Socially induced sex change is orchestrated by a novel combination of genes and epigenetic factors that govern sex differentiation and cell fate. Approximately 500 fish species can change sex in adulthood. The molecular basis for this astonishing transformation remains broadly unknown. Genetic regulation of embryonic sex differentiation is well established in vertebrates but also appears influential in sequential hermaphrodites. Recent work indicates that epigenetic effects and genes regulating cell fate are also important drivers of sex change. Here we use the spotty wrasse to investigate gonadal sex change at a molecular level. While the expression of some sex differentiation genes (dmrt1, foxl2a, ctnnb1) in spotty wrasse follow established sex-biased patterns, others (sox9a, znrf3, rspo1) oppose typical vertebrate-models. We propose that gene neofunctionalisation due to teleost whole-genome duplication may explain these counter-intuitive expression profiles. Significant epigenetic reprogramming within the transitional spotty wrasse gonad is implied through the dynamic expression of methyltransferases and the chromatin-modifying Jumonji family genes, jarid2b and kdm6bb. Furthermore, our results show that fancl and pou5f3, two genes associated with either DNA repair pathways or stem cell pluripotency, are downregulated as sex change advances. This highlights genetic factors that may underlie a functional change of cell fate trajectory. Collectively, this work demonstrates the diversity of genetic pathways that are dynamically activated in a phased, sex-specific manner to co-ordinate vertebrate sex change.

The stunning sexual transformation commonly triggered by age, size or social context in some fishes is one of the best examples of phenotypic plasticity thus far described. To date our understanding of this process is dominated by studies on a handful of subtropical and tropical teleosts, often in wild settings. Here we have established the protogynous New Zealand spotty wrasse, Notolabrus celidotus, as a temperate model for the experimental investigation of sex change. Captive fish were induced to change sex using aromatase inhibition or manipulation of social groups. Complete female-to-male transition occurred over 60 days in both cases and time-series sampling was used to quantify changes in hormone production, gene expression and gonadal cellular anatomy. Early-stage decreases in plasma 17β-estradiol (E2) concentrations or gonadal aromatase (cyp19a1a) expression were not detected in spotty wrasse, despite these being commonly associated with the onset of sex change in subtropical and tropical protogynous (female-to-male) hermaphrodites. In contrast, expression of the masculinising factor amh (anti-Müllerian hormone) increased during early sex change, implying a potential role as a proximate trigger for masculinisation. Collectively, these data provide a foundation for the spotty wrasse as a temperate teleost model to study sex change and cell fate in vertebrates.

The stunning sexual transformation commonly triggered by age, size or social context in some fishes is one of the best examples of phenotypic plasticity thus far described. To date our understanding of this process is dominated by studies on a handful of subtropical and tropical teleosts, often in wild settings because sex change has been challenging to achieve in captivity. Here we have established the protogynous New Zealand spotty wrasse, Notolabrus celidotus, as a temperate model for the experimental investigation of sex change. Captive fish were induced to change sex using either aromatase inhibition or manipulation of social groups. Complete transition from female to male occurred over 60 days and time-series sampling was used to quantify changes in hormone production, gene expression and gonadal cellular anatomy using radioimmunoassay, nanoString nCounter mRNA and histological analyses, respectively. Early-stage decreases in plasma 17β-estradiol (E2) concentrations or gonadal aromatase (cyp19a1a) expression were not detected in spotty wrasse, despite these being commonly associated with the onset of sex change in subtropical and tropical protogynous (female-to-male) hermaphrodites. In contrast, expression of the masculinising factor amh (anti-Müllerian hormone) increased during early sex change, implying a potential role as a proximate trigger for masculinisation. Expression of male-related genes responsible for androgen production cyp11c1 and hsd11b2 increased from mid sex change. Gonadal expression of the glucocorticoid and mineralocorticoid receptors nr3c1 and nr3c2, putative mediators of the stress hormone cortisol, increased in late stages of sex change. Collectively, these data provide a foundation for the spotty wrasse as a temperate teleost model to study sex change and cell fate in vertebrates. Summary statement The spotty wrasse, Notolabrus celidotus, is a new temperate model for the study of vertebrate sex change, this work characterises endocrine and genetic markers based on laboratory induced sex change.

