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Histological stages of gonadal development in spotty wrasse: (A) female; (B) early transitioning fish; (C) mid-transitioning fish; (D) late transitioning fish; (E) terminal phase male. AtO atretic oocyte, BlV blood vessel, Col collagen, GoC gonial cells, Lum gonadal lumen, PvO previtellogenic oocyte, Oog oogonia, StC stromal cells, SCD sperm collecting duct, Spc spermatocytes, Spg spermatogonia, Spz spermatozoa, TAl tunica albuginea, YBB yellow–brown body consistent with melanomacrophage centres. Scale bars in A, B, E = 200 µm; C, D = 100 µm.
Source publication
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 protogy...
Citations
... temperate wrasse Notolabrus celidotus, which can take 60 days to change sex (Goikoetxea et al., 2021). ...
This study explored the extent to which the hermaphroditic and reproductive characteristics vary among five congeneric (Choerodon) and co-occurring species of the Labridae, a family in which hermaphroditism is widespread. Sampling was located in the large sub-tropical Shark Bay (26° S, 113° E), one of only twenty World Heritage Properties in Australia. Choerodon cephalotes, C. cyanodus, C. cauteroma and C. schoenleinii live within the main body of the embayment, which approaches the southern limit of their tropical, geographical ranges, whereas C. rubescens lives along its outer western boundary, close to the northern limit of its warm, temperate distribution. A combination of comprehensive length and age compositions, histological analyses of gonads and logistic regressions of lengths and ages at maturity and sex change are consistent with all five species being monandric protogynous hermaphrodites. Choerodon cephalotes, C. rubescens and C. schoenleinii are sexually dichromatic, with the lengths and ages of males at the extreme upper end of those of females and low proportions of males among adults (adult sex ratios, ASR: 11–33%). It is proposed that these species have polygynous mating systems, such as leks or harems. Although C. cauteroma has an ASR of essentially parity, its pronounced sexual dichromatism and the ages of its males typically lying at the upper end of those of its females indicate that this species also has a polygynous mating system. Limited sexual dichromatism and an ASR as high as 71% suggest that C. cyanodus is not polygynous. As the timing of the main spawning period of the four species within Shark Bay increases sequentially between early spring and late summer, the temperature trigger for spawning by those species also presumably increases progressively. This phasing of spawning, allied with some interspecific spatial variation, reduces the potential for competition among the larvae and early juveniles of the five Choerodon species. Choerodon rubescens spawns earlier in Shark Bay than in the cooler waters of the Abrolhos Islands, 230 km further south. The management implications of data for Choerodon species, and particularly of the size compositions and the ASRs of the two largest species, C. rubescens and C. schoenleinii, are discussed. The variations in the hermaphroditic and reproductive features of five congeneric species of labrid, in the region in which their distributions overlap, highlight the evolutionary/biological plasticity of this family and thereby contributes to its diversity and success.
... Like most wrasses, they have dimorphic initial phase (IP) and terminal phase (TP) colour morphs (Choat 1965). Sexually mature fish will spawn in captivity and sex change is induced in IP fish through the manipulation of social structure (Thomas et al. 2019;Goikoetxea et al. 2021). The ability to complete natural sex change under laboratory conditions is especially important as other model species such as the bluehead wrasse adapt poorly to captivity, thus most sex change experiments have to be done in wild populations (Liu et al. 2015). ...
... The brains were then rinsed with 1xPBS, flash-frozen in liquid nitrogen and stored at −80°C. Although terminal phase males are easily distinguishable from females, initial phase males are more difficult to distinguish (Goikoetxea et al. 2021). To confirm sex of the females, a gonadal sample was dissected from each fish, fixed in 4% paraformaldehyde for 24 h, and subsequently stored in 1xPBS. ...
For most vertebrates, sexual fate is genetically determined and remains fixed throughout life. However, for some teleost fishes sex is more plastic. Significant progress has been made in characterising the cellular and molecular processes that underpin gonadal sex change. The brain-mediated mechanisms that underlie and initiate this transformation, however, remain poorly understood. One reason for this is the current lack of a neuroanatomical reference work for sex changing fishes. Here, we present a brain atlas for the New Zealand (NZ) spotty wrasse (Notolabrus celidotus), developed from 10 wild caught specimens (2 male and 8 female). From each fish, coronal cryosections were collected and stained with cresyl violet solution. Photomicrographs were taken of each section using an inverted bright field microscope and 3D reconstructions were rendered of each brain for annotation and volumetric comparison of specific brain regions. The brain atlas describes the general features of the NZ spotty wrasse brain as well as its specificities. This atlas provides the necessary foundation for further investigation of the brain mechanisms driving protogyny in this species.
