Abstract
Ligand bound nuclear estrogen receptor (ER) acts as a transcription factor regulating the expression of estrogen dependent genes. There are four nuclear ER isoforms in rainbow trout (Oncorhynchus mykiss). The objective of this study was to measure whole body mRNA levels of the two ERalpha isoforms (alpha1/alpha2) and the two ERbeta isoforms (beta1/beta2) in male and female embryos from 50 to 600 degree-days (DD; days post-fertilizationxwater temperature) and in embryos exposed to vehicle or 17beta-estradiol (E2) for 2h at 230, 240 and 250 DD. All four isoforms were detected at every time point in both sexes. Sexual dimorphism was rarely observed; at 50 DD the level of ERalpha2 mRNA was significantly greater in males than in females and at 100 DD the level of ERbeta1 mRNA was significantly greater in females than in males (p<0.05). Expression profiles of the two ERalpha isoforms were slightly different from one another, whereas the ERbeta isoforms exhibited similar expression patterns. The effect of E2 was not different between male and female embryos. The level of ERalpha1 mRNA increased significantly at 240 DD; a similar but not statistically significant trend was observed at 230 and 250 DD. Despite the critical role of estrogen during sex differentiation in rainbow trout, the receptivity to this hormone as measured by the response in mRNA levels of ER appears to be largely the same between males and females and ERalpha1 is the only E2 responsive isoform.