Aeromoniasis is a devastating bacterial disease that affects aquaculture of this and many other carp species throughout the world. Breeding programmes can make use of these new markers to select rohu carp broodstock with genetic variants that make them more resistant to disease. This would develop a breed of fish that survives and thrives even in the presence of the disease. The small rural communities in India that farm this fish would benefit as production and profitability per pond would be substantially improved.
The research has been funded by the Norwegian Research Council and the Department of Biotechnology of India, and carried out by Nofima (the Norwegian Institute of Food, Fisheries and Aquaculture research) and CIFA (Central Institute of Freshwater Aquaculture, India).
Rohu is farmed in India, Bangladesh, Burma and Thailand and over 1.2 million tonnes of rohu is farmed in India per year.
Aeromoniasis disease, caused by the bacteria Aeromonas hydrophila, causes haemorrhaging and ulceration when fish are stressed. It is widespread and difficult to control and treat. Previous work by Nofima and CIFA showed that some fish inherit genes that give them higher resistance to the disease.
The scientists knew that the immune system of fish is complex and that the reaction to bacterial infections is likely to be controlled by many different genes, most with small effects on the animal’s ability to resist the outbreak of disease.
In the search for the markers, the genetic code (DNA sequence) of the genes present in different individuals was analysed. The inheritance of genetic code variants in over 3,000 genes was compared in large families that had been challenged with exposure to the disease. The study looked for associations between the number of hours individual rohu survived after infection with the disease and the genetic code of the genes that were inherited. In this way, several gene markers associated with disease resistance were identified. Some of the markers either occurred in, or mapped closely to, genes with known roles of immune function in fish.
High throughput sequencing and genotyping technologies were used to read the genetic code of the 3000 genes which were mapped to positions on the 25 chromosome pairs present in rohu.
One of the genes identified by the scientists in Norway was the pore forming protein, perforin, which is produced by T-cells and natural killer-cells, and thought to be inserted into the membrane of invading cells such as bacteria, allowing enzymes to enter and destroy these foreign cells. Animals inheriting a particular perforin variant survive 2-3 hours longer on average than other animals challenged with the disease. The scientists in India found that expression of this gene in the spleen of rohu was increased twenty-fold around 12 hours after Aeromonas hydrophila infection. Greater expression of this gene was also found in gill tissue after infection.
The project has produced many other interesting leads about genes affecting immunity in fish which the scientists in Norway and India are keen to pursue in future projects.