Communal spawning behaviour in marine aquaculture species often results in a few individuals contributing disproportionate amounts of gametes. This can lead to a reduction in genetic variability and increases the risk of inbreeding among successive generations. Therefore, long term sustainability of captive breeding programmes for such species partly depends upon maintaining a sufficiently high proportion of parents contributing high quality gametes during spawning. The current study was conducted to evaluate if the use of slow-release gonadotropin-releasing hormone analog (GnRHa) implants could increase the number of females spawning high quality gametes, and thus increase genetic variation in a captive population of yellowtail kingfish Seriola lalandi (Valenciennes, 1833). Broodstock fish received implants with or without 500 μg of GnRHa during the spawning season. GnRHa treatment was associated with a higher proportion of females contributing to spawning. However, compared to eggs from non-GnRHa-treated broodstock, GnRHa significantly decreased the floating rate, fertilisation rate, number of viable eggs and egg oil globule diameter. Overall, the use of slow-release GnRHa implants is a useful tool to increase parental contribution to spawning, but this benefit must be carefully balanced against lower egg quality.

Wild Ballan wrasse Labrus bergylta were sampled monthly over 2 years in western Norway to identify the natural process of sex change in this species. Light microscopy of standard histological-stained and immunohistochemistry-treated gonad tissue showed that spermatogonial germ cells tended to proliferate around the periphery of the lamellae before filling into the slowly receding, apoptotic central areas of the lamellae. Sex change occurred following the breeding season. From July to September, fish were most often in an early state of gonadal transition (ET), characterized by degenerating previtellogenic oocytes and pockets of proliferating spermatogonia in the germinative epithelia. The majority of fish with late transitional gonads, that were typically dominated by spermatogenic cells, developing efferent ducts and the beginning of lobule formation, were found between October and November. Sex steroid profiles of fish representing the different sexual phases showed that breeding females had the highest concentrations of 17β oestradiol (E2 ) and the lowest concentration of 11 ketotestosterone (11KT). Concentrations of E2 decreased greatly in ET fish at the beginning of sex change and remained low in all subsequent phases. The opposite trend was demonstrated in 11KT profiles. Initial-phase female fish had minimal concentrations of 11KT, but these increased during subsequent transitions. Sex change occurred most often in fish 34-41 cm total length (L(T)) and the median of fish in the size-frequency overlap of female and male fish was 36 cm L(T).

This 2 year study examined the reproductive cycle of wild female Ballan wrasse Labrus bergylta in western Norway as a precursor to captive breeding trials. Light microscopy of ovarian histology was used to stage gonad maturity and enzyme-linked immuno-absorbent assay (ELISA) to measure plasma concentrations of the sex steroids testosterone (T) and 17beta-oestradiol (E(2)). Ovarian recrudescence began in late autumn to early winter with the growth of previtellogenic oocytes and the formation of cortical alveoli. Vitellogenic oocytes developed from January to June and ovaries containing postovulatory follicles (POF) were present between May and June. These POF occurred simultaneously among other late maturity stage oocytes. Plasma steroid concentration and organo-somatic indices increased over winter and spring. Maximal (mean +/-s.e.) values of plasma T (0.95 +/- 0.26 ng ml(-1)), E(2) (1.75 +/- 0.43 ng ml(-1)) and gonado-somatic index (I(G); 10.71 +/- 0.81) occurred in April and May and decreased greatly in July when only postspawned fish with atretic ovaries occurred. Evidence indicates that L. bergylta are group-synchronous multiple spawners with spawning occurring in spring and peaking in May. A short resting period may occur between late summer and autumn when previtellogenic oocytes predominate and steroid levels are minimal.