Part B: Morphometric and transcriptomic responses to sub-chronic exposure to the polycyclic aromatic hydrocarbon phenanthrene in the fathead minnow (Pimephales promelas)

Abstract

Phenanthrene is a tricyclic polycyclic aromatic hydrocarbon and environmental contaminant found in high concentrations around urban catchments and in the vicinity of oil extraction activities. Fish exposed to phenanthrene can exhibit altered reproductive hormone profiles and/or differences within gonadosomatic index and altered gamete proportions, but the mechanisms underlying these changes are not fully understood. In this study, we conducted a sub-chronic bioassay and measured transcriptional responses in the liver, the major tissue involved in generating lipids for oocyte growth. Adult male and female fathead minnow (Pimephales promelas) were exposed to an average measured concentration of 202 μg phenanthrene/L for a 7 week period. Condition factor was reduced in both males and females, while female fish also showed decreased gonadosomatic index relative to control females. In females exposed to phenanthrene, perinucleolar proportions were increased ∼1.9-fold relative to the control group whereas the proportions of vitellogenic oocytes decreased ∼8.8 fold. In males exposed to phenanthrene, spermatogonia proportions were increased ∼2.3 fold in testicular tissues compared to control fish. Thus, gametes were at an earlier stage of maturation in phenanthrene-treated fish compared to controls. However, no differences were detected in the production of 17β-estradiol or testosterone from the gonad in either sex. Catalase activity was also assessed in the liver as a measure of oxidative stress and this biomarker did not change in activity in either sex. In addition to endpoints in the ovary, the female hepatic transcriptome was measured, as this tissue produces lipids for oocyte maturation. Transcriptomic responses to phenanthrene exposure suggested a reduction in vitellogenin mRNA, and lipid metabolism and immune system pathways. Comparisons of hepatic transcriptome responses with Part A (72 h phenanthrene exposure) showed that energy homeostasis pathways were consistently altered following phenanthrene exposure over multiple durations and concentrations. We suggest that altered energy homeostasis may be adversely affecting reproductive efforts, as impaired reproduction has been observed in other studies investigating polycyclic aromatic hydrocarbons.