Endocrine Control of Reproduction in Fish

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Endocrine Control of Reproduction, Fish

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DOI: 10.1016/B978-0-12-809633-8.20579-7

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Endocrine Control of Reproduction, Fish

Jakob Biran,Agricultural Research Organization, Rishon Letziyon, Israel Berta Levavi-Sivan,The Hebrew University of Jerusalem, Rehovot, Israel

Introduction

Fish comprise more than 40% of all vertebrate species and are extremely diverse. Nevertheless, the general endocrine components and pathways that regulate ﬁsh reproduction are highly conserved and as in other vertebrates, they are generated by the hypothalamus-pituitary-gonadal (HPG) axis (Levavi-Sivanet al., 2010; Zoharet al.,2010). Sensory information from the internal and external environments as well as social cues (such as presence of a potential mate, temperature, photoperiod, metabolism, energy stores, gonadal state etc.), are processed by theﬁsh brain and integrated by the hypothalamus to initiate and regulate reproductive activity. The hypothalamus reacts to environmental and physiological conditions by synthesizing and releasing neuropeptides (primarily gonadotropin-releasing hormone; GnRH) and monoamines (mainly dopamine) which regulate the activity of endocrine cells in the anterior pituitary. That region of the pituitary produces the gonadotropins follicle stimulating hormone (FSH) and luteinizing hormone (LH). Upon stimulation, gonadotrope cells located in the anterior pituitary produce and release their hormones into the blood (Levavi-Sivanet al., 2010). Both gonadotropins reach the gonads, where they stimulate the synthesis of steroidal hormones that, in turn, regulate maturation of gametes, as well as convey feedback to the hypothalamus and pituitary.

In spite of the diversity of reproductive strategies among the approximately 29,000 recognizedfish species,fish share several distinct endocrine characteristics: (i) Thefish neurohypophysis is positioned dorsally to the adenohypophysis, in contrast to the anterior-posterior organization of the mammalian pituitary. Moreover, unlike the mammalian adenohypophysis, the teleostean adenohypophysis does not contain a cleft (a remnant of the mammalian Rathke’s pouch), and so there is no clear morphological separation into a pars distalis and a pars intermedia as in mammals (Pogoda and Hammerschmidt, 2007); (ii) Fish lack both median eminence and a hypophyseal portal system. Instead, both neurohypophysis and adenohypophysis are directly innervated by hypothalamic nerve terminals (Ball, 1981) and apply a dual mode of gonadotrope regulation by GnRH, combining both neuro- glandular and neurovascular components (Golan et al., 2015); (iii) Cells producing distinct gonadotropin are well defined, spatially arranged, and grouped together in particular regions of the gland. In addition, while a single mammalian gonadotrope may produce both FSH and LH,fish gonadotropins are produced and secreted by distinct cell types (Levavi-Sivanet al., 2010).

LH cells exhibit close cell-to-cell contacts and form a continuous network throughout the gland while FSH cells are more loosely distributed, still maintaining some degree of cell-to-cell contact by cytoplasmic processes (Golanet al.,2015); (iv) Due to events of whole-genome duplications that have occurred duringfish evolution, as many as three GnRH-ligand andfive GnRH-receptor (GnRH-R) genes have been identified in certainfish species. By contrast, only one or two such variants occur in mammals. The gene duplication in GnRH ligands has led to segregation in the expression pattern and central function of each ligand. A species-specific variant (seabream-type GnRH; GnRH1) that occurs infish, is mainly located in the preoptic area, and plays a central role in regulating reproduction. A highly preserved variant (chicken-type cGnRH-II; GnRH2) in the midbrain may act as a neurotransmitter and/or neuromodulator. A salmon-type GnRH (sGnRH; GnRH3) that has been primarily identified in the terminal nerve ganglion and olfactory bulbs, is reported to have a neuromodulatory function (Zoharet al., 2010). GnRH receptors are G-protein coupled receptors (GPCR). Piscine GnRH-Rs lack their C-terminal tail; therefore, no GnRH desensitization exists (Levavi-Sivanet al., 2010).

