Seasonal changes and endocrine regulation of pejerrey (Odontesthes bonariensis) spermatogenesis in the wild

Influence of sewage discharge on dissolved oxygen concentration and fish diversity in the Girado stream and Chascomús lake

Hypoxic aquatic environments have increased in recent decades mainly by human activities that generate pollution. The objective of this work was to study the influence of the discharge of sewage effluents from Chascomús city on the concentration of dissolved oxygen (DO) in the Girado stream and its connection with Chascomús lake and analyze the impact on fish biodiversity. Four sites were selected to measure DO and temperature weekly for one year: discharge of sewage effluent, union of the sewage effluent with the Girado stream, Girado stream and Chascomús lake. Also, ichthyological samplings were carried out in a presumed hypoxic zone and a normoxic zone of the Girado stream. The results showed that there were two hypoxic zones corresponding to the sewage effluent and the union of the effluent with the Girado stream (~3 mg l-1). Also, two normoxic zones were identified, the course of the Girado stream and the union with Chascomús lake (~ 8 mg l-1). In these areas the water temperature varied according to the seasonality of the region (from 10 to 30 °C) without differences between zones. It is important to note that these areas received a constant flow of sewage effluent from Chascomús city (~ 885,600 l per day). In the fish sampling carried out in the 4 seasons of the year, 14 typical species of the Pampas lakes were captured with species more tolerant to low DO. Pejerrey were captured in a small number in both sites but with no signs of abnormalities in the gonads.

Sex reversal of pejerrey (Odontesthes bonariensis), a species with temperature-dependent sex determination, in a seminatural environment

This study examined the changes in sex ratios and sex reversal rates in pejerrey Odontesthes bonariensis that occur with the progression of the spawning season in a seminatural setting. Four groups of hatchery-produced pejerrey larvae were stocked in floating cages in La Salada de Monasterio lake (Pampas region), a natural habitat of this species, and reared from hatching beyond gonadal sex determination with minimum human interference. Cage 1 was stocked at the beginning of the spring spawning season and the other cages were stocked with monthly delays until cage 4 in early summer. The genotypic (amhy+, XY/YY; amhy-, XX) and phenotypic (testis, male; ovary, female) sex ratios and proportions of genotype/phenotype mismatched individuals were estimated and their relation to water temperature and daylength during the experiment was analysed by generalized linear modelling. Water temperature varied between 11 and 30.5°C, and daylength duration between 11 h 22 min and 14 h 35 min. Sex genotyping revealed nearly balanced sex ratios of XY/YY (46%-49.1%) and XX (50.9%-54%) fish in cages 2-4 whereas the genotypic sex ratio in cage 1 was clearly biased towards XY/YY fish (60.6%). Phenotypic males ranged from 42% to 54.4% in cages 1-3. Cage 4, in turn, had significantly more phenotypic males (66%). The percentage of XX males (phenotypic male/genotypic female) was 23.1% in cage 1, decreased to a minimum of 5.4% in cage 2 and gradually increased in cages 3 and 4 to a maximum of 40.7% in the latter. The percentages of XY/YY females (phenotypic female/genotypic male) were highest in cage 1 (30%) and decreased progressively in the other cages to a significantly lower value (4.3%) in cage 4. These results generally support the findings of laboratory studies on the effect of temperature on the sex determination of this species and also provide novel evidence of a XX genotype-specific masculinizing effect of short daylength.

Effects of Anthropic Pollutants Identified in Pampas Lakes on the Development and Reproduction of Pejerrey Fish Odontesthes bonariensis

