Transcriptional Response of Coho Salmon (Oncorhynchus kisutch) to the Intracellular Pathogen Piscirickettsia salmonis, Reveal Several Pathways Related with Disease Resistance

The coho salmon has been intensively cultured for 3 decades in Chile, and still the bacterium Piscirickettsia salmonis is one of the most serious problems faced by this aquaculture species. However, little is known about host-pathogen interactions, from a molecular point of view. In this study we sequenced mRNA from susceptible and resistant fish after an extensive challenge test with P. salmonis; the fish were classified as susceptible or resistant on the basis of bacterial loads. The heritabilities were relatively high ranging from 0.3 to 0.35. Illumina-reads from 20 individuals were quality filtered and processed in order to produce a de novo, annotated, transcriptome. We found 6458 differentially expressed genes (DEG) (≥ 2 fold change, p-adj ≤ 0.01) using Deseq2. A remarkable relationship comparing up- and down-regulated genes were found in 134 terms, which appear to be related with biological process such as immune responses. Several immune-related genes were down-regulated, including those related to MHC class II, MHC I, and bactericidal permeability. Others, such as chemokines, were up.-regulated, possibly in response to bacterial injury in susceptible fish. At the sequence level some of the genes appeared to be under selection. The results obtained with this information will be crucial for evaluating the host-pathogen interaction and in this manner evaluate the effects of genomic selection on the disease response and resistance. Corfo 14ITE2-3665; FONDEF: ID14I10090.