Spawning kinetics and egg/larval quality of greater amberjack (Seriola dumerili) in response to multiple GnRHa injections or implants

Gonadotropin expression, pituitary and plasma levels in the reproductive cycle of wild and captive-reared greater amberjack (Seriola dumerili)

We compared the endocrine status of the pituitary-gonad axis of wild and captive-reared greater amberjack (Seriola dumerili) during the reproductive cycle (April - July), reporting on the expression and release of the two gonadotropins for the first time in the Mediterranean Sea. Ovaries from wild females were characterized histologically as DEVELOPING in early May and SPAWNING capable in late May-July, the latter having a 3 to 4-fold higher gonadosomatic index (GSI). SPAWNING capable wild females exhibited an increase in pituitary follicle stimulating hormone (Fsh) content, plasma testosterone (T) and 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P), while almost a 10-fold increase was observed in pituitary luteinizing hormone (Lh) content. An increasing trend of plasma 17β-estradiol (E2) was also recorded between the two reproductive stages in wild females. Captive-reared females sampled during the reproductive cycle exhibited two additional reproductive categories, with REGRESSED females having extensive follicular atresia and fish in the REGENERATING stage having only primary oocytes in their ovaries. Pituitary content of Fsh and Lh, fshb and lhb expression and plasma levels of Fsh and Lh remained unchanged among the four reproductive stages in captive females, in contrast with plasma E2 and T that decreased in the REGENERATING stage, and 17,20β-P which increased after the DEVELOPING stage. In general, no significant hormonal differences were recorded between captive-reared and wild DEVELOPING females, in contrast to SPAWNING capable females, where pituitary Lh content, plasma Fsh and T were found to be lower in females in captivity. Overall, the captive females lagged behind in reproductive development compared to the wild ones and this was perhaps related to the multiple handling of the sea cages where all the sampled fish were maintained. Between wild males in the DEVELOPING and SPAWNING capable stages, pituitary Lh content, plasma T and 17,20β-P, and GSI exhibited 3 to 4-fold increases, while an increasing trend of pituitary Fsh content, lhb expression levels and plasma 11-ketotestosterone (11-KT) was also observed, and an opposite trend was observed in plasma Lh. Captive males were allocated to one more category, with REGRESSED individuals having no spermatogenic capacity. During the SPAWNING capable phase, almost all measured parameters were lower in captive males compared to wild ones. More importantly, captive males showed significant differences from their wild counterparts throughout the reproductive season, starting already from the DEVELOPING stage. Therefore, it appears that captivity already exerted negative effects in males prior to the onset of the study and the multiple handling of the cage where sampled fish were reared. Overall, the present study demonstrated that female greater amberjack do undergo full vitellogenesis in captivity, albeit with some dysfunctions that may be related to the husbandry of the experiment, while males, on the other hand, may be more seriously affected by captivity even before the onset of the study.

Effects of different hormonal treatments on spermatogenesis advancement in hatchery-produced greater amberjack Seriola dumerili (Risso 1810)

In earlier studies, wild-caught greater amberjack Seriola dumerili (Risso, 1810) males reared in sea cages showed gametogenesis impairment and low sperm production and quality. Here, we (a) examined if F1 hatchery-produced males reared in sea cages also exhibit reproductive dysfunctions and (b) evaluated the effects of gonadotropin releasing hormone agonist (GnRHa) administration through injections (GnRHainj) or sustained-release implants (GnRHaimpl), and human chorionic gonadotropin (hGC) injections on spermatogenesis/spermiation enhancement. Fish were given a hormone treatment just prior to the spawning season, and were transferred to land-based tanks, according to an established spawning induction protocol. Blood samples (n = 6) were obtained on Days 0, 7 and 13 after treatment. Testis samples were obtained on Days 0 (n = 4) and 13 (n = 2 per treatment). The fish prior to their transfer from the sea cages to the land-based tanks, exhibited a low gonadosomatic index, altered sex steroid hormone profile and high density of testicular apoptotic cells. After transfer to tanks, there was a general depression of sex steroid plasma levels parallel to an increase in cortisol concentrations. Despite the negative effect on steroidogenesis by the transfer from the sea, the hormonal treatments increased the number of fish from where sperm could be obtained, as well as testis growth, and reduced testicular apoptosis. Treatment with hCG resulted in the most significant changes in spermatogenesis, while GnRHaimpl appeared to induce less intense, but likely longer-lasting effects. The study indicated that F1 hatchery-produced males also exhibited reproductive dysfunctions as wild-caught captive-reared greater amberjack, and that the observed positive effects of the hormone treatments on spermiation/spermatogenesis were likely mediated by factors other than sex steroid hormones.

