Adaptive radiation in fishes has been actively investigated over the last decades. Along with numerous well-studied cases of lotic radiation, some examples of riverine sympatric divergence have been recently discovered. In contrast to the lakes, the riverine conditions do not provide evident stability in the ecological gradients. Consequently, external factors triggering the radiation, as well as developmental mechanisms underpinning it, remain unclear. Herein, we present the comprehensive

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... of external and internal drivers of the riverine adaptive divergence of the salmonid fish Salvelinus malma. In the Kamchatka River, this species splits in the reproductively isolated morphs that drastically differ in ecology and morphology: the benthivorous Dolly Varden (DV) and the piscivorous Stone charr (SC). To understand why and how these morphs originated, we performed a series of field and experimental work, including common-garden rearing, comparative ontogenetic, physiological and endocrinological analyses, hormonal "engineering" of phenotypes. Results We revealed that the type of spawning ground acts as the main external factor driving the radiation of S. malma . In contrast to DV spawning in the leaf krummholz zone, SC reproduces in the zone of coniferous forest, which litter has a toxic impact on developing fishes. SC enhances resistance to the toxicants via metabolism acceleration provided by the elevated thyroid hormone content. These physiological changes lead to the multiple heterochronies resulting in a specific morphology and SC's expansion into a piscivorous niche. Conclusions S. malma represents a notable example of how the thyroid axis contributes to the generation of diverse phenotypic outcomes underlying the riverine sympatric divergence.