AHR2 as a Potential Integrator of Eco‐Evo‐Devo in Non‐Mammalian Vertebrates: A Focus on Fishes

ABSTRACT Aryl hydrocarbon receptor 2 (AHR2) is increasingly recognized as a distinctive member of the AHR family in non‐mammalian vertebrates, where it acts as a ligand‐activated transcription factor linking environmental cues to developmental and physiological outcomes. The most extensive AHR2‐specific evidence for these functions currently comes from fishes. Although the AHR pathway has traditionally been framed through responses to environmental contaminants, it also responds to endogenous, dietary, microbiota‐derived and other exogenous small molecules. This review focuses on AHR2 because its distinctive evolutionary history and the convergence of receptor‐specific evidence across development, chemical responsiveness and adaptation make it a particularly informative candidate interface linking chemical sensing, development and evolution. We first consider the origin and diversification of the AHR2 paralog, its ligand spectrum, signaling partners and regulatory context. Organismal studies, with primary emphasis on fishes, are then used to examine AHR2‐dependent roles in craniofacial, cardiac, skeletal, reproductive and behavioral development, as well as variation in chemical sensitivity among paralogs, species and populations. Evidence from pollution‐adapted killifish and Atlantic tomcod directly connects heritable AHR‐pathway variation with altered chemical responses and local adaptation. By contrast, proposed links among natural ligand variation, AHR2‐dependent developmental plasticity and morphological divergence remain incompletely tested. Examples from other vertebrate groups provide comparative context for receptor retention, loss and functional divergence. By expanding the attention on development, ecology and evolution rather than on the toxicological endpoints alone, this fish‐centred perspective presents AHR2 as a useful model for testing how chemical environments may influence developmental reaction norms, evolutionary responses and population resilience.