Timing of puberty in F1-generation hatchery-produced greater amberjack (Seriola dumerili)

We evaluated the onset of puberty of first-generation (F1) hatchery-produced greater amberjack (Seriola dumerili) reared in sea cages for 5 years. Fish were sampled every year in June, at the expected peak of the spawning period in the Mediterranean Sea. No sexual dimorphism in body weight was observed in the study. The ovaries of 1 and 2-year-old (yo) females consisted of primary oocytes only, while at the age of 3-yo early vitellogenic (Vg) oocytes were also identified, but with extensive follicular atresia. At the age of 4-yo, late Vg oocytes were observed, but again extensive follicular atresia characterized the ovaries of 50 % of females. At the age of 5-yo, follicular atresia of Vg oocytes was very limited. In males, gametogenesis was evident already in 1- and 2-yo fish, and 100 % of sampled 3-yo males produced collectable viable sperm. Plasma testosterone (T), 17β-estradiol (E2), and 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) remained similar in 3 - 5-yo females, with T and E2 levels being highest in females in advanced vitellogenesis or with significant follicular atresia, compared to immature females. In males, plasma T declined over the years, while 11-ketotestosterone (11-KT) and 17,20β-P were highest in 4 and 5-yo males, with spermatozoa motility characteristics being improved from the 4th year onwards. The administration of GnRHa implants to 5-yo fish induced only two spawns, albeit no fertilized eggs were obtained. The results indicate that hatchery-produced greater amberjack males mature well and within the same age observed in the wild, however with smaller gonad size. On the contrary, females mature later than in the wild, also with a smaller gonad size. Spawning in response to GnRHa treatment was not effective, suggesting that Mediterranean hatchery-produced broodstocks may be dysfunctional, and further research is needed to document any improvement as the fish get older, or to determine if the results may be related to the specific stock of fish.

Dysregulation of testis mRNA expression levels in hatchery-produced vs wild greater amberjack Seriola dumerili

Reproductive dysfunctions have been recently documented in male greater amberjack Seriola dumerili caught from the wild and reared in captivity. In the present study, we compared testis transcriptome in wild fish (WILD), hatchery-produced fish with apparently normal spermatogenesis (Normal Farmed; NormalF) and hatchery-produced fish with evident reproductive dysfunction (Dysfunctional Farmed; DysF). Gene expression analysis identified 2157, 1985 and 74 differentially expressed genes (DEGs) in DysF vs WILD, NormalF vs DysF and NormalF vs WILD comparisons, respectively. In DysF, a dysregulation of several interconnected biological processes, including cell assembly, steroidogenesis and apoptosis was found. Gene enrichment of progesterone-mediated oocyte maturation, oocyte meiosis and cell cycle pathways were identified in the DysF vs NormalF comparison. Most of the DEGs involved in the enriched pathways were downregulated in DysF. The comparison of NormalF vs WILD showed that most of the DEGs were downregulated in NormalF, including a gene that encodes for a regulatory protein with a protective role in apoptosis regulation (ptpn6), indicating that spermatogenesis was dysfunctional also in the apparently "normal" hatchery-produced fish. Hence, rearing of male greater amberjack in captivity, from eggs produced by captive breeders, did not prevent the appearance of reproductive dysfunctions, and these dysfunctions involved several biological processes and metabolic pathways.

Sex-Inclined Piwi-Interacting RNAs in Serum Exosomes for Sex Determination in the Greater Amberjack (Seriola dumerili)

The greater amberjack (Seriola dumerili) is a gonochoristic fish with no sexual dimorphism in appearance, making sex identification difficult. Piwi-interacting RNAs (piRNAs) function in transposon silencing and gametogenesis and are involved in various physiological processes, including sex development and differentiation. Exosomal piRNAs can be indicators for the determination of sex and physiological status. In this study, four piRNAs were differentially expressed in both serum exosomes and gonads between male and female greater amberjack. Three piRNAs (piR-dre-32793, piR-dre-5797, and piR-dre-73318) were significantly up-regulated and piR-dre-332 was significantly down-regulated in serum exosomes and gonads of male fish, compared to female fish, consistent with the serum exosomal results. According to the relative expression of four marker piRNAs derived from the serum exosomes of greater amberjack, the highest relative expression of piR-dre-32793, piR-dre-5797, and piR-dre-73318 in seven female fish and that of piR-dre-332 in seven male fish can be used as the standard for sex determination. The method of sex identification can ascertain the sex of greater amberjack by blood collection from the living body, without sacrificing fish. The four piRNAs did not show sex-inclined expression in the hypothalamus, pituitary, heart, liver, intestine, and muscle tissue. A piRNA-target interaction network involving 32 piRNA-mRNA pairs was generated. Sex-related target genes were enriched in sex-related pathways, including oocyte meiosis, transforming growth factor-beta signaling pathway, progesterone-mediated oocyte maturation, and gonadotropin releasing hormone signaling pathway. These results provide a basis for sex determination in greater amberjack and improve our understanding of the mechanisms underlying sex development and differentiation in the species.

