Dosage-dependent differences in the effect of aromatizable and nonaromatizable androgens on the resulting phenotype of coho salmon (Oncorhynchus kisutch).

This study was conducted to investigate whether aromatization to estrogen could be the cause for the paradoxical feminization of gonads of sexually-undifferentiated fish after treatment with androgen at either high doses or for long periods. The aromatizable androgen 17α-methyltestosterone (MT) and the nonaromatizable androgen 17α-methyldihydrotestosterone (MDHT) were administered to groups of newly hatched coho salmon (Oncorhynchus kisutch) in a single 2h immersion at concentrations ranging from 6.25 to 6,400µg/l. The effects of treatment were evaluated by determining the resultant proportion of males in each experimental group. The effects of steroid administration on the final mean weight, length and condition factor were also determined. An increase in all these three variables was observed in the groups treated with the higher doses of MT. Regarding the resultant sexual phenotype, the response to both androgens was similar at the majority of doses tested. However, at the highest dose, the proportion of females increased with respect to that of males for MT, but not for MDHT. Since the major difference between the two androgens tested is their capacity to be aromatized, it seems that aromatization to estrogen, rather than inhibition of the biosynthesis of endogenous androgen, may explain the paradoxical feminization encountered.