Aromatase, 5α- and 5β-reductase in brain, pituitary and skin of the sex-role reversed Wilson's phalarope

ABSTRACT While intrasexual competition for mates is generally considered to be an androgen-dependent characteristic of reproductively active males, in the Wilson's phalarope (Phalaropus tricolor) it is the female that acquires the brighter nuptial plumage and aggressively competes for access to the less aggressive males. Despite this pronounced sex-role reversal, circulating sex steroid hormones of breeding phalaropes are similar to those of avian species displaying traditional male–female reproductive roles. To investigate whether these behavioural and morphological steroid-dependent differences may be due to differences in target organ metabolism of circulating androgen, [3H]androstenedione in the presence of an NADPH-generating system was incubated with homogenates of brain, pituitary and skin of male and female Wilson's phalaropes collected from a naturally breeding population. Oestrone, 5α-androstanedione and 5β-androstanedione were measured as endpoints of aromatization, 5α-reduction and 5β-reduction respectively. Aromatase activity in the anterior hypothalamus/preoptic area (AHPOA) and posterior hypothalamus was greater in breeding males with high circulating concentrations of testosterone than in females, and activity in the AHPOA was greater in breeding than in non-breeding males (with low circulating testosterone). Aromatase levels did not differ in septum, archistriatum, hyperstriatum or pituitary. 5α- and 5β-reductase were detected in all neuroendocrine tissues sampled and although there were no significant male–female differences, 5α-reductase was greater in the AHPOA of breeding than of non-breeding males. We infer from this that the behavioural sex-role reversal of phalaropes is unlikely to be accounted for by differences in androgen metabolism in neural targets, although the capacity to form greater quantities of oestrogenic and 5α-reduced metabolites in the AHPOA of breeding males may be linked to the expression of masculine copulatory behaviours. Aromatase activity was not detected in skin containing a sexually dimorphic feather tract; however, 5α- and 5β-reductase activities were significantly higher in females than in males and may account for the brighter nuptial plumage of females. These data suggest that alternate determinants of neural responsiveness such as sex-steroid receptor abundance or neural circuitry may underlie atypical sexual behaviours in phalaropes. Journal of Endocrinology (1989) 122, 573–581