Spawning Performance and Sex Steroid Levels in Female Pikeperch Sander lucioperca Treated with Poly(lactic-co-glycolic acid) Microparticles

Simple Summary

Pikeperch Sander lucioperca is a promising candidate for intensive aquaculture. However, controlled reproduction has become the major bottleneck in pikeperch production. To improve and optimize its artificial reproduction, an effective method for hormone treatment is needed. The use of a poly(lactic-co-glycolic acid) microparticle-sustained-release system to administer gonadotropin-releasing hormone agonist to pikeperch resulted in acceptable reproductive output. Our results establish the potential of poly(lactic-co-glycolic acid) microparticle as a novel tool for hormone treatment in fish.

Abstract

Pikeperch Sander lucioperca is a piscivorous species considered a promising candidate for the diversification of intensive aquaculture. This study aimed to determine the effect of a sustained-release delivery system incorporating mammalian gonadotropin-releasing hormone agonist (mGnRHa) into poly(lactic-co-glycolic acid) (PLGA) microparticles on the sex steroid levels and aspects of artificial reproduction of pikeperch. Fish were divided into four groups and injected with 20 µg mGnRHa/kg, 5-day release microparticles encapsulated with 5 µg GnRHa/kg BW (PLGA 5), 20 µg GnRHa/kg (PLGA 20), or 1 mL/kg 0.9% NaCl (control). Cumulative percentage ovulation was 100% in the PLGA 5 group, significantly higher than in other tested groups. No differences among groups were observed in latency or fecundity. The level of 11-ketotestosterone (11-KT) peaked at 40 h post-injection, and was sustained during ovulation, in all treated groups. The 17β-estradiol (E2) concentration increased in the mGnRHa-only group immediately after hormone injection, while both PLGA groups showed a reduction in E2 after injection, continuing to decrease until ovulation. A low dose of mGnRHa in PLGA microparticles significantly improves induction of ovulation and results in acceptable reproductive performance, which may positively affect pikeperch production under controlled conditions.

Sperm quality of greater amberjack Seriola dumerili throughout the reproductive season and in response to GnRHa treatment with controlled release implants

A wild-caught stock of greater amberjack Seriola dumerili reared in sea cages in two commercial aquaculture facilities in Greece was administered gonadotropin-releasing hormone agonist (GnRHa), loaded in controlled release implants, and was used in spawning induction experiments throughout the reproductive season. Sperm quality was evaluated using computer-assisted sperm analysis (CASA) before and after GnRHa administration and the extent of the spermiation period was determined. Males were in spermiation throughout the monitoring period from May 30 until July 18, at temperatures between 19 and 24 °C. However, lower sperm motility duration, density, and survival under cold storage were observed from early July onwards. Sperm quality did not recover after the end of spawning induction experiments in tanks and the return of the fish for 14-28 days to the sea cage in mid-July, which could be related to the high temperatures of this period. An improvement trend was observed in the quality of the milt collected on day 7 after a single GnRHa administration, but a significant decrease was observed on day 21 in sperm density, survival under cold storage, and straight line velocity (VSL). On the contrary, a double GnRHa administration spaced 14 days apart maintained the same sperm quality for a longer period of 29 days. Further spermiation enhancement experiments should be conducted in the future to describe in more detail the kinetics of sperm production after GnRHa therapy and its effects on sperm quality.

Increased melanomacrophage centres in the liver of reproductively dysfunctional female greater amberjack Seriola dumerili (Risso, 1810)

The greater amberjack Seriola dumerili is a new aquaculture fish that may display reproductive dysfunctions. During extensive follicular atresia, which is a common reproductive dysfunction in females during vitellogenesis, part of the reabsorbed yolk returns to the liver to be metabolized and recycled. Melanomacrophage centres (MMCs) are aggregates of macrophage-like cells that play a role in the destruction, detoxification and recycling of endogenous and exogenous materials, and have been associated with systemic stress. Wild and captive-reared greater amberjack were sampled in the Mediterranean Sea during two different phases of the reproductive cycle. The liver of reproductively dysfunctional captive-reared females sampled during the spawning season showed a high density of both MMCs and apoptotic cells. A weak liver anti-cytochrome P450 monooxygenase 1A immunoreactivity was observed, suggesting that the examined fish were not exposed to environmental pollutants. We propose that the observed increase in MMCs and apoptosis in captive-reared fish was related to the hepatic overload associated to the metabolism of yolk proteins reabsorbed during extensive follicular atresia. Since follicular atresia is a frequent physiological and pathological event in teleosts, we suggest that the reproductive state should be always assessed when MMCs are used as markers of exposure to stress or pollutants.