Owing to their commercial value, the reproductive biology of the variousﬁsh species has been well studied. Thus, interest constantly increases inﬁsh as model organisms for basic science, as well as for the agricultural aspects.

Reproductive Strategies

The high variability offish and the ecological niches they inhabit has led to the adoption of various reproductive strategies including viviparity (Poecilia), sequential hermaphroditism (the protandrous seabream or the protogenous orange spotted grouper) and even simultaneous hermaphroditism (Black hamlet). Nevertheless, mostfish species are gonochoristic and oviparous, meaning that the sexes are separate and fertilization and embryonal development are external (Yaronet al., 2003). Fish may have synchronous ovaries and spawn only once in a season (carp) or even once in their lifetime (river eel and Atlantic salmon); or they can possess asynchro- nous (or group-synchronous) ovaries, producing several spawning events during the breeding season. During this period, their ovaries contain several generations of oocytes at various developmental stages. Spawning cycles of asynchronousfish may vary from 24 h in the gilthead seabream, up to one month in tilapia, (Yaronet al., 2003).

Fish with synchronous ovaries undergo sharp metabolic changes during the vitellogenic period that differ from those occurring duringﬁnal oocyte maturation and spawning. Endocrine changes that support each step reﬂect relatively simple and well-timed hormonal patterns. In contrast, simultaneous production and maintenance of several oocyte generations in asyn- chronous ovaries makes the sequential hormone release more complex. These endocrine differences are discussed in the following sections.

362 Encyclopedia of Reproduction, 2nd edition, Volume 6 https://doi.org/10.1016/B978-0-12-809633-8.20579-7

Encyclopedia of Reproduction, Second Edition, 2018, 362–368

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The Hypothalamus-Pituitary-Gonadal (HPG) Axis

The HPG axis consists of three major sites of endocrine regulation. A plethora of neuropeptides are secreted from speciﬁc neurons in the brain (primarily but not only in the hypothalamus) and form an interface between the central nervous system and the endocrine system. These neuropeptides may directly affect the gonadotropes in the pituitary, or may exert their action via the GnRH neurons.

This action stimulates the production and release of two distinct but chemically related gonadotropins: FSH and LH. FSH and LH are released into the circulation, and stimulate the gonads to secrete sex-speciﬁc steroids that are responsible for gametogenesis, vitellogenesis andﬁnal oocyte maturation in females, as well as spermatogenesis and spermiogenesis in males (Fig. 1).

The Hypothalamus and its Neurohormones

The plethora of neuropeptides and monoamines released by the brain were shown to regulate piscine reproductive functions. These include: GnRH, pituitary adenylate cyclase-activating peptide (PACAP), kisspeptins, Neurokinin B (NKB), Neurokinin F (NKF), Neuropeptide Y (NPY), ghrelin, secretoneurin, leptin, gonadotropin-inhibitory hormone (GnIH), dopamine, serotonin,g-amino- butyric acid (GABA), spexin, dynorphin, and more.

GnRH wasfirst isolated from mammals, and later from other vertebrates (Sherwood et al.,1983). To date, 30 structurally different forms of GnRH have been identified, while 12 structural variants of the GnRH molecule have been found in different fish species. The N-terminal amino acid sequence (pGlu-His-Trp-Ser) and carboxyl terminal amino acid sequence (Pro-Gly-NH₂) have been conserved over 400 million years of chordate evolution (Kochman, 2012). As indicated by its name, chicken GnRH II (GnRH-2) was first isolated from chicken brain and found to be the most ubiquitous form of GnRH molecule. The brain of most teleosts contains three forms of GnRH, encoded by three distinct genes and differ in their developmental origin, spatial distribution, and function. GnRH1 neurons are hypophysiotropic and are positioned in the preoptic area (POA); GnRH2 neurons reside in the midbrain tegmentum, and are suggested to be involved in food intake and reproductive behavior; and the GnRH3 neuronal population is localized in the terminal nerve and has no direct innervation to the pituitary (Zoharet al., 2010). Some teleost species have only two GnRH forms: the salmon and zebrafish genomes do not contain the GnRH1 form, while catfish and eel lack the GnRH3. The lack of these forms is compensated by the axonal projections of the remaining forms (Abe and Oka, 2011). For example, in zebrafish, some of the GnRH3 neurons developing in the terminal nerve migrate into the hypothalamic POA and inner- vate the pituitary, suggesting that they acquire the role GnRH1 fulfills in“three-form”fish (Zoharet al.,2010).

