The impacts of 2,4-D herbicide DMA® 4 IVM on reproductive health and gene expression along the hypothalamic-pituitary-gonad-liver [HPGL] axis in the fathead minnow (Pimephales promelas).

Aquatic herbicides are commonly used to control a variety of non-native plants. One common active ingredient used in commercial herbicide formulations globally is 2,4-dichlorophenoxyacetic acid (2,4-D). Though 2,4-D is used in aquatic ecosystems, no studies have investigated cellular, biochemical, and transcriptional effects or mechanisms of 2,4-D exposure on fathead minnows (Pimephales promelas) throughout juvenile development. Herein, we aim to evaluate the impacts of chronic ecologically relevant 2,4-D exposure on reproductive systems of fathead minnow. Juvenile fathead minnows were exposed to ecologically relevant concentrations of 2,4-D in DMA4 herbicide under laboratory conditions in a flow through system. Male plasma testosterone concentrations, testicular and ovarian histology, and differential gene expression along the HPGL axis in three tissue types (brain, liver, and gonad) were assessed after a chronic 120 day exposure period. We observed significantly decreased plasma testosterone concentration in male fish exposed to environmentally relevant concentrations (0.50, 2.00, and 4.00 mg/L) of 2,4-D. We observed a significant increase in ovarian severity grading and oocyte atresia in female fish exposed to 4.00 mg/L. Of differential expression analysis in fish exposed to 2.00 mg/L 2,4-D in DMA4, we identified significantly decreased expression of the steroid hormone receptors ESR1, ESR2b, and AR in males. Expression of male steroidogenic genes 3βHSD and 11βHSD2 along with expression of genes regulating steroid metabolism, SULT1st2 and CBR1l were increased. Altogether, these data suggest that 2,4-D could act as an endocrine disrupting chemical that alters expression of primary genes regulating hormone receptors, steroidogenesis, and steroid metabolism along the hypothalamic-pituitary-gonad-liver (HPGL) axis. The use of 2,4-D herbicides for weed control in aquatic ecosystems could present risks to the reproductive health of non-target aquatic species.