Olive flounder (Paralichthys olivaceus) is one of the most economically valuable fish species in Korea. However, the olive flounder aquaculture industry is suffering massive losses due to the outbreaks of various diseases. In particular, the scuticociliatosis caused by Miamiensis avidus infection is responsible for more than 50% of the mortality of juvenile flounders. To develop effective scuticociliatosis treatment strategy and to understand the molecular mechanisms underlying the host defense response after parasitic infection, we performed this study to clarify the genes and pathways that are involved in the protective response of olive flounder after Miamiensis avidus infection via different routes.
Healthy juvenile flounders with average body weight of 30 g were experimentally infected by Miamiensis avidus via either intraperitoneal injection of 4 x105 cell/fish or immersion of 1 x 104 cell/ml for 6 hours. The skin, gill and head kidney tissue samples were collected at 0 h, 9h, 48h and 96h after infection. The total RNA of each samples was extracted, concentration/integrity determined, prepared for cDNA library and then paired-end sequenced using an Illumina HiSeq 2500 system. The raw sequencing data was processed by Trinity assembly and unigene annotation according to the Gene Ontology (GO) and BLAST analysis. Finally, over 5000 differentially expressed genes (DEGs) were determined by normalized ratio of reads between the PBS control and Miamiensis avidus infected fish in a tissue specific manner. Among the DEGs, a number of genes were clustered into immune related pathways, including immunoglobulin production pathway, cytokine/chemokine mediated signaling pathway, regulation of hematopoietic progenitor cell differentiation, complement and coagulation cascades pathway. In conclusion, this study uncovered the key immune genes and pathways closely related to scuticociliatosis and provide insights into the development of effective vaccine and immunostimulants to prevent the parasitic diseases in flounder aquaculture.