Dose-Dependent Early Life Stage Toxicities in Xenopus laevis Exposed In Ovo to Selenium

Abstract

Selenium (Se) is a developmental toxicant in oviparous vertebrates. The adverse reproductive effects of Se toxicity have been predominantly investigated in fishes and birds with only a few studies focusing on amphibians. The objective of this study was to determine tissue-based toxicity thresholds for early life stage Se toxicities in Xenopus laevis as a consequence of in ovo exposure through maternal transfer of dietary Se. Following a 68-day dietary exposure to food augmented with l-selenomethionine (SeMet) at measured concentrations of 0.7 (control), 10.9, 30.4, or 94.2 μg Se/g dry mass (d.m.), adult female X. laevis were bred with untreated males, and resulting embryos were incubated until 5 days postfertilization (dpf). The measured Se concentrations in eggs were 1.6, 10.8, 28.1, and 81.7 μg Se/g d.m., respectively. No biologically significant effects were observed on fertilization success, hatchability, or mortality in offspring. Frequency and severity of morphological abnormalities were significantly greater in 5 dpf tadpoles from the highest exposure group when compared to the control, with eye lens abnormalities being the most prominent of all abnormalities. The estimated EC10 value for frequency of total early life stage abnormalities was 44.9 μg Se/g egg d.m., which suggests that this amphibian species is less sensitive to in ovo Se exposure than most of the fish species studied to date.

Bioaccumulation and Biodistribution of Selenium in Metamorphosing Tadpoles

“…15,66−69 In particular, a recent study describing maternal transfer of dietary SeMet in X. laevis showed that lens abnormalities were the most sensitive indicator of in ovo exposure (EC 10 = 43.4 μg Se/g egg dw). 15 Ocular lens malformation reported in that study included a reduction in spherical shape, cataract-like opaqueness and ectopy from the ocular globe. 15 Severe craniofacial abnormalities related to the eyes were also documented at the highest exposure concentration (90 μg Se/g dw in maternal diet).…”

“…3,4 However, fish and birds have been the principal focus of Se toxicity studies, 4 and much less research has considered amphibians and reptiles. 3,15,16 Amphibians have been shown to accumulate Se from contaminated water during larval development, 11,17 but Se toxicokinetics, particularly during sensitive larval developmental stages, is poorly under- stood. This is important since amphibians may be at greater risk to dissolved contaminants compared to other aquatic vertebrates because of their highly permeable and vascularized skin, which presents an additional route of ion uptake.…”

Embryo Microinjection of Selenomethionine Reduces Hatchability and Modifies Oxidant Responsive Gene Expression in Zebrafish

“…Selenium (Se) is a well-documented teratogen in oviparous vertebrates including fishes, aquatic birds and amphibians 1 2 3 4 . As an essential trace element, adequate intake of Se is required to maintain physiological homeostasis in all animals, whereas a marginal increase in Se intake can lead to Se poisoning 1 5 .…”

Synchrotron-Based Imaging Reveals the Fate of Selenium in Striped Marsh Frog Tadpoles

“…Tadpoles have a chondrified skeleton during early premetamorphosis stages with ossification generally starting in most species during late premetamorphosis (developmental stage sampled in this experiment). Previous studies have reported spinal and craniofacial deformities in tadpoles and fish exposed to Se, which may relate to the Se accumulations in these structures. ,,− A recent study found that exposure to selenomethionine altered the expression of genes involved in the skeletogenesis of fish during early development . The study found a decreased expression of melatonin receptor ( Mtr ) in exposed fish (10–500 μM selenomethionine) and suggests that Se-induced skeletal deformities are caused by a blockage or inhibition of melatonin.…”

“…Indeed, it has been proposed that the distinct morphological changes and growth patterns of the lens during development may be indicative of the timing and concentration of exposure. , Further research on the fate of Se during tadpole growth and development under various exposure conditions is required to test this hypothesis. Improving our understanding of Se distribution in ocular tissues is important as excess Se in the lens has been linked to deformities and cataractogenesis, ,− which could have severe implications on tadpoles’ capacity to find food and avoid predators.…”