As in mammals, GnRH1 neurons in thefish POA display irregular and episodic spontaneous electrical activity with circadian fluctuations in the episodicfiring frequency. These irregular activities are in contrast to those of the non-hypothalamic populations

Fig. 1 Schematic diagram displaying HPG axis inﬁsh. Schematic diagram showing potential regulatory signals conveyed by hypothalamic neurons to pituitary cells, leading to the gonads (HPG axis).

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of GnRH neurons, i.e., GnRH2 and GnRH3, which display regular pacemaker activities, further supporting their functions other than–hypophysiotropic (Oka, 2010). In contrast to the similarities in the GnRH neurons’electrical activity betweenﬁsh and mammals, a striking difference can be seen in the structure and function of the GnRH-R. In mammals, constant delivery of high GnRH doses leads to hypoactivity of the gonad and disrupted estrous cycles, due to desensitization of the GnRH-R. In contrast, chronic administration of high doses of GnRH inﬁsh leads to increased gonadotropin release and even to spawning. (Zohar et al.,2010).

While hypothalamic GnRH is a positive regulator of the HPG axis, neuroendocrine release of hypothalamic dopamine serves as a negative effector of HPG activity in many, albeit not all,fish species. Dopamine is a neurotransmitter, synthesized from tyrosine by tyrosine hydroxylase (the rate-limiting enzyme) and Dopa-decarboxilase. Although dopamine is known to have pleiotropic effects on both brain and pituitary functions, its involvement in the endocrine regulation offish reproduction has been demonstrated in severalfish species, including goldfish, zebrafish, carp, African catfish, tilapia, eel, grey mullet, sturgeon, and rainbow trout [reviewed byLevavi-Sivanet al.(2010);Zoharet al.(2010)]. Contrasting these effects, dopaminergic inhibition was not observed in several marine species, suggesting that its involvement in reproductive function is mainly important in freshwaterfish. Dopamine receptors are GPCRs and are classically divided into two principal subtypes, according to their ability to activate (D1-like subtype) or inhibit (D2-like subtype) adenylyl cyclase, the key enzyme in the conversion of adenosine triphosphate to 3⁰5⁰-cyclic AMP. Various agonists and antagonists with high specificity to D1 or D2 receptors were employed to demonstrate that the inhibitory effects of dopamine onfish reproduction is due to specific activation of D2-like receptors. Fontaine and colleagues have demonstrated the expression of three D2-like receptors in LH cells in the adenohypophysis of the female zebrafish, and that the dopaminergic innervation of the adenohypophysis originates from the hypothalamic POA (Fontaineet al., 2015). Thesefindings provide neuroanatom- ical support for the existence of dopaminergic inhibition in piscine reproduction.

