Hepatic transcriptomic responses to benzo[a]pyrene in early-life stage Japanese quail and double-crested cormorant

Avian species can exhibit markedly different responses to aryl hydrocarbon receptor (AHR) ligands such as dioxin-like compounds (DLCs) and polycyclic aromatic hydrocarbons (PAHs). The molecular basis for this interspecies variability is well characterized for DLCs but remains poorly understood for PAHs. In the present study, we used transcriptomic analysis to investigate our recent observation that double-crested cormorants (Nannopterum auritum) are >30-fold more sensitive than Japanese quail (Coturnix japonica) to the embryolethal effects of a potent PAH, benzo[a]pyrene (BaP). Graded concentrations of BaP were injected into the air cell of fertilized, unincubated eggs. Nominal concentrations were 0, 50, and 500 ng/g for quail, and 0, 0.5, and 5 ng/g for cormorant, with the highest concentration targeted to the lethal dose 20% (LD20) in each species. Livers of mid-incubation embryos (quail, embryonic day (ED) 9; cormorant, ED14) were preserved for chemical residue analysis and RNA sequencing. Low BaP concentrations in livers were suggestive of metabolic clearance by mid-incubation. Differential expression analysis revealed a higher number of differentially expressed genes (DEGs) at higher doses for both species (quail: 18 and 75 DEGs; cormorant: 0 and 9 DEGs), with no DEGs overlapping between species. We observed DEGs and pathways related to inflammation and genotoxicity in both species, and differential expression of AHR-responsive genes only in quail. Cormorant, the species that was more sensitive in vivo, was transcriptionally less responsive at concentrations of BaP near the LD20. Future studies may help to explain this observation by examining species differences in toxicokinetic processes occurring in avian eggs, and the nature of the interaction between PAHs and the avian AHR.