الفهرس 
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Abstract
This study conducted in Alexandria branch, Baltim station of National Institute of Oceanography and Fisheries and at Genetics laboratory of Animal Husbandry and Animal Wealth Development Department, Faculty of Veterinary Medicine, Alexandria University through the period April 2016 - September 2017.
The aim of this work was to study the effect of different salinity concentrations (6 ppt, 16 ppt, and 20 ppt) on expression of selected genes in Nile tilapia fish including
Stress related was
a. genes in gills and liver as GST, HSP27, and HSP70 genes.
b. Inflammatory related genes in gills and liver as IL1, IL8, CC and C-X-C
chemokines genes.
c. Immune-related genes as IgMH in gills and kidney, TLR7 in liver, MHC and MX genes in kidney.
Nile tilapia (Oreochromis niloticus) were obtained from Lake Brullus Kafr Elsheikh governate (water salinity is 6 ppt) where the fish were acclimated for one week. The fish were divided into 3 groups; G1: was maintained at 6 ppt salinity (control group). G2: was maintained at 16 ppt salinity concentration. G3: was maintained at 20 ppt salinity concentration.
The experiment duration was about two months after acclimation week, which include the period of increasing salinity concentration for 20 and 28 days in G2 and G3 )half ppt per day). The samples were collected from all groups about one month from the end of salinity adaptation of G2 and G3. The tissue samples (Gills, liver and kidney) were collected from three fish in each group, then RNA was isolated and cDNA was synthesized. Quantitative real time PCR were done to detect the expression level of target genes in different salinity concentration. The result showed that:
1. Stress related genes:
- The stress related genes (GST and HSP27) expression in gills was the highest at 16 ppt salinity concentration compared to the control group (6 ppt) and to 20 ppt which has only slight up regulatory effect.
- HSP27 expression in gills was higher than in liver.
- Mild up regulation of HSP70 in gills by 16 ppt salinity concentration compared to the control group (6 ppt). Meanwhile, its expression was up regulate to 73.5 fold in liver by 16 ppt, while 20 ppt salinity concentration induced only slight down regulation of the gene compared to the control group (6 ppt).
2. Inflammatory related genes:
- The expression of inflammatory related genes IL1, IL8, and CC-chemokine in gills was lower than their levels in liver.
- The effect of 16 ppt salinity concentration on liver inflammatory genes expression was higher than 20 ppt salinity concentration and the control group (6 ppt).
- The inflammatory related gene C-X-C chemokine increased by 20 ppt more than 16 ppt salinity concentration and the control group (6 ppt).
3. Immune-related genes:
- The IgMH expression in gills was significantly down regulated by 16 ppt and 20 ppt salinity concentration compared to the control group (6 ppt). Meanwhile, its expression was increased in kidney to 17 fold by 16 ppt salinity concentration and 5 fold by 20 ppt salinity concentration compared to the control group (6 ppt).
- The expression of TLR7 in liver was higher at 16 ppt salinity concentration than 20 ppt salinity concentrations and the control group (6 ppt).
- The expression of other immune-related genes in kidney as MHC and MX was higher with 20 ppt than 16 ppt salinity concentration and the control group (6 ppt).
- from the obtained results, it was concluded that 16 ppt salinity concentration is better than 20 ppt. As the inflammatory and stress response in liver and gills are higher at 20 ppt, supporting by observation that salinity stress lead to increase of the metabolic rate in the whole body of the stressed fish especially in organs involved in the osmoregulatory mechanism such as gills, liver, and kidney.
- Higher expression of immune-related genes in kidney with higher salinity concentration (20 ppt), may indicate that the fish grown in higher salinity concentration are predisposed to any infectious agent present in water.
- If we would consider the brackish water dependent aquaculture as a solution for fresh water shortage, we must include an immune stimulant food additive to the regime in order to overcome the immune suppressive effect of hyper-saline stress upon the fish.