Anthropic activities can seriously affect the health of the organisms inhabiting them, and the observation of any alteration in the reproduction of fish could be associated with the presence of endocrine disruptors. In this manuscript we have collected information on the adverse effects of pollutants (heavy metals, environmental steroids, and agrochemicals), present in Chascomús lake, Argentina, either at environmentally relevant and pharmacological concentrations on reproduction, embryonic development, and larval survival of pejerrey fish Odontesthes bonariensis. During development, it has been reported that 17β-estradiol (E₂) feminized and reduced larval survival, while 17α-ethinyl-estradiol (EE₂) not only feminized but also affected both embryo and larval survival. In adult male fish, treatments with EE₂ and E₂ + EE₂ were able to increase mRNA abundance of gnrh3 and cyp19a1b and decreased those of gonadotropin receptors (fshr and lhcgr). Heavy metals such as cadmium, chromium, and copper negatively affected sperm quality, diminishing the motility. Also, a decrease in the percentage of hatching rate and larval survival was also observed with the same metals, highlighting zinc as the most detrimental metal. Furthermore, all these metals altered the expression of hypothalamic and pituitary genes related to reproduction in male pejerrey (gnrh1,2,3; cyp19a1b; fshb; lhb; fshr and, lhcgr). Moreover, in all cases pyknotic cells, corresponding to the degeneration of the germ cells, were observed in the testes of exposed fish. For agrochemicals, exposure of male pejerrey to environmental concentrations of glyphosate did not cause alterations on the endocrine reproductive axis. However, male pejerrey with gonadal abnormalities such as the presence of intersex (testis-ova) gonads were found in other Pampa´s lakes with high concentrations of atrazine and glyphosate associated with soybean and corn crops near their coasts. These types of studies demonstrate that pejerrey, an endemic species with economic importance inhabiting the Pampas shallow lakes, can be used as a sentinel species. It should be noted that increased pollution of aquatic ecosystems and the effects on the reproduction of organisms can lead to a decline in fish populations worldwide. Which, added to overfishing and other external factors such as global warming, could cause an eventual extinction of an emblematic species.

Gonadal development in pejerrey (Odontesthes bonariensis) during spawning season in relation with sex steroids and temperature variation in Gómez lake (Pampas region, Argentina)

Gómez lake (34 ° 39 'S 61 ° 01' W) is a typical shallow lake of Pampas region placed in the upper area of the Salado river. The most abundant fish species in this lake is the pejerrey (Odontesthes bonariensis) valued due to the quality of its flesh and its attractiveness as a game fish. The aim of this study was to describe for the first time in this pejerrey wild population the gonadal stages during three consecutive spawning seasons (August to December) in relation with sexual steroids and temperature in this lake. In general, pejerrey gonadal development, the gonadosomatic index and the plasma levels of estradiol and testosterone varied in relation to air temperature. During the sampling period, pejerrey females started to ovulate in early August (winter), with a peak in October and ending in December with some of them with atretic oocytes. For males, it was possible to find spermiating animals during the whole spawning season and some arrested animals only in December. Our results confirm the relationship with pejerrey maturation and temperature and can be useful for decision making in the management of this natural resource.

Regulation of spermatogenesis and reproductive capacity by Igf3 in tilapia

A novel insulin-like growth factor (igf3), which is exclusively expressed in the gonads, has been widely identified in fish species. Recent studies have indicated that Igf3 regulates spermatogonia proliferation and differentiation in zebrafish; however, detailed information on the role of this Igf needs further in vivo investigation. Here, using Nile tilapia (Oreochromis niloticus) as an animal model, we report that igf3 is required for spermatogenesis and reproduction. Knockout of igf3 by CRISPR/Cas9 severely inhibited spermatogonial proliferation and differentiation at 90 days after hatching, the time critical for meiosis initiation, and resulted in less spermatocytes in the mutants. Although spermatogenesis continued to occur later, more spermatocytes and less spermatids were observed in the igf3-/- testes when compared with wild type of testes at adults, indicating that Igf3 regulates spermatocyte to spermatid transition. Importantly, a significantly increased occurrence of apoptosis in spermatids was observed after loss of Igf3. Therefore, igf3-/- males were subfertile with drastically reduced semen volume and sperm count. Conversely, the overexpression of Igf3 in XY tilapia enhanced spermatogenesis leading to more spermatids and sperm count. Transcriptomic analysis revealed that the absence of Igf3 resulted in dysregulation of many genes involved in cell cycle, meiosis and pluripotency regulators that are critical for spermatogenesis. In addition, in vitro gonadal culture with 17α-methyltetosterone (MT) and 11-ketotestosterone (11-KT) administration and in vivo knockout of cyp11c1 demonstrated that igf3 expression is regulated by androgens, suggesting that Igf3 acts downstream of androgens in fish spermatogenesis. Notably, the igf3 knockout did not affect body growth, indicating that this Igf specifically functions in reproduction. Taken together, our data provide genetic evidence for fish igf3 in the regulation of reproductive capacity by controlling spermatogenesis.

