In the present transcriptomic analysis, two aquaporin 4 (aqp4) clones were found in cluster 3. Real-time PCR data confirmed that rainbow trout aqp4 gene exhibited a strong over expression in the preovulatory ovary. In mammals, AQP4 is also known as mercurial insensitive water channel (MIWC). It was previously shown that water permeability was strongly increased in African clawed frog oocytes expressing MIWC [42] . In marine fish, a strong oocyte hydration occurs during oocyte maturation [43, 44] . In addition, it was recently shown that this oocyte hydration involves an aquaporin1-like protein in seabream [45] . In freshwater species, data on oocyte hydration is more controversial. However, a limited but