Differential expression of subunits of 20β-hydroxysteroid dehydrogenase during gametogenesis in rainbow trout (Oncorhychus mykiss).

The patterns of expression of two subunits of 20β-hydroxysteroid dehydrogenase (20β-HSD), key enzyme involved in the biosynthesis and activation of steroid hormones, were examined in rainbow trout by using a combination of quantitative real-time PCR and in-situ hybridization. The expression of targeted genes was examined in mRNA extracted from different tissues at different gonadal stages in male and female trout. Both subunits of 20β-HSD were found to be widely distributed in tissues. The highest expression of 20β-HSD A was found in intestine followed by skin, stomach, liver and gills, whereas, the highest expression of 20β-HSD B was observed in stomach followed by head kidney, ovary - at late vitellogenesis stage- and trunk kidney. In ovarian tissue 20β-HSD A was highly expressed in mature oocytes, and the highest expression of 20β-HSD B was in ovary at late vitellogenesis stage. There were no differences in the level of expression of either subunit among groups of rainbow trout at different stages of maturational competence. In male fish, 20β-HSD A was highly expressed in testis stage I in contrast to 20β-HSD B which was highly expressed in testis stage VIII. In situ- hybridization results showed that the 20β-HSD gene was highly expressed in gastrointestinal organs, while only slightly expressed in the gonadal tissue of fish at stage 62day-post-fertilization (dpf). Overall, the results confirm the ubiquitous presence of 20β-HSD among tissues in rainbow trout with relatively minor fluctuations in expression associated with reproductive cycles which collectively suggests a wider metabolic role of these enzymes than just an association with the synthesis of control hormones for reproduction.