During recent years, a large surfeit of novel neuropeptides have been found to regulate the secretion of GnRH in mammals and other non-mammalian vertebrates. However, it was the identification of kisspeptin as an upstream regulator of the hypophysiotropic GnRH neurons that led to a significant change in the way that endocrinologists perceive neuroendocrine regulation of the HPG axis (Tena-Sempereet al., 2012). The discovery that GnRH neurons have Kiss1-receptors (a.k.a. GPR54) was thefirst indication that the kisspeptin system is involved in the control offish reproduction (Parharet al., 2004). Unlike most mammals, which possess only one kisspeptin system (KISS1), the majority of teleosts have two kisspeptin systems, comprised of Kiss1 and Kiss2; and two cognate receptors, Kiss1r and Kiss2r (Biranet al., 2008). Nevertheless, certainfish species have only one kisspeptin gene and two Kiss1R genes; in others, two kisspeptin genes were identified with only one Kiss1R gene. This inconsistency can be explained by the fact that bonyfish underwent two complete genome duplications during evolution, and many of these duplicated genes were subsequently lost (Mechalyet al., 2013). As in the case of GnRH, chronic administration of kisspeptin was found to suppress reproductive activity in rats and monkeys (Seminaraet al., 2006; Thompsonet al.,2006), while chronic administration of kisspeptin infish leads to increased reproductive functions (Nocilladoet al.,2013).

Deletion and site-directed mutagenesis of an estrogen-responsive element (ERE) from the kiss promoters of the protogynous orange-spotted grouper (Epinephelus coioides), indicated that Kiss1 is regulated by estradiol 17b(E2), via the classical pathway utilized by Erb1, as well as via an activator protein 1 (Ap1)-dependent, non-classical pathway utilized by Erb2. Kiss2 was also regulated by E2 through the Creb transcription factor, as well as by Erb1 and Erb2 pathways. The effects of gonadal steroids on kisspeptin in vivowere also demonstrated. In the medaka, Kiss1 neurons drastically change theirﬁring activity according to E2 levels, and in the European sea bass expression levels of Kiss2 were affected by circulating testosterone levels (Alvaradoet al.,2016). Taken together, accumulating data clearly support the assumption that E2 is involved in the feedback regulation of piscine kisspeptins via various estrogen receptors.

A relatively new neuropeptide found to be involved infish reproduction is Neurokinin B (NKB, encoded by the genetac3). NKB is a member of the tachykinin peptide family. The mammalian NKB is unique in that its prepro-hormone encodes a single mature peptide, while other prepro-tachykinins encode two mature peptides. Mammals also have one high affinity receptor for NKB (NKBR/NKR3). In contrast,fish have up to three NKB receptors and two genes encoding NKB. Importantly, prepro-tachykinin3 offish matures into two peptides. Because the additional peptide appears only infish and frogs, it was designated neurokinin F (NKF;Fig. 1) (Biranet al.,2012). Moreover, while kisspeptin and NKB are co-expressed by the same neurons in the mammalian hypothalamus, the piscine kisspeptin and NKB are expressed by different neuronal subsets of thefish hypothalamus (Ogawa et al.,2012). NKB-R are expressed in LH but not FSH cells, suggesting regulation offinal oocyte maturation by NKB via LH surge (Biranet al., 2014).In vivoadministration of NKB/NKF peptides increased reproductive functions in zebrafish, tilapia, goldfish and striped bass (Biranet al., 2012, 2014;Zmoraet al., 2017). However,tac3gene products in carp do not play a role in LH synthesis at the pituitary level, but may serve as novel stimulators for prolactin and somatolactin synthesis.

The Pituitary and its Gonadotropins

As in other vertebrates, the regulation offish reproduction involves the synthesis and secretion of hormones from the adenohypophysis, including FSH (GTH-I), LH (GTH-II), growth hormone (GH), thyroid-stimulating hormone (TSH) and adrenocortico- tropic hormone (ACTH), with FSH and LH serving as the key regulators of gonadal development and function. FSH and LH are heterodimeric hormones with a distinctbsubunit and a commonasubunit, which is also common to TSH). Over the years, gonadotropin subunit genes were isolated in more than 56fish species from at least 14 teleost orders, allowing a vast characterization of fish gonadotropin synthesis and release. Signals from hypothalamic neurohormones and gonadal steroids are integrated in 364 Endcocrine Control of ReproductionjEndocrine Control of Reproduction, Fish

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gonadotrope cells to regulate synthesis and release of both gonadotropins. Nonetheless, GnRH regulation of FSH synthesis and release is less clear than that of LH (Zoharet al.,2010; Yaronet al.,2003).

