Developmental and tissue-specific expression of AHR1, AHR2, and ARNT2 in dioxin-sensitive and -resistant populations of the marine fish Fundulus heteroclitus

Fundulus heteroclitus is a well-characterized marine fish model for studyin g aryl hydrocarbon toxicity. The F. heteroclitus population in New Bedford Harbor (NBH), a Superfund site in southeastern Massachusetts, exhibits heri table resistance to the toxic effects of planar halogenated aromatic hydroc arbons (PHAHs), including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TC DD) and p olychlorinated biphenyls (PCBs). To investigate the role of the aryl hydroc arbon receptor (AHR) signal transduction pathway in PHAH resistance, we mea sured the relative levels of AHR1, AHR2, and ARNT2 mRNA in whole embryos at different developmental stages and in dissected tissues of adults, compari ng expression of these genes in NBH fish with fish from a reference site (S corton Creek, MA [SC]). Expression of both AHR1 and AHR2 mRNA increased dur ing development, achieving maximum levels Drier to hatching. Maximal embryo nic expression of AHR1 was delayed relative to AHR2. Whole NBH and SC embry os exhibited no discernable differences in expression of these genes. As we have previously observed, adult SC fish expressed AHR2 and ARNT2 mRNA in a ll tissues examined, while AHR1 was expressed predominantly in brain, heart , and gonads. In contrast, AHR1 mRNA was widely expressed in NBH fish, appe aring with unusual abundance in gill, gut, kidney, liver, and spleen. This AHR1 expression pattern was not observed in the lab-reared progeny of NBH f ish, demonstrating that constitutive AHR1 expression in gill, gut, kidney, liver, and spleen is not a heritable phenotype. Furthermore, widespread AHR 1 expression was not induced in reference-site fish by TCDD or PCB mixtures , suggesting that aberrant AHR1 expression is not simply a normal physiolog ical response of contaminant exposure. These results identify ubiquitous AH R1 expression as an attribute unique to feral NBH F. heteroclitus, and they represent a first step in determining the regulatory mechanisms underlying this expression pattern and its possible role in TCDD resistance.