The bluehead wrasse (Thalassoma bifasciatum) changes sex from female to male when the dominant male of a social group is lost. Todd et al. (2019) described this transformation transcriptomically and methylomically and proposed a mechanistic cascade: the social cue acts through a stress-axis cortisol surge that suppresses the aromatase gene cyp19a1a, upregulates the male-promoting factor amh and androgen-biosynthesis genes, and is accompanied by replacement of DNA methyltransferases, a peak in TET demethylases, deactivation of the PRC2 chromatin complex, and an intermediate gonadal state molecularly resembling mammalian pluripotent cells. Every causal link in this model remains untested in the source species; later work in other protogynous fishes is correlative or context-dependent. The problem is to establish, by functional intervention, which links of the proposed cascade are necessary and/or sufficient for socially controlled sex change, and whether the intermediate gonad is genuinely a pluripotent or germline-stem-cell-based state.

Background

Sequential hermaphroditism — changing sex as part of the life cycle — occurs in several families of marine teleost fishes. The bluehead wrasse (Thalassoma bifasciatum), a Caribbean coral-reef fish, is a textbook protogynous (female-first) hermaphrodite: when the dominant terminal-phase (brightly colored, territorial) male of a social group disappears, the largest female completes a transformation of behavior within hours and converts mature ovaries into functional testes within roughly 8-10 days. Because the trigger is a purely social cue, this system exposes how a committed adult developmental program can be reopened by the environment.

Todd et al. (2019, Science Advances 5:eaaw7006) profiled gene expression (RNA-seq) and DNA methylation (whole-genome bisulfite sequencing, a method that reveals methylation of cytosines across the genome) across the entire time course of sex change in brain and gonad. Their proposed model is: (1) perception of the missing dominant male activates the stress axis, raising cortisol, the main glucocorticoid stress hormone in fishes; (2) cortisol down-regulates cyp19a1a, the gene encoding aromatase, the enzyme converting androgens to estrogens, so estrogen (17β-estradiol) production collapses and feminizing gene expression falls, while amh (anti-Müllerian hormone, a male-promoting factor) and androgen-biosynthesis genes (cyp11c1, hsd11b2) rise, driving ovarian atresia and spermatogonial recruitment; (3) the brain side is only partly resolved — isotocin (the fish homolog of oxytocin, linked to territorial aggression) was up-regulated early, but expected changes in arginine vasotocin (AVT, the fish vasopressin homolog), gonadotropin-releasing hormone (GnRH) and norepinephrine (NE) signaling were not detected at whole-brain transcriptome resolution; (4) the gonad passes through an intermediate state whose expression of epigenetic machinery — DNA methyltransferases (DNMTs, which add methyl marks), TET enzymes (which oxidize and remove them), and the PRC2 histone-marking complex — resembles mammalian naïve pluripotent stem cells and primordial germ cells (PGCs), suggesting an active erasure and rewriting of epigenetic sexual-fate memory; and (5) several duplicated sex-pathway genes (e.g. paralogs of wnt4, foxl2 and fstl) show reversed, male-associated expression, and JAK-STAT pathway genes (a signaling cascade known to maintain male fate in Drosophila but previously unlinked to vertebrate sexual development) are progressively up-regulated.

The authors were explicit that this model rests on correlations: no step had been functionally tested, and key predictions (an early cortisol surge in this species, localized neuropeptide signaling, pluripotency of the intermediate gonad, necessity of the epigenetic reprogramming) lacked direct evidence. Earlier endocrine experiments in other protogynous species — cortisol or aromatase-inhibitor treatment in the sandperch (Frisch et al. 2007) and three-spot wrasse (Nozu & Nakamura 2015) — support components of the hypothesis but did not test the socially cued cascade of the bluehead wrasse, and no glucocorticoid-receptor blockade or epigenetic-enzyme intervention during natural sex change has been reported in any hermaphroditic fish.

Problem Statement

Establish, by functional intervention in the bluehead wrasse, which links of the Todd et al. (2019) model of socially controlled sex change are causal, jointly answering: (i) does the social cue (loss of the dominant male) initiate sex change through a cortisol surge that directly represses cyp19a1a (aromatase) and up-regulates amh and the androgen-biosynthesis genes cyp11c1 and hsd11b2; (ii) what neuroendocrine signaling (isotocin, arginine vasotocin, GnRH, norepinephrine) carries the social cue from brain to gonad during the earliest stages; (iii) what the functional significance of the neofunctionalized gene duplicates (e.g. wnt4b, the male-specific foxl2 paralogue, fstl4) and of JAK-STAT pathway up-regulation is; (iv) whether the epigenetic reprogramming — DNMT replacement, the TET expression peak, PRC2 deactivation and the global methylation increase — is necessary for sex change or a consequence of it; and (v) whether the intermediate gonad is genuinely a pluripotent or germline-stem-cell-based state, rather than a correlative molecular signature. A full resolution requires addressing all five links jointly; independently establishing or refuting any single link is a reportable partial result but does not by itself constitute a complete resolution. The objective is a causally validated (or refuted and revised) chain from social stimulus to complete gonadal reprogramming, not merely additional correlative omics descriptions.

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