As previously described, there are two main modes of gonadal development in fish. Synchronous gonadal development is accompanied by a clear duality of gonadotropin secretion; while the adenohypophysis of immature salmon contains mainly FSH cells, the adenohypophysis of sexually mature rainbow trout contains numerus LH-producing cells. In addition, only FSH is released into the blood of immature coho salmon, with increased levels during the gonadal recruitment step, and reduced levels towardsfinal oocyte maturation and spawning. While both gonadotropins induce E2 secretion, LH is more potent than FSH in raising 17a,20bdihydroxy-4-pregnen-3-one (DHP) levels from post vitellogenic oocytes (Swanson, 1991). However, in carp, LH alone is sufficient to regulate both vitellogenesis andfinal oocyte maturation while FSH may have another, yet undefined role.

In contrast, both FSH and LH are synthesized and released during all stages of reproduction in multi-spawningﬁsh with asynchro- nous ovaries like the gilthead seabream, goldﬁsh and tilapia (reviewed byLevavi-Sivanet al.(2010);Yaronet al.(2003)). These differences are probably due to the need to regulate the development of several oocyte generations in parallel. In this situation, the differential regulation of each oocyte generation may be attributed to differences in the expression of gonadotropin receptors.

Thefish FSH and LH receptors are GPCRs belonging to the Class A of rhodopsin-like receptors. Mammalian FSHR and LHR show very high selectivity to their cognate ligands, leading to functional specificity. However, ligand-receptor promiscuity was shown in severalfish species. Structural modeling combined with in vitro assays of gonadotropin receptor activation have demonstrated that in somefish species, FSHR may bind both FSH and LH, while in almost allfish species tested, LHR is highly specific for LH (Aizen et al., 2012). Concomitant with their importance in induction of ovarian steroidogenesis, gonadotropin receptors are expressed by follicular cells of the ovary (theca and granulosa cells) and by interstitial and nurse cells of the testis (Leydig and Sertoli cells) (Yaron and Sivan, 2006).

Gonadal steroids transmit their feedback to regulate the synthesis and release of FSH and LH. This feedback can be of a positive or negative nature, depending on the reproductive state of thefish. Gonadectomy in sexually mature goldfish, catfish, Atlantic croaker, bass and salmonids results in increased LH secretion, while treatment with gonadal steroids may reverse this effect. Never- theless, during earlier stages of gonadal development, gonadal steroids can induce LHbsubunit expression and protein levels (Levavi-Sivanet al., 2010; Yaronet al., 2003). More anatomical work is required to understand whether these effects are direct or indirect.

LH acts on the ovarian follicle to produce DHP, the maturation-inducing Hormone, (MIH) in mostﬁshes. The dramatic increase in the capacity of post vitellogenic follicles to produce DHP in response to LH is correlated with a decrease in P450c17 (P450c17-I) and P450 aromatase (oP450arom) mRNA and increase in the novel form of P450c17 (P450c17-II) and 20b-hydroxysteroid dehy- drogenase (20b-HSD) mRNA.

The Gonads and Their Steroids

Mostfish species used in research and aquaculture are gonochoristic. Therefore, a sexually maturefish would have either a functional testis (male) or a functional ovary (female). In most gonochoristicfish, FSH and LH stimulate the synthesis of three key sex steroids:

E2 serves as the principal estrogen and stimulates germ-cell proliferation and growth and vitellogenesis; 11 ketotestosterone (11-KT) serves as the central androgen that regulates spermatogenesis and spermiogenesis; DHP regulatesﬁnal oocyte maturation and ovulation in females as well as spermatozoa maturation and spermiation in males (Yaron, 1995). In addition, DHP is involved in the meiosis onset of ogonia in the ovary and of spermatogonia in the testis. Androgens and E2 (under FSH) are involved in the appear- ance of lipid droplets in previtellogenic oocyes. It is noteworthy that higher vertebrates utilize testosterone and not 11-KT as their main androgenic hormone. These pathways are regulated in parallel to the above-mentioned feedback regulation of gonadal steroids on reproductive functions of the hypothalamus and pituitary (Fig. 2).

