Physiological and molecular responses of the chorioallantoic membranes to diluted bitumen exposures in multiple bird species

Embryotoxicity is a well-known consequence of polycyclic aromatic compound (PAC) exposure, but the molecular mechanisms of action of PAC mixtures, especially for unconventional crude petroleum types such as diluted bitumen (dilbit), remain to be fully elucidated in birds. To explore the mechanism of action of PAC in birds, the egg-injection model was used. Eggs of domestic chicken (Gallus gallus), double-crested cormorant (Nannopterum auritum), and northern gannet (Morus bassanus) were injected with 0.5 μL/g of average species-specific egg weight of 1:10-1:10,000 dilutions of Clearwater or Cold Lake Blend dilbits into the air cell on embryonic Day 0 and were artificially incubated until the liver was formed. The injections of <0.16-335 ng total PAC/g egg were consistent with PAC concentrations measured in wild bird eggs and in embryos exposed to dilbit through eggshell oiling. Mortality and frequency of malformations were low across treatments. The expression of genes involved in xenobiotic detoxification in both liver and chorioallantoic membrane (CAM) differed among species in response to dilbit exposure. Cytochrome P450 1a (cyp1a) in the CAM of dilbit-exposed chickens was induced to a higher fold-change at a lower PAC concentration than the liver, but this pattern was not consistent in wild birds. The expression of additional genes involved in the aryl hydrocarbon receptor activation adverse outcome pathway were variable in the double-crested cormorant and northern gannet CAMs. Our study demonstrates the usefulness of CAM as a target tissue for PAC metabolism in embryotoxicity. Future studies should address the differential CAM physiology across bird species to better understand the variation of species responses to contaminants and consider the use of CAM in addition to liver.