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Growth and immunity are essential physiological functions that are each energetically costly. Thus, energetic allocation into these systems must be appropriately balanced throughout life. Such 'tradeoffs' require molecular-level cross talk between growth and immunity. Limited data suggests that the growth hormone (GH) – insulin-like growth factor (IGF) axis, a key pathway regulating vertebrate growth, also has a role in immune function. Here I exploit salmonid fishes to characterise the regulation of GH-IGF axis genes under multiple experimental contexts where growth and immunity were being traded-off. Specifically, I examined the transcriptional response of the GH-IGF axis to in vivo immune-stimulation at different stages of ontogeny, considering several tissue contexts (i.e. whole fish, immune tissues, and skeletal muscle) and animals with distinct growth rates driven by GH-overexpression. To better contextualise the data, I established the regulation of key gene markers for muscle growth and immune status. Further, I considered gene duplicates retained from whole-genome duplication events in salmonid evolutionary history, exploiting new genomic resources for gene characterisation (e.g. novel IGF-IR paralogues). Four major conclusions arose from my studies. First, up-regulation of IGFBP-1A1 and IGFBP-6A2 (known IGF inhibitors), following disease challenge, suggests that IGF signalling is repressed during infection, promoting energetic reallocation towards immunity. Second, the combined regulation of multiple IGF system genes in immune tissues suggests that while IGF signalling is repressed during infection, IGFBP-6A2 may directly …