Male

In spermiogenesis, non-functional sperm undergo the process of sperm maturation to become mature spermatozoa, fully capable of vigorous motility and fertilization. These processes are mainly controlled by sex steroid hormones. Spermatogonial renewal is controlled by E2 through the expression of platelet-derived endothelial cell growth factor. The proliferation of spermatogonia toward meiosis is initiated by 11-KT, which is produced by FSH stimulation. 11-KT prevents the expression of anti-Müllerian hormone (AMH), which functions to inhibit proliferation of spermatogonia and induce expression of activin B, which functions in the induction of spermatogonial proliferation. Meiosis is induced by DHP through the action of trypsin. DHP also regulates sperm maturation through the regulation of seminal plasma pH. Gonadotropic stimulation of Leydig cells in the testis induce the synthesis and release of 11-KT, which leads to the activation of Sertoli cells, resulting in stimulation of spermatogenesis.

With the advancement of spermatogenesis, 11-KT levels start to decline and DHP levels rise, leading to induction of spermiogenesis.

In someﬁsh species, DHP was shown to further regulate sperm motility (Miura and Miura, 2011;Schulzet al., 2010) (Fig. 2).

Female

In the case of femalefish, the endocrine roles of gonadal steroids are more complex; alongside their roles in feedback regulation and gonadal development, gonadal estrogens induce the synthesis and release of vitellogenin (VTG), the primary storage protein infish oocytes, by the female liver (Lubzenset al.,2017). Infish with synchronous ovaries, theca cells of the follicle respond to FSH to produce testosterone, which is then aromatized into E2 by the granulosa cells. E2 stimulates the production of VTG and in parallel

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regulates its accumulation in the yolk. Following the vitellogenic growth of the oocyte, FSH levels decline concurrently with the rise in LH levels. This leads to shifts in steroidogenesis that leads the ovary to produce mainly DHP (Yaron, 1995).

In addition to the classic genomic mechanism of steroid action mediated by activation of intracellular nuclear receptors, there is now extensive evidence that steroids also activate receptors on the cell surface to initiate rapid intracellular signaling and biological responses that are often non-genomic. Two recently discovered novel proteins with seven-transmembrane domains, GPCR30 (GPR30), and membrane progestin receptors (mPRs), have the ligand binding and signaling characteristics of estrogen and progestin membrane receptors, respectively. During theﬁrst phase–at the end of vitellogenic oocyte growth–the ovarian follicles produce large amounts of estrogens and minor quantities of progestins, resulting in activation of the inhibitory pathway regulating oocyte maturation, but not the stimulatory pathway. The estrogens act through GPR30 to activate a stimulatory G protein (Gs), resulting in stimulation of adenylyl cyclase activity and increases in cAMP production. The high levels of cAMP maintain meiotic arrest ofﬁsh oocytes, possibly through downstream signaling molecules such as protein kinase A. In addition, estrogens upregulate GPR30 expression to potentiate the inhibitory pathway and down-regulate mPRaexpression to block the stimulatory one, through activation of GPR30 via unknown pathways. As a result, the oocytes remain in meiotic arrest. In the second phase, the steroidogenic pathway is switched to the production of progestins and estrogen production is declined. At the same time, progestins, through mPRa, cause down-regulation of GPR30 expression, which together with the reduction in estrogen levels, causes suppression of the inhibitory pathway. Progestin binding to mPRaresults in activation of an inhibitory G protein (Gi) decreasing adenylyl cyclase activity and cAMP levels, leading to inhibition of protein kinase A, thereby releasing the oocyte from meiotic arrest and allowing oocyte maturation to proceed (Thomas, 2012).

