Abstract

The monitoring of sea farming centers at the south of Chile was performed to obtain data on the expression of 39 immune-related genes related of Salmo salar in three fish organs: gill, spleen and head kidney. The data was obtained from farms with and without occurrences of outbreaks of Pis- cirickettsia salmonis, and a generalized mixed linear model (GMLM) was established, considering the environmental variables and fish necropsy. This model allowed (i) the establishment of a baseline of expression of immune response genes, and (ii) molecular gene markers of susceptibility to the pathogen, which is, if fish are P. salmonis positive. For the identifi- cation of the baseline expression of immune genes, the condition of normal (healthy) fish was defined and differences in the expression of these genes were established in the gill, spleen and head kidney. In addition, genes exhibiting temporal variation in their expression were identified and therefore, an annual historical reference of this value was considered to the baseline determination. Regarding the genes proposed as markers of P. salmonis infection, it was determined that the expression of the genes coding for TNF-a, cathelicidin, NLRX1 and IL-1b, in gills; cathelicidin and hepcidin in anterior kidney; and hepcidin and IL-10 in spleen are indicators of infected fish, and thus, susceptible to P. salmonis. This result also highlights the data obtained at the gill level, an easy- sampling organ in the field with validated molecular indicators. While these studies were based on the genes expression levels, we also obtained result on the availability at the protein level of several of these molecules, using specific antibodies obtained in this project. The GMLM will also allow to propose molecules whose increase in expression over time can be predictive indicators of P. salmonis infection. One of the molecular markers with the best application perspective and whose availability was also evaluated at the protein level is cathelicidin expressed in gills. The use of the proposed molecular tools in sea farming centers to evaluate the expression of gene markers will be useful for the identification of critical windows for therapeutic treatment. Consequently, if the expression of gene markers is detected between reference values, it will indicate infection by P. salmonis with an associated probability, and industry could applied productive strategies such as the use of medicated diets.