Cis-regulatory inactivation of vitamin D signaling confers extreme calcium tolerance in Jialing fish.

The turquoise karst lakes of China's Jiuzhaigou Nature Reserve harbor among the highest natural calcium concentrations known in freshwater, around threefold higher than adjacent rivers. Yet the Jialing fish (Schizopygopsis kialingensis) thrives there with serum calcium levels indistinguishable from river-dwelling conspecifics. We show that this calcium tolerance arose through evolutionary loss of vitamin D signaling. Lake and river populations diverged ∼410,000 years ago. Gill transcriptomics revealed >90% suppression of vitamin D receptor b (vdrb) and 99% reduction of its downstream calcium transporter calb2a in lake fish. Zebrafish knockout of either gene enhanced survival under hypercalcemia. We traced this suppression to specific mutations in a vdrb enhancer that reduce its activity. These findings reveal that regulatory dismantling of a canonical endocrine pathway can drive vertebrate adaptation to extreme environments, providing a clear example of how cis-regulatory evolution can drive ecological adaptation.