Benzo(a)pyrene exposure in early life suppresses spermatogenesis in adult male zebrafish and association with the methylation of germ cell-specific genes.

Polycyclic aromatic hydrocarbons (PAHs) are environmental contaminants that are widely present in aquatic ecosystems. To assess the impact of early-life exposure to benzo[a]pyrene (BaP), a representative PAH, on reproductive ability in adult male zebrafish (Danio rerio), fertilized embryos were exposed to 0.05, 0.5, 5 and 50 nM of BaP for 96 h, and then the hatched larvae were raised to adulthood in clean water. In one-year-old male fish, the percentage of spermatozoa in testis was significantly reduced in the 0.5, 5 and 50 nM treatments. When the treated fish were mated with untreated fish, significantly decreased rate of egg fertilization and hatching success and significantly elevated malformation rate the F1 larvae were observed in the 0.5, 5 and 50 nM treatments. The transcriptional levels of genes along the brain-pituitary-gonadal axis, involving gnrh3, gnrhr3, fshß, lhß, lhγ, lhrγ and ar, were downregulated. In addition, embryonic BaP exposure upregulated the promotor methylation of germ cell-specific genes in the testis of adult fish. The upregulated methylation of ddx4, dnd1, nanos2 in the testis might be associated with the downregulated mRNA levels of these genes, which could be another reason for the inhibition of spermatogenesis. These results indicate that early-life exposure to BaP suppress the reproductive capability of adult male fish, which would cause a decrease in fish population.

Embryonic exposure to benzo(a)pyrene inhibits reproductive capability in adult female zebrafish and correlation with DNA methylation.

This study was conducted to investigate the effects of embryonic short-term exposure to benzo(a)pyrene (BaP), a model polycyclic aromatic hydrocarbon, on ovarian development and reproductive capability in adult female zebrafish. In 1-year-old fish after embryonic exposure to BaP for 96 h, the gonadosomatic indices and the percentage of mature oocytes were significantly decreased in the 0.5, 5 and 50 nmol/L treatments. The spawned egg number, the fertilization rate and the hatching success were significantly reduced, while the malformation rate of the F1 unexposed larvae were increased. The mRNA levels of follicle-stimulating hormone, luteinizing hormone, ovarian cytochrome P450 aromatase cyp19a1a and cyp19b, estrogen receptor esr1 and esr2, and hepatic vitellogenin vtg1 and vtg2 genes, were down-regulated in adult female zebrafish that were exposed to BaP during embryonic stage. Both 17ß-estradiol and testosterone levels were reduced in the ovary of adult females. The methylation levels of the gonadotropin releasing hormone (GnRH) gene gnrh3 were significantly increased in the adult zebrafish brain, and those of the GnRH receptor gene gnrhr3 were elevated both in the larvae exposed to BaP and in the adult brain, which might cause the down-regulation of the mRNA levels of gnrh3 and gnrhr3. This epigenetic change caused by embryonic exposure to BaP might be a reason for physiological changes along the brain-pituitary-gonad axis. These results suggest that short-term exposure in early life should be included and evaluated in any risk assessment of pollutant exposure to the reproductive health of fish.