver and caudal fin tissues. We identified differentially expressed gene transcripts in all marine diesel treated groups and developed gene expression assays targeting robust bioindicators associated with polycyclic aromatic hydrocarbon (PAH) metabolism. The gene ontology profile demonstrated classical cellular response to general and oil-induced stress. Notable enrichment related to hormone and fatty acid metabolism, morphogenesis, apoptotic signaling, and response to toxins and reactive oxygen species. In quantitative real-time polymerase chain reaction (qPCR)-based assessments, transcript abundance of oil response-associated genes cytochrome P450-1a (cyp1a) and aryl-hydrocarbon receptor repressor (ahrra) were significantly upregulated in marine diesel WAF treatments. Our findings demonstrate the utility of transcriptomics for molecular-level monitoring of contaminant exposure and health and produce novel resources for further gene expression response-based studies for threatened Oolichan.