Relative mRNA abundance Ovarian expression profiles of angiotensin-converting enzyme 2 (ace2), coagulation factor V (cf5), CXC chemokine L14 (cxcl14), aquaporin 4 (aqp4), pendrin (slc26), vasotocin (avt), serine protease 23 (sp23), ADAM22 (adam22), and elongation fac-tor 1 alpha (ef1α) genes during rainbow trout late oogenesis (mean ± SEM) Figure 3 Ovarian expression profiles of angiotensin-converting enzyme 2 (ace2), coagulation factor V (cf5), CXC chemokine L14 (cxcl14), aquaporin 4 (aqp4), pendrin (slc26), vasotocin (avt), serine protease 23 (sp23), ADAM22 (adam22), and elongation factor 1 alpha (ef1α) genes during rainbow trout late oogenesis (mean ± SEM). Ovaries were sampled from separate females during late vitellogenesis (LV, N = 6), post-vitellogenesis (PV, N = 6) and oocyte maturation (MAT, N = 6). The mRNA abundance of each gene was determined by real-time PCR and normalized to the abundance of 18S. Abundance was arbitrarily set to 1 for LV stage and data are expressed as a percentage of the transcript abundance at this stage. Bars sharing the same letter(s) are not significantly different (p < 0.05). significant hydration was also observed in several freshwater species including rainbow trout [46] . Our data suggest that, similarly to marine species, the oocyte hydration occurring during oocyte maturation could also be aquaporin-mediated in freshwater species such as rainbow trout. In addition to aqp4 gene, we also observed a dramatic over expression of slc26 gene at the time of meiosis resumption. Solute carrier family 26 member 4 (slc26) is also known as sodium-independent chloride/ iodide transporters or pendrin. The over expression of slc26 gene at the time of oocyte maturation is dramatic, as demonstrated by real-time PCR. Together, the strong upregulation of aqp4 and slc26 genes at the time of meiosis resumption stresses the importance of water and ion transports in the rainbow trout preovulatory ovarian functions. In marine species, the major oocyte hydration occurring before ovulation is probably important for adjusting egg buoyancy. In contrast, in freshwater species laying demersal eggs such as rainbow trout, it has been hypothesized that the limited (25%) oocyte hydration occuring before ovulation could be necessary for the completion of the ovulation process [46] . Thus, the increase of oocyte volume could facilitate the rupture of the follicular walls and subsequently, the release of the oocyte from its follicular layers.