Abstract
This paper describes the finding of genes displaying differential expression in Nile tilapia ( Oreochromis niloticus ) adapted to saltwater and discusses their potential involvement in saltwater tolerance. The availability of fresh water is severely limited in many countries; therefore, the use of brackish‐water areas may present an opportunity to expand the tilapia aquaculture industry. Increased knowledge of genes involved in saltwater tolerance will facilitate selection for this trait in tilapia. A set of fish from four different families of Nile tilapia was divided into two freshwater containers, the different families equally represented in the two experimental groups, and salinity was gradually increased in one of them. A differential expression study identified a number of genes expressed differently in saltwater and fresh water. Based on these expression analyses and on a judgement of potential candidate genes probably to be involved in adaptation to changes in salinity, four genes were selected and their levels of expression further analysed by real‐time polymerase chain reaction. The selected genes beta haemoglobin , Ca 2+ transporting plasma membrane ATPase , pro‐opiomelanocortin and beta‐actin showed changes in expression. The genes were mapped and assigned to an already existing, high‐resolution linkage map.