Resistance of Adult Kryptolebias marmoratus Hermaphrodites to Irreversible Sex Change by Exogenous Androgens

The Kryptolebias marmoratus species complex contains the only known self-fertile hermaphroditic vertebrates. All three taxa in this clade live in mangrove forests of the Caribbean basin, and all have individuals with both testis and ovarian tissue in their gonads. Two, K. marmoratus and K. hermaphroditus, use self-fertility as their main mode of reproduction and have colonized remote islands. K. marmoratus also has well-documented production of fertile males through sex change of adult hermaphrodites. The control of sex change in K. marmoratus is poorly understood. Individuals believed to be genetically identical can be raised in the same environment, yet change sex at drastically different times or not at all. However, juvenile fish can be permanently masculinized by immersion in the androgen methyltestosterone (MT). Here, we first document substantial individual variation in overall gonad size and in testis content within and between the gonads of adult K. marmoratus hermaphrodites. This led us to hypothesize that variation in testis-derived androgen may create a positive feedback that drives sex change in some individuals, but not others. We used endocrine manipulations to test a prediction of this hypothesis, namely that sex change in adults could be triggered by exogenous androgens. While exposure to MT led to full masculinization of pigmentation and partial masculinization of the gonad, it was not enough to maintain a permanent transition like that seen in a natural sex change. Additional treatments with 11-ketotestosterone and the aromatase inhibitor fadrozole had weaker effects, which were also impermanent. In this way, adult K. marmoratus differ from larvae, and from adults of other gonochoric teleosts (i.e., with true females and males), in which permanent masculinization of females has been reported after administration of both androgens and aromatase inhibitor. This resistance may reflect changes in adult sexual physiology that evolved to address the underlying need of hermaphrodites to maintain many female traits despite the presence of elevated androgens in a common circulatory system.