Changing sex: it's a fishy thing.

Abstract

Sex is a much more fluid affair for fishes than for terrestrial vertebrates. Many species (around 2%) show sequential hermaphroditism, switching from male to female (protandry), from female to male (protogyny) or - more rarely - from one to the other then back again. Such sexual plasticity likely confers an adaptive advantage during changes in the social milieu. Protogynous fishes, such as groupers and wrasses, were previously thought to remain as males after having made the switch once. However, Jiaxing Chen and colleagues from Sun Yet-Sen University (Guangzhou, China) show in the current issue of Journal of Fish Biology that, in the orange-spotted grouper Epinephelus coioides, some males will make the reverse switch back to females when placed in male-only groups. Interestingly, it was the smaller males, likely lower in the pecking order, that tended to make this switch. As might be expected, plasma levels of 11-keto testosterone (11-KT; the main androgen in teleosts) fell significantly in those males that reversed sex, whilst there was a peak of 17β-estradiol (the major estrogen) at six weeks in the same fish. Expression of genes coding for other sex hormones, namely gonadotropin-releasing hormone (GnRH) in the hypothalamus, plus follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in the pituitary glands, of sex-reversed fish were similar to that of females, as were those of some other genes thought to be involved in gonadal sex-change (cyp191a1a, the gene coding for aromatase, for example). Intriguingly, however, this last example only became elevated after the sex-reversal was complete, raising doubts as to its involvement in the initiation of the process. The authors suggest that, in this species at least, FSH may be more important than LH in the regulation of sex-reversal, the latter more important in maintaining male gametes. Whilst the involvement of the hypothalamus-pituitary-gonadal (HPG) axis in sex-change is beyond doubt, the precise details may be species or, as the current study suggests, context-dependent. As Mark Lokman (University of Otago, New Zealand), who works on sex-change in wrasse (but was not involved in the current study) comments, “The ability to change sexual phenotype is not uncommon in fishes..”. However, since the work of Chen et al. in groupers, and Kuwamura et al. (2007) in wrasse (also published in Journal of Fish Biology),, he continues, “…it is possible that sexual phenotype in fish is more plastic than generally accepted or appreciated.”