TOXICITY OF AQUEOUS L-SELENOMETHIONINE EXPOSURE TO EARLY LIFE-STAGES OF THE FATHEAD MINNOW (PIMEPHALES PROMELAS) AND ZEBRAFISH (DANIO RERIO)
Gerhart, Allyson Kristi 1994-
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Selenium (Se) is an important micronutrient in all vertebrate organisms due to its involvement in the synthesis of select proteins and enzymes involved in redox metabolism. However, its narrow range between essentiality and toxicity has generated concern as certain industrial activities have increased the loading of Se into the aquatic environment. Oviparous vertebrates such as fishes are particularly susceptible to elevated dietary Se concentrations during early life stages, as Se is readily transferred from mother to embryo. Research has shown that exposure to selenomethionine (SeMet), the predominant form of Se in the diet, during sensitive developmental stages can result in teratogenic (abnormal physiological or developmental) effects in developing fish larvae. More recent Se research employing aqueous SeMet embryo exposures generated similar effects as those observed in maternal transfer and yolk microinjection studies, potentially offering an alternative to the standard methods of Se exposure. However, these aqueous embryo exposures are a relatively new concept and thus warrant further investigation. As such, the overarching objective of this research was not to replicate an environmentally relevant exposure scenario to produce results with regulatory implications, but to further investigate the mechanism of Se toxicity, as it relates to oxidative stress, through waterborne fathead minnow (Pimephales promelas) and zebrafish (Danio rerio) embryo exposures to SeMet, tert-butyl hydroperoxide (tBOOH), and tert-butyl hydroquinone (tBHQ). In addition, this research aimed to critically evaluate the utility of aqueous exposures as a method for determining the effects of SeMet on developing embryos. In my first experiment , newly fertilized fathead minnow embryos were exposed for six days to 30, 90, 270, 810, 2430, 7290, 21870, and 65610 µg Se/L (as SeMet). In the second experiment, newly fertilized zebrafish embryos were first exposed for 5 days to 5, 25, 125, and 625µg Se/L (as SeMet). These exposures informed the following experiment in which embryos were exposed to 75 mg/L tBOOH and two concentrations of SeMet (25 and 125 µg Se/L) either with (tBOOH-t, 25-t, 125-t) or without (tBOOH, 25, 125) a 4h 100 µg/L tBHQ pre-treatment. Survival, hatchability, time to hatch, the frequency and severity of deformities (total and type), and changes in the expression of seven selected antioxidant-associated genes were determined. SeMet exposure reduced hatchability and survival, and increased the incidence and severity of deformities in both fathead minnow and zebrafish in a concentration-dependent manner. In terms of time to hatch, opposite responses were observed, with a reduced time to hatch in fathead minnow and increased time to hatch in zebrafish. In the second experiment, exposure to tBHQ did not affect any of the endpoints evaluated; however, exposure to tBOOH increased time to hatch and the incidence/severity of deformities, decreased hatchability, and significantly increased expression of glutathione-disulfide reductase (gsr) mRNA abundance in the pre-treated tBOOH treatment group. No significant differences were observed between embryos pre-treated with tBHQ and embryos exposed to either tBOOH or SeMet alone. The results of this study suggest that aqueous embryo exposures to SeMet can impact embryo-larval development, as evidenced by the production of deformities, and thus, the survivability of fish. In terms of the pro-oxidant tBOOH, exposure resulted in significant increases in both the incidence and severity of deformities such as those observed in the SeMet treated embryos, potentially suggesting a similar pathway (i.e. oxidative stress) plays a role in the generation of deformities. Furthermore, my results suggest that early life-stage zebrafish are more sensitive than fathead minnows to aqueous embryo SeMet exposures, inspiring more questions about factors such as developmental pattern and chorion physiology, and how this potentially influences toxicity across fish species.
DegreeMaster of Science (M.Sc.)
SupervisorJanz, David M
CommitteeHecker, Markus; Niyogi, Som; Ortega, Van A
Copyright DateOctober 2019