Sex Steroid Levels in XY Males and Sex-Reversed XX Males, of Rainbow Trout (Oncorhynchus mykiss), During the Reproductive Cycle


Author’s address (for correspondence): Emilio Espinosa, Departamento de Patología Animal, Facultad de Veterinaria, Miguel Servet 177, 50013 Zaragoza, Spain. E-mail:


In this study, the annual cycle of the gonadal steroids testosterone (T), 11-ketotestosterone (11-KT), 17β-oestradiol (E2) and 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) was determined using radioimmunoassay and then compared, for XY males (n = 35) and sex-reversed XX males (n = 27) rainbow trout, to establish possible endocrinology differences. Both in XY males and sex-reversed XX males, significant correlation was shown between body weight and T (r = 0.5046 and 0.34078, respectively; p < 0.0001) or KT (r = 0.52494 and 0.43545, respectively; p < 0.0001) concentrations. Plasma androgen levels in XY and sex-reversed XX males were similar and showed an intense seasonal variation. The highest levels for T and 11-KT were detected from December to April with a peak in January (51.67 ± 5.11 and 61.95 ± 4.25 ng/ml, for XY males and 57.1 ± 5.82 and 59.27 ± 4.84 ng/ml, respectively, for XX males). In addition, there was a positive correlation (p < 0.0001) between T and 11-KT levels for XY males (r = 0.7533) and sex-reversed XX males (r = 0.6019). Concentrations of DHP in XY males also showed seasonal variation with a peak in February (25.18 ± 12.99 ng/ml). However, DHP levels in sex-reversed XX males were undetectable (<0.1 ng/ml) over the year. Levels of E2 were undetectable through the year in both groups of trout. In conclusion, the androgenic and oestrogenic profiles of sex-reversed XX males were similar to those observed in XY males. The only difference in the annual gonadal steroid cycle between XY and sex-reversed XX males was in the DHP profile.