Impairment of oocyte maturation as a mechanism of decreased fecundity in Japanese medaka (Oryzias latipes) exposed to the brominated flame retardant, 1,2,5,6-tetrabromocyclooctane (TBCO)

Yamin Raza; Chloe Devoy; Paul D Jones; Jon A Doering; Steve Wiseman

doi:10.1016/j.aquatox.2023.106695

Impairment of oocyte maturation as a mechanism of decreased fecundity in Japanese medaka (Oryzias latipes) exposed to the brominated flame retardant, 1,2,5,6-tetrabromocyclooctane (TBCO)

Aquat Toxicol. 2023 Oct:263:106695. doi: 10.1016/j.aquatox.2023.106695. Epub 2023 Sep 11.

Authors

Yamin Raza¹, Chloe Devoy¹, Paul D Jones², Jon A Doering³, Steve Wiseman⁴

Affiliations

¹ Department of Biological Sciences, University of Lethbridge, Lethbridge, AB T1K 3M4, Canada.
² Toxicology Centre, University of Saskatchewan, Saskatoon, SK S7N 5B3, Canada; School of Environment and Sustainability, University of Saskatchewan, Saskatoon, SK S7N 5B3, Canada.
³ Department of Biological Sciences, University of Lethbridge, Lethbridge, AB T1K 3M4, Canada; Department of Environmental Sciences, Louisiana State University, Baton Rouge, Louisiana 70803, United States.
⁴ Department of Biological Sciences, University of Lethbridge, Lethbridge, AB T1K 3M4, Canada. Electronic address: steve.wiseman@uleth.ca.

PMID: 37716316
DOI: 10.1016/j.aquatox.2023.106695

Abstract

Inhibition of oocyte maturation is an understudied mechanism by which chemical stressors can impair fecundity of female fishes. The primary objective of the present study was to develop an assay to assess oocyte maturation disruption by chemical stressors in Japanese medaka (Oryzias latipes). First, an in vitro assay to assess maturation inducing hormone (MIH)-stimulated oocyte maturation in zebrafish was validated for use with Japanese medaka. Next, using the brominated flame retardant, 1,2,5,6-tetrabromocyclooctane (TBCO), which previously was shown to decrease fecundity of Japanese medaka and inhibit oocyte maturation in zebrafish, effects on oocyte maturation were quantified using in vitro and in vivo exposure. Adaptation of the protocol for in vitro MIH-stimulated maturation of stage IV oocytes from zebrafish was successful in inducing greater than 80% of stage IX oocytes from female Japanese medaka to mature. To assess effects of in vitro exposure, stage IX oocytes were exposed to 0, 2, 20, and 200 μg/L of TBCO, followed by exposure to MIH. The in vitro exposure caused a significant decrease in maturation of oocytes exposed to 20 and 200 μg/L of TBCO. To assess effects of TBCO on fecundity and oocyte maturation following in vivo exposure, sexually mature fish were fed a control, 100 μg/g, or 1000 μg/g concentration of TBCO-spiked fish food for 21 days, where fecundity was measured daily, and following the exposure, stage IX oocytes were excised to assess MIH-stimulated maturation. Fecundity and oocyte maturation were significantly decreased at either concentration of TBCO. Plasma concentrations of 17β-estradiol (E2) and hepatic abundances of transcripts of vitellogenin (vtgI and vtgII) were quantified, but there were no significant differences between treatments. Results suggest that inhibition of oocyte maturation is a mechanism by which TBCO decreases fecundity, and that in vitro assays of oocyte maturation might be predictive of fecundity in this species.

Keywords: Endocrine disruption; Fecundity; Maturation inducing hormone; Oogenesis; Vitellogenin.