From gametogenesis to spawning: How climate-driven warming affects teleost reproductive biology

Ambient temperature modulates reproductive processes, especially in poikilotherms such as teleosts. Consequently, global warming is expected to impact the reproductive function of fish, which has implications for wild population dynamics, fisheries and aquaculture. In this extensive review spanning tropical and cold-water environments, we examine the impact of higher-than-optimal temperatures on teleost reproductive development and physiology across reproductive stages, species, generations and sexes. In doing so, we demonstrate that warmer-than-optimal temperatures can affect every stage of reproductive development from puberty through to the act of spawning, and these responses are mediated by age at spawning and are associated with changes in physiology at multiple levels of the brain-pituitary-gonad axis. Response to temperature is often species-specific and changes with environmental history/transgenerational conditioning, and the amplitude, timing and duration of thermal exposure within a generation. Thermally driven changes to physiology, gamete development and maturation typically culminate in poor sperm and oocyte quality, and/or advancement/delay/inhibition of ovulation/spermiation and spawning. Although the field of teleost reproduction and temperature is advanced in many respects, we identify areas where research is lacking, especially for males and egg quality from "omics" perspectives. Climate-driven warming will continue to disturb teleost reproductive performance and therefore guide future research, especially in the emerging areas of transgenerational acclimation and epigenetic studies, which will help to understand and project climate change impacts on wild populations and could also have implications for aquaculture.

Cooling of pejerrey Odontesthes bonariensis (Teleostei, Atherinidae) embryos at sub-zero temperatures

Pejerrey fish (Odontesthes bonariensis) is a seasonal multiple spawner with great economic importance and an adequate species for Aquaculture. For these reasons, it is necessary to apply biotechnologies to optimize its reproduction in captivity. In this context, the aim of this work was to develop a cooling protocol for pejerrey embryos at sub-zero temperatures. Two cryoprotective solutions (CSs: S1 and S2), two cooling curves (a fast and a slow one) and two storage temperatures (-14 and -20 °C) were evaluated for 1 h. High percentages of embryo survival (80-100%) were obtained in all cases. In particular, for cooling at -14 °C, the most suitable protocol was the slow temperature decrease in combination with S1 (2.5 M methanol, 1.4 M Me2SO, 0.3 M sucrose, and 0.08 M NaCl). The hatching rate (86.67 ± 11.55%) and the larval survival observed did not differ from those of the control group, and about 30% of normal-looking larvae were obtained. Besides, the slow cooling was also the best way to reach -20 °C, obtaining a hatching rate of around 60%. However, all the larvae had different kind of malformations. Finally, in order to improve the results obtained at -20 °C, the CSs were incorporated into the embryos by microinjection. In this case, it was observed that the most convenient combination was the microinjection of S2 (same composition as S1 but without Me2SO) in the perivitelline space followed by rapid cooling. Although the hatching rate was not improved (67.93 ± 8.31%), the microinjection allowed to obtain at least 4.5% normal-looking larvae. These results showed that the cooling of pejerrey embryos at zub-zero temperatures was feasible. Moreover, the microinjection of cryoprotectants within the pejerrey O.bonariensis embryos was employed for the first time in this species.

Kisspeptins and their receptors in the brain-pituitary-gonadal axis of Odonthestes bonariensis: Their relationship with gametogenesis along the reproductive cycle

In vertebrates, the reproduction is controlled by the brain-pituitary-gonadal (BPG) axis and kisspeptin has emerged as a key player of this axis. In this study, we analyzed changes in the expression levels of kiss1, kiss2, and their receptors, kissr2 and kissr3 during gametogenesis in the BPG axis of feral Odontesthes bonariensis. In females, levels of brain kiss1 showed an increase at final maturation (Fm), while kiss2 levels were shown to be high at primary growth (Pg) stage, with no differences in the expression of their receptors. In the pituitary, kiss1 and kiss2 peaked at the cortical alveoli (Ca) stage, and kissr3 at initial vitellogenesis. In parallel, there was an increase of kiss1, kissr2 and kissr3 in the ovary during the Ca stage and both receptors again at Fm stage. In males, the four genes were highly expressed in the brain at the arrested (A) stage. In the pituitary, kiss2 peaked at spermatogonial (SG) and spermatocytary (SC) stages; while kissr3 reached a peak at the spermiogenic stage (SP). In testes, kiss1 and kiss2 significantly increased during the SG and SC stages; meanwhile, kissr2 increased at SG and SC, whereas kissr3 levels were significantly high at SC and SP stages. Taken together these results showed that the kisspeptin system in pejerrey is expressed in the three levels of the BPG axis with different expression profiles during the gonadal cycle. These findings pointed that kisspeptins have different roles in gametogenesis in this species.

