Tilapia aquaculture has shown significant development in the last 5 years in Brazil, becoming the 5th largest producer of this species in the world. Intensive fish farming implies several management challenges in the health, reproduction and nutrition fields. To allow timely and properly decision-making biological indicators are required. In the health framework, there are several infectious diseases that increase the use of antibiotics and generate quality losses in fillets due to muscle injuries. A prophylactic measure is the administration of effective vaccines along with having diets that help strengthen the immune system and improve production yields. At the same time, to increase the efficacy of hormonal treatment for sexual reversion, more accurate, sensitive, validated, simple and rapid methods are required.
The aim of the present study was to show the application of predictive and quantitative transcriptomic biomarkers for the evaluation of vaccines, hormonal treatment for sex reversal, and diets in Nile Tilapia (Orechromus niloticus). In the first case a trial under controlled conditions was carried out injecting fish intraperitoneally with a commercial vaccine against Streptococcus agalactiae. Gene expression was performed by RT-qPCR in head-kidney for il1b, il8, infg, cd3e, cd4 and cd8. Our results showed a strong activation of the innate immune response during the first 24 hours after injection followed by an activation of the cell-mediated immune response 5 days after injection. Additionally, a food restriction challenge was carried out under controlled conditions and its impact on growth indicators and expression of oxidative stress related genes in the liver was evaluated. Significant overexpression of genes related to oxidative stress (gpx and sod1) was observed from day 7 onwards in fish subjected to a dietary restriction higher than 40%. Besides, a lower expression of growth-related genes (igf1) was observed. Finally, we identified 3 genes with significant differential expression between male and female Nile tilapia, which were validated to assess sex reversion in fish as small as 2 g.
We can conclude that the application of transcriptomic tools is useful to provide a comprehensive diagnosis of the immunological and physiological status in cultured tilapia promoting timely and strategic decision making.