Conserved androgenic differentiation pathways are repurposed during the evolution of adult sexual plasticity

Early exposure to androgens is necessary to organize male phenotype, and inhibition of androgen signaling adversely affects external genitalia development. Nonetheless, vertebrates that remain sexually plastic as adults require mechanisms that induce changes and prevent fixation of genital morphology. In Lythrypnus dalli, a highly social hermaphroditic fish, sex differences in genitalia morphology are maintained in the absence of dimorphic levels of potent fish androgen 11-ketotestosterone. Since a functional androgen receptor (AR) is critical to the masculinization of phenotype across all vertebrates, local differences in the AR may be one mechanism to maintain dimorphic genitalia in this species. We demonstrate that AR expression is high throughout the male genitalia, but low or absent in female genitalia. Animals with ambiguous genitalia exhibited higher AR expression than females within the body of the papilla. In addition, both male and female developing L. dalli exhibited high mesenchymal AR expression suggesting that elevated that levels of AR in this region of ambiguous animals may be characteristic of multipotentiality, allowing for genitalia morphogenesis in either direction. To examine the necessity of AR in maintaining male-typical genitalia, nesting males were intraperitoneally injected with 50μg/g/day of flutamide (AR antagonist) or a vehicle for five days. Compared to control animals, flutamide treated males had a significant percent decrease in papilla length to width ratio. In a species that must remain neutral to sexual canalization, regional differences in AR expression appear to mediate this phenotypic difference. INDEX WORDS: Androgen receptor, Sexual plasticity, Sexual differentiation CONSERVED ANDROGENIC DIFFERENTIATION PATHWAYS ARE REPURPOSED DURING THE EVOLUTION OF ADULT SEXUAL PLASTICITY