Genome Editing Era in Fish Reproduction

The emergence of the novel methods of transcription activator-like effector nucleases (TALEN) (Bedellet al., 2012) and clustered regularly interspaced palindromic repeats-associated (CRISPR)/Cas9 (Gagnonet al.,2014) systems, commenced a technological revolution infish genome editing, allowing reverse genetics infish. These methods are now applicable in severalfish species of commercial importance, including Atlantic salmon (Edvardsen et al., 2014), channel catfish (Khalilet al., 2017), Nile tilapia (Wuet al., 2016), sea lamprey (Squareet al., 2015) and modelfish species zebrafish and medaka (Bedellet al., 2012; Gagnon et al.,2014; Luoet al., 2015). These tools have already provided some excitingfindings regarding the endocrine regulation of fish reproduction.

At the hypothalamic level in zebrafish, no phenotypes were found infish carrying single or double mutations in genes encoding kisspeptin ligands or their receptors (Tanget al.,2015). Similarly, no phenotypes were identified in GnRH3 mutant zebrafish, and even triple knockoutfish for both kisspeptin ligands and GnRH3 resulted in normal reproductive development (Spiceret al., 2016;

Liuet al., 2017). Theseﬁndings strongly challenge the current dogma of neuroendocrine regulation ofﬁsh reproduction.

At the pituitary level, zebraﬁsh carrying homozygous mutation in the gene encoding FSHb-subunit, display delayed gonadal maturation; and mutants in the gene encoding LHb-subunit show normal gonadal development but fail to spawn and hence are infertile (Zhanget al., 2015). Double mutant zebraﬁsh for both gonadotropins or both gonadotropin receptors develop an Fig. 2 Endocrine control of spermatogenesis A schematic diagram summarizing the possible control mechanisms of spermatogenesis in the Japa- nese eel. FSH, follicle-stimulating hormone; LH, luteinizing hormone; 17a,20b-DHP, 17a,20b-dihydroxy-4-pregnen-3-one; PD-ECGF, platelet-derived endothelial cell growth factor; AMH, anti-Müllerian hormone; CAll, carbonic anhydrase; eSRS, spermatogenesis related substances. Reproduced from Miura, C., Miura, T., 2011. Analysis of spermatogenesis using an eel model. Aqua-Biosci. Monogr. 4, 105–129.

366 Endcocrine Control of ReproductionjEndocrine Control of Reproduction, Fish

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infertile all-male population phenotype. However, while FSHR mutant females are infertile, FSHR mutant males show no signiﬁ- cant phenotype (Chuet al., 2015).

At the gonadal level,“loss of function”mutations in the zebraﬁsh aromatase gene resulted in an all-male phenotype (Lauet al., 2016); and mutating the nuclear progesterone receptor in zebraﬁsh leads to female infertility due to ovulation failure with no reproductive phenotype in males (Tanget al., 2016).

Taken together, thesefindings demonstrate the high importance and strength of genomic editing infish. Nevertheless, it also highlight the need for additional knockout lines in other commercially importantfish models.

Concluding Remarks

As in other vertebrates, inﬁsh, endocrine regulation of reproduction is governed by the HPG axis. In the current article, we brieﬂy reviewed the endocrine components of the axis with emphasis on the unique and different characteristics of these components.

Taken together, the data presented show the high evolutionary importance ofﬁsh as model organisms for endocrine research in the reproductiveﬁeld; they are an exciting model for studying vertebrate reproduction. Moreover, some features in the piscine HPG axis may provide explanations to phenomena that cannot be addressed in higher vertebrates. For example, the anatomical separation between FSH and LH enables the characterization of regulatory pathways for each gonadotropin, while their co- expression in mammals would hamper such efforts.

Finally, there is no doubt that novel genomic tools will significantly advance endocrine research infish, which will lead to important breakthroughs. Nevertheless, adopting these methodologies should be advanced in concert with development of additional and more classical endocrine tools, such as specific hormone ELISAs, antibodies and anatomical research.

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