... The protogynous New Zealand spotty wrasse, Notolabrus celidotus, is a convenient temperate water model for sex change studies (Goikoetxea et al. 2021). These fish grow to approximately 250 mm and will undergo sex change in captivity following male removal from the social group. ...
... Following fixation, samples were transferred to 70% ethanol, processed for paraffin-embedded sectioning (3-4 µm) and stained with haematoxylin and eosin. Each fish was classified into one of five sexual stages (Table 1) according to their gonadal histology (Goikoetxea et al. 2021). ...
... Fifteen candidate genes implicated previously in sex change Goikoetxea et al. 2021), together with two housekeeping genes (Table 2), were analysed using nanoString nCounter™ CodeSet gene expression quantification at the Otago Genomics Facility, Biochemistry Department, University of Otago, New Zealand. For this experiment, RNA was extracted with Direct-zol RNA MiniPrep Plus (Zymo Research) and RNA concentration was assessed using a Qubit 2.0 Fluorometer (Life Technologies). ...
Summary statement
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.
... IP males and IP females cannot be distinguished by their external morphology [47]. Sex change can be reliably induced in spotty wrasse females by the removal of dominant TP males from established social groups in captivity [48][49][50] (Muncaster et al., submitted). ...
... The effect of stress as a potential trigger for sex change in spotty wrasse was investigated by implanting cortisol pellets into IP spotty wrasse individuals under socially inhibitory conditions between the months of June-September 2017. This is during the breeding season, which lasts from late July until November, but outside of the optimal window for natural sex change, which occurs between November-May (i.e., outside the breeding season) [49]. Fish were captured around high tide by hook and line off the coast of Tauranga, Bay of Plenty, New Zealand (37.6878˚S, 176.1651˚E) and kept at the Aquaculture Centre at Toi Ohomai Institute of Technology, Tauranga. ...
... Fish were fed frozen green-lipped mussels (Perna canaliculus) three times a week for the duration of the experiment (ten weeks) (standard practice for this species). The duration of the experiment and the sampling times were estimated based on previous data on the completion of sex change in captive spotty wrasse using an aromatase inhibitor (60 days) or social inhibition (66 days) [49]. ...
Sex change occurs as a usual part of the life cycle for many teleost fish and the modifications involved (behavioural, gonadal, morphological) are well studied. However, the mechanism that transduces environmental cues into the molecular cascade that underlies this transformation remains unknown. Cortisol, the main stress hormone in fish, is hypothesised to be a key factor linking environmental stimuli with sex change by initiating gene expression changes that shift steroidogenesis from oestrogens to androgens but this notion remains to be rigorously tested. Therefore, this study aimed to experimentally test the role of cortisol as an initiator of sex change in a protogynous (female-to-male) hermaphrodite, the New Zealand spotty wrasse ( Notolabrus celidotus ). We also sought to identify potential key regulatory factors within the head kidney that may contribute to the initiation and progression of gonadal sex change. Cortisol pellets were implanted into female spotty wrasses under inhibitory conditions (presence of a male), and outside of the optimal season for natural sex change. Histological analysis of the gonads and sex hormone analyses found no evidence of sex change after 71 days of cortisol treatment. However, expression analyses of sex and stress-associated genes in gonad and head kidney suggested that cortisol administration did have a physiological effect. In the gonad, this included upregulation of amh , a potent masculinising factor, and nr3c1 , a glucocorticoid receptor. In the head kidney, hsd11b2 , which converts cortisol to inactive cortisone to maintain cortisol balance, was upregulated. Overall, our results suggest cortisol administration outside of the optimal sex change window is unable to initiate gonadal restructuring. However, our expression data imply key sex and stress genes are sensitive to cortisol. This includes genes expressed in both gonad and head kidney that have been previously implicated in early sex change in several sex-changing species.
