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Sex determination and differentiation varies widely across vertebrates, but is most dramatically diverse in fishes. Among fishes sex reversal and sex change are observed in 41 teleost families spanning 7 orders. These sex-changing fish perhaps highlight better than any other system that sex determination is not the narrow and fixed construct we once thought, but a plastic trait that is better viewed as a reaction norm. However, while this stunning transformation is increasingly understood, a fundamental question arises, which is why some fish species have retained this inherent plasticity in sexual fate, while others have not? Here, we explore our current understanding of sex change in fish, some of the factors that permit and constrain sex reversal, and posit that gene duplication and neofunctionalization contribute to the sexual lability observed in fish.

Sexual fate is no longer seen as an irreversible deterministic switch set during early embryonic development but as an ongoing battle for primacy between male and female developmental trajectories. That sexual fate is not final and must be actively maintained via continuous suppression of the opposing sexual network creates the potential for flexibility into adulthood. In many fishes, sexuality is not only extremely plastic, but sex change is a usual and adaptive part of the life cycle. Sequential hermaphrodites begin life as one sex, changing sometime later to the other, and include species capable of protandrous (male-to-female), protogynous (female-to-male), or serial (bidirectional) sex change. Natural sex change involves coordinated transformations across multiple biological systems, including behavioural, anatomical, neuroendocrine, and molecular axes. We here review the biological processes underlying this amazing transformation, focussing particularly on its molecular basis, which remains poorly understood, but where new genomic technologies are significantly advancing our understanding of how sex change is initiated and progressed at the molecular level. Knowledge of how a usually committed developmental process remains plastic in sequentially hermaphroditic fishes is relevant to understanding the evolution and functioning of sexual developmental systems in vertebrates generally, as well as pathologies of sexual development in humans.