Suppression and recovery of reproductive behavior induced by early life exposure to mercury in zebrafish

doi:10.1016/j.cbpc.2020.108876

Comparative Biochemistry and Physiology Part C: Toxicology & Pharmacology

Volume 239, January 2021, 108876

https://doi.org/10.1016/j.cbpc.2020.108876 Get rights and content

Highlights

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Early life Hg exposure suppressed reproductive behavior in adult zebrafish.
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Early life Hg exposure impaired reproductive outcomes in adult zebrafish.
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Early life Hg exposure altered reproductive behavior-related genes' expressions.
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Disruption of reproductive behavior could be recovered in the F₁ generation.

Abstract

While mercury (Hg)-induced reproductive impairments have been demonstrated in fishes, the effects of exposure to Hg²⁺ during early life stages on the reproductive behavior in adulthood and the persistency of these effects in the next generation remain largely unknown. In this study, zebrafish embryos were exposed to 0.6, 3, or 15 μg·L⁻¹ Hg²⁺ for 5 days and then reared for an additional 115 days in clean water, from which embryos were obtained and cultured in clean water for 120 days as the F₁ generation. Increased Hg levels in brains and decreased survival and growth were observed in individuals exposed to Hg²⁺ during early life stages. Early life exposure to Hg²⁺ reduced the frequency of touching in males as well as the frequency and duration of visits to the spawning area by females, males, or both sexes simultaneously, and resulted in lesser spawning and fertilization. Moreover, early life exposure to Hg²⁺ interfered with the transcription of genes encoding neuropeptides and hormones related to reproduction, which could be responsible for diminished sexual behavior and reduced reproductive outcomes. In the F₁ generation, such alterations were not observed in either females or males, indicating that the disruption of normal patterns of reproductive behavior caused by early life exposure to Hg²⁺ did not persist and was recovered. Overall, this study demonstrated that exposure to Hg²⁺ during early life stages suppressed the reproductive behavior of adult fish but this disruption could be recovered in the F₁ generation.

Graphical abstract

Introduction

Metals are a major class of pollutants in the water environment that can negatively affect the health of aquatic organisms. Among the metals, mercury (Hg) is of particular concern because of its considerable toxicity to aquatic organisms even at low concentrations (Vieira et al., 2009). Anthropogenic activities such as industry and agriculture, together with natural sources such as volcanic eruptions, have released large amounts of Hg into the water environment (Monteiro et al., 2013). In the aquatic system, fish accumulate Hg from surrounding water and other organisms via the food chain transfer, which in turn induces histopathological changes and disturbs metabolic processes and endocrine status (Zhang et al., 2016a, Zhang et al., 2016b; Sun et al., 2018).

For animals that undergo sexual reproduction, reproductive behavior is one of the most key events during the life cycle. Normal reproductive behavior is essential to maintain the size and age structure of the population (Weis et al., 2001), and altered reproductive behavior can be act as an ecologically relevant biomarker of endocrine dysfunction caused by contaminants (Sandheinrich and Miller, 2006). In teleost fish, a series of special sexual behaviors occurs during gamete release and fusion, such as nesting, courtship, chasing, mating, egg release, and/or sperm release (Munakata and Kobayashi, 2010). These behaviors can be performed in an innate manner after attaining puberty and are influenced by various environmental factors such as depth gradient, temperature, and contaminants (Sessa et al., 2008; Lorenzi et al., 2016; Schram and Steele, 2016). If any of these behaviors are blocked, normal patterns of reproductive behavior may be disrupted and ultimately impair reproductive success. Reproductive behavior is regulated by physiological and endocrine processes. Several hormones and neuropeptides play key roles in regulating the occurrence of reproductive behavior of fish, such as gonadotropin-releasing hormone (GnRH), gonadotropin-inhibitory hormone (GnIH), and kisspeptin (Munakata and Kobayashi, 2010; Parhar et al., 2016). The GnRH system can mediate reproductive behaviors by regulating neuroendocrine, while GnIH and kisspeptin can inhibit and stimulate the secretion of gonadotropins as well as the activity of GnRH, respectively (Zhao et al., 2014; Parhar et al., 2016; Li et al., 2017). Therefore, the disturbance of these endocrine hormones and neuropeptides is closely associated with changes in reproductive behavior.