Comparative transcriptome analysis of male and female gonads reveals sex-biased genes in spotted scat (Scatophagus argus)

Scatophagus argus is a new emerging aquaculture fish in East and Southeast Asia. To date, research on reproductive development and regulation in S. argus is lacking. Additionally, genetic and genomic information about reproduction, such as gonadal transcriptome data, is also lacking. Herein, we report the first gonadal transcriptomes of S. argus and identify genes potentially involved in reproduction and gonadal development. A total of 136,561 unigenes were obtained by sequencing of testes (n = 3) and ovaries (n = 3) at stage III. Genes upregulated in males and females known to be involved in gonadal development and gametogenesis were identified, including male-biased dmrt1, amh, gsdf, wt1a, sox9b, and nanos2, and female-biased foxl2, gdf9, bmp15, sox3, zar1, and figla. Serum estradiol-17β and 11-ketotestosterone levels were biased in female and male fish, respectively. Sexual dimorphism of serum steroid hormone levels were interpreted after expression analysis of 20 steroidogenesis-related genes, including cyp19a1a and cyp11b2. This gonadal transcript dataset will help investigate functional genes related to reproduction in S. argus.

Effect of GnRHa on plasma levels of Fsh and Lh in the female greater amberjack Seriola dumerili

The effects of gonadotropin-releasing hormone agonist (GnRHa) on plasma levels of follicle-stimulating hormone (Fsh) and luteinising hormone (Lh) are reported for female greater amberjack Seriola dumerili with post-vitellogenic ovarian oocytes. Five females were implanted with pellets containing GnRHa (600 μg kg^-1 body weight), while five other females were injected with saline. All females implanted with GnRHa-containing pellets ovulated 36-42 h post-implantation. The GnRHa implants elevated Lh, but not Fsh plasma levels within 42 h of GnRHa administration.

Vitellogenin receptor and fatty acid profiles of individual lipid classes of oocytes from wild and captive-reared greater amberjack (Seriola dumerili) during the reproductive cycle

The greater amberjack Seriola dumerili (Risso, 1810) is a large migratory pelagic fish occurring in tropical and temperate waters with a great potential for the world aquaculture industry. Previous studies showed that wild-caught female greater amberjack reared in sea cages and handled during the reproductive season, underwent extensive ovarian atresia. This atresia, however, was not related to an insufficient liver transcription or oocyte uptake of vitellogenin (Vtg). In the present study, the structure of two greater amberjack vitellogenin receptors, namely Vtgr (Lr8-) and Lrp13, was characterized. Moreover, vtgr and lrp13 gene expression and the fatty acid profiles of specific phospholipids and neutral lipids were compared in the ovaries of wild and captive-reared greater amberjack during different phases of the reproductive cycle (i.e. early gametogenesis, advanced gametogenesis and spawning). Ovarian vtgr and lrp13 transcription was more active during early gametogenesis, suggesting that vitellogenin receptor transcripts were synthesized by previtellogenic oocytes and remained in the cellular mRNA pool until oocytes resumed meiosis and entered into secondary growth (i.e. vitellogenesis). Rearing of wild-caught greater amberjack in captivity together with handling during the reproductive season was associated with a reduced vtgr and lrp13 transcription and with a diminished capacity of oocytes in the early phase of gametogenesis (primary oocyte growth) to enter into vitellogenesis. During early gametogenesis, remarkable differences in the fatty acid composition were observed between wild and captive-reared individuals: all phospholipids of captive fish displayed dramatic increases of saturates (16:0 and 18:0) and decreases of arachidonic acid (ARA) and docosahexaenoic acid (DHA). The present study confirms the susceptibility of greater amberjack reproductive function to handling stress and suggests that the consequent extensive atresia of vitellogenic follicles originated during the primary oocytes growth when the capacity of oocytes to synthesize vitellogenin receptors was reduced. The study also suggests that this reduced capacity was associated with an altered oocyte phospholipid fatty acid composition during early gametogenesis.