... Consequently, there is clear behavioural control over sex change, where social or mating systems confer differential advantages for each sex under particular social and ecological conditions 1 . We now have a solid mechanistic understanding of sex change, including hormonal, histological and molecular mechanisms of the process 12 , especially in sparid, gobiid and serranid fishes where sequential hermaphroditism is common 13 . These fishes are typically marine, long-lived and may form relatively compact social groups. ...
Sex change (sequential hermaphroditism) has evolved repeatedly in teleost fishes when demographic conditions mediate fundamentally different sex-specific returns for individuals of particular age and size. We investigated the conditions for potential sex change in an annual killifish (Millerichthys robustus) from temporary pools in Mexico. In natural populations, we detected adults with intersex colouration and gonads. Therefore, we experimentally tested whether this apparent sex change can be generated by manipulation of ecological and social conditions, rather than being caused by environmental disturbance. We demonstrated functional protogynous (female-to-male) sex change in 60% replicates, when groups of five females interacted and had a visual and olfactory cue of a male. Only one female changed sex in any given replicate. The sex change never occurred in isolated females. Protandrous (male-to-female) hermaphroditism was not recorded. We characterized gradual changes in behaviour, colouration and gonad structure during the sex change process. The first behavioural signs of sex change were observed after 23 days. Secondary males spawned successfully after 75 days. We discuss the adaptive potential of sex change in short-lived annual fishes through the seasonal decline of males, and during colonization of new habitats. This is the first observation of functional hermaphroditism in an annual killifish.
Determining the origins of novel genes and the genetic mechanisms underlying the emergence of new functions is challenging yet crucial for understanding evolutionary innovations. The novel fish antifreeze proteins, exemplifying convergent evolution, represent excellent opportunities to investigate the evolutionary origins and pathways of new genes. Particularly notable is the near-identical type I antifreeze proteins (AFPI) in four phylogenetically divergent fish taxa. This study tested the hypothesis of protein sequence convergence beyond functional convergence in three unrelated AFPI-bearing fish lineages, revealing different paths by which a similar protein arose from diverse genomic resources. Comprehensive comparative analyses of de novo sequenced genome of the winter flounder and grubby sculpin, available high-quality genome of the cunner, and those of 14 other relevant species found that the near-identical AFPI originated from a distinct genetic precursor in each lineage, and independently evolved coding regions for the novel ice-binding protein while retaining sequence identity in the regulatory regions with their respective ancestor. The deduced evolutionary processes and molecular mechanisms is consistent with the Innovation-Amplification-Divergence (IAD) model applicable to AFPI formation in all three lineages, a new Duplication-Degeneration-Divergence (DDD) model we propose for the sculpin lineage, and a DDD model with gene fission for the cunner lineage. This investigation illustrates the multiple ways by which a novel functional gene with sequence convergence at the protein level could evolve across divergent species, advancing our understanding of the mechanistic intricacies in new gene formation.
The gonadal sexual fate of vertebrates is either defined by genetics or environment, or a combination of both factors. Interestingly, in sequential hermaphroditism, the animal can undergo natural sex changes from female-to-male, male-to-female, and bidirectional way throughout their lives. This change exhibits the process which shifts between oogenesis and spermatogenesis and is regarded as an ideal instance of sexual plasticity. To develop the experimental model for studying the sexual plasticity of protogynous fish, the social conditions that induce sex changes were defined in wrasse, Pseudolabrus sieboldi. When six females were kept together in a tank, the largest female became a male, whereas a similar conversion did not occur when only two females were present in a tank. A semi-gonadectomy analysis developed in the present study verified the direct relationship between gonadal sex and body coloration. In P. sieboldi, the sex change is controlled by the relative body size of an individual within a group, rather than by absolute body size. When six females were kept in smaller sized tanks, delayed sex change or unchanged individuals was observed. Overall, more than 90% of the largest females demonstrated sex change after being housed with five smaller females in different sizes of tanks ranging from 80 to 500 L. Furthermore, the experiment using a transparent barrier suggested that visual stimuli are one of the major cues to initiate sex change. Our findings on the laboratory conditions leading to the initiation of sex change in wrasse suggest the usefulness of this species as a model organism for comparative studies in molecular, cellular, and physiological mechanisms of sexual plasticity, as well as on social and reproductive behaviors.