In fish, early life stages (embryo-larva) are particularly sensitive to contamination, and the short-term toxicity test at early life stages is particularly important for assessing the long-term effects of pollutants on fish. In addition, this test does not require exposure throughout the life cycle, which can be performed in a more cost- and labor- efficient manner (Arcand-Hoy and Benson, 1998). Previous studies have demonstrated that behaviors of fish can be affected by Hg exposure, showing aberrant swimming behavior and depression of predation or predator avoidance performance (Zhou and Weis, 1998; Ho et al., 2013; Harayashiki et al., 2018). Moreover, Hg-induced reproductive impairment and its underlying mechanism have been widely studied in fish (Crump and Trudeau, 2009; Guchhait et al., 2018). It is unclear whether the reproductive behavior of adult fish is affected by early life exposure to Hg²⁺ and whether this effect can be recovered in offspring.

Zebrafish (Danio rerio) is one of the excellent models for toxicology studies. The main objective of this study was to investigate the effects of short-term exposure of zebrafish to Hg²⁺ during early life stages on reproductive behavior in adulthood and the next generation. Zebrafish embryos were exposed to 0.6, 3, or 15 μg·L⁻¹ Hg²⁺ for 5 days, and then grown-out in clean water until 120 days. The survival and growth of fish and the accumulation of Hg in brains were determined. The reproductive behavior was videotaped and typical parameters of sexual behavior were examined. Furthermore, expressions of genes encoding neuropeptides and hormones related to reproductive behavior were detected. In addition, these parameters were also examined in the F₁ generation to determine whether they could be recovered.

Section snippets

Chemical and exposure doses

Mercuric chloride (HgCl₂, purity ≥99.5%) was purchased from Sinopharm Group Corporation (Shanghai, China). Stock solutions were prepared in double-distilled water, from which small aliquots were added to dechlorinated water in order to achieve the desired concentrations of 0 (control), 0.6, 3, or 15 μg·L⁻¹ Hg²⁺. The actual concentration of total Hg (tHg) for each treatment were 0.06 ± 0.01, 0.54 ± 0.05, 2.50 ± 0.27 and 13.43 ± 0.53 μg·L⁻¹, respectively. These concentrations were selected based

Survival and growth

In the F₀ generation, exposure to 15 μg·L⁻¹ Hg²⁺ during early life stages reduced fish survival (Fig. 1A) and the body length and mass of adult males and females (Fig. 1B and C). In the F₁ generation, exposure of F₀ individuals to Hg²⁺ during early life stages did not significantly affect survival and growth of fish (Fig. 1D, E, and F).

Concentrations of tHg in brain

In the F₀ generation, compared with the control, greater concentrations of tHg were observed in brains of adult females exposed to 15 μg·L⁻¹ Hg²⁺, and in brains

Discussion

Reproductive toxicity is one of the most sensitive and crucial toxic endpoints of contaminations, and reproductive behavior is an obligatory component of successful breeding. Disruption of normal patterns of reproductive behavior can impair the reproductive output and consequently population dynamics (Oshima et al., 2003). Although previous studies have reported negative effects of exposure to Hg on fish reproduction, such as impairment of gonadal histology, disturbance of hormones balance,

Conclusions

The present study showed that early life exposure of zebrafish to Hg²⁺ suppressed subsequent reproductive behavior in adulthood, manifested by lessened courtship activities of males and apparent avoidance of the spawning area by both males and females, which resulted in lesser spawning and fertilization. Interference with the transcription of genes encoding neuropeptides and hormones related to reproduction might be responsible for diminished sexual behavior and reduced reproductive outcomes.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (31901183), the Basic Research and Frontier Exploration Project of Chongqing (cstc2018jcyjAX0123), and the Science and Technology Research Program of Chongqing Municipal Education Commission (KJQN201900512).

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¹: These authors contributed equally to this work.

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Suppression and recovery of reproductive behavior induced by early life exposure to mercury in zebrafish

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Chemical and exposure doses

Survival and growth

Concentrations of tHg in brain

Discussion

Conclusions

Declaration of competing interest

Acknowledgments

Aquat. Toxicol.

Environ. Pollut.

Aquat. Toxicol.

Gen. Comp. Endocrinol.

Mar. Environ. Res.

J. Aquat. Res.

Sci. Total Environ.

Mar. Pollut. Bull.

Methods

Gen. Comp. Endocrinol.

Chemosphere

Aquat. Toxicol.

Gen. Comp. Endocrinol.

Environ. Pollut.

Neurotoxicol. Teratol.

Chemosphere

Ecotoxicol. Environ. Safe.

Aquat. Toxicol.

Comp. Biochem. Physiol. C Toxicol. Pharmacol.

Neurotoxicol. Teratol.

Aquat. Toxicol.

Aquat. Toxicol.

Aquat. Toxicol.

Fish reproduction: an ecologically relevant indicator of endocrine disruption

Environ. Toxicol. Chem.