Transcriptomics and proteomics provide insights into the adaptative strategies of Tibetan naked carps (Gymnocypris przewalskii) to saline-alkaline variations.

Gymnocypris przewalskii is an exclusively cyprinid fish that inhabits Lake Qinghai, which is characterized by high salinity and alkalinity. To elucidate the molecular basis of the adaptation of G. przewalskii to a wide range of salinity‒alkalinity conditions, we performed morphological, biochemical, transcriptomic and proteomic analyses of the major osmoregulatory organs of the gills and kidney. Morphological examination revealed that mitochondria-rich cells were replaced by mucus cells in the gills during the transition of G. przewalskii from freshwater to lake water. In the kidney, the tight junction formed dense structure in the renal tubules under lake water condition compared with the loose structure in freshwater. The results of the biochemical assays revealed an increased content of total amino acids, indicating their potential roles as osmolytes and energy supplies in freshwater. The decreased urea concentration suggested that urea synthesis might not be involved in the detoxicity of ammonia. The transcriptomic and proteomic data revealed that genes involved in ion absorption and ammonia excretion were activated in freshwater and that genes involved in cell junction and glutamine synthesis were induced in lake water, which was consistent with the morphological and biochemical observations. Together with the higher levels of glutamine and glutamate, we proposed that G. przewalskii alleviated the toxic effect of ammonia direct excretion through gills under freshwater and the activation of the conversion of glutamate to glutamine under high saline-alkaline condition. Our results revealed different expression profiles of genes involved in metabolic pathways, including the upregulation of genes involved in energy production in freshwater and the induction of genes involved in the synthesis of acetylneuramic acid and sphingolipid in soda lake water. In conclusion, the appearance of mitochondria-rich cells and increased energy production might contribute to ion absorption in G. przewalskii to maintain ion and solute homeostasis in