Pituitary-thyroid axis development during the larval-juvenile transition in the pejerrey Odontesthes bonariensis

The morphological development of the thyroid gland of pejerrey Odontesthes bonariensis during larval-juvenile transition was studied and related to whole-body concentrations of thyroxine (T₄ ) and tri-iodothyronine (T₃ ). A complementary (c)DNA fragment of the thyroid-stimulating hormone β-subunit (tshb) was sequenced and transcript levels quantified during this period. Follicles with eosinophilic and T₄ -immunoreactive colloids were detected at hatching together with tshb transcript levels and whole-body concentrations of T₄ and T₃ hormones. Thyroid follicles were located in the subpharyngeal region associated with the ventral aorta below the hyoid bone. Follicle structure switched from the rounded form at hatching to oval in juveniles. Significant increase of follicle number per larva, mean colloidal area and total colloidal area was observed throughout development with maximum values at the end of the larval-juvenile transition. A significant decrease of tshb expression together with a significant increase in T₄ and T₃ whole-body concentrations was observed prior to achieving the juvenile phenotype. These results are in accordance with a negative feedback regulation of tshb expression by thyroid hormones and a possible association between thyroid hormone levels and the acceleration of metabolic processes necessary to complete metamorphosis.

Sex hormone binding globulin: Expression throughout early development and adult pejerrey fish, Odontesthes bonariensis

Sex hormone binding globulin (Shbg) is a plasma glycoprotein that binds and transports steroids in the blood of all vertebrate classes apart from birds. In the present study we characterized shbg from pejerrey, a fish species with a well characterized temperature-dependent sex determination. The pejerrey shbg mRNA comprises 1185bp encoding for a 395 amino acid Shbg precursor protein that includes a leader sequence for secretion. Relative quantification of shbg transcript abundance revealed expression early in development coinciding with the sex-determining period and probably in association with temperature leading to male determination. The hepatopancreas was the main site of shbg expression, which varied according to the sex cycle in females. It was also expressed in gills, gonads, gut and taste buds during both larval stages and in adult fish. The presence of Shbg in organs in close contact with the environment such as gills, pseudobranchs, gut and taste buds suggests that these are potential sources of uptake or release of steroids/xenosteroids to and from the aquatic environment.

Effects of re-stripping on the seminal characteristics of pacu (Piaractus mesopotamicus) during the breeding season

Seminal characteristics in teleost fish with an annual reproductive period, such as pacu (Piaractus mesopotamicus), may vary during the breeding season. The sperm formed before the beginning of the spawning period may be stored for a long time, causing damage to the cells. Therefore, re-stripping may be an important way to eliminate the "old" and allow for the collection of "new" spermatozoids. In this study, we analyzed the seminal characteristics of hormonally induced pacu at the beginning, middle and end of the breeding season, and we analyzed samples from re-stripped males (stripped first at the beginning, re-stripped in the middle, and re-stripped again at the end of the season) during two breeding seasons. The sperm density, ionic composition, pH, and osmolality were similar among the groups. The semen volume, seminal plasma protein concentration and incidence of morphologically anomalous sperm increased over time. In addition, some parameters that are associated with good-quality semen decreased, such as sperm motility, viability and DNA integrity. Moreover, we observed a positive association among motility, viability and DNA integrity for sperm with elevated 11-ketotestosterone, but there was no such association for fshb or lhb mRNA levels in the pituitary. The semen that was obtained earlier (at the beginning) or from re-stripped males exhibited better characteristics than the other samples collected. In conclusion, collecting semen from pacu at the end of breeding season should be avoided; it is preferable to strip early and then re-strip later in the season, and this approach may be used for diverse aquaculture purposes.