PHYLOGENETIC DISTRIBUTION OF THE AH RECEPTOR IN NONMAMMALIAN SPECIES - IMPLICATIONS FOR DIOXIN TOXICITY AND AH RECEPTOR EVOLUTION

The mammalian Ah receptor (AhR) controls the expression of the CYP1A1 gene and may also mediate the toxic effects of chlorinated dioxins and related compounds. Little is known about the presence and characteristics of the AhR in non-mammalian species. Recent studies have identified an AhR in fish, consistent with their sensitivity to dioxin toxicity. However, the AhR may be lacking in many species of marine invertebrates. Thus, invertebrate species may be less susceptible to dioxin effects, or may show different structure-activity relationships from those seen in vertebrates. The AhR may have evolved more than 450 million years ago in response to compounds present in the environment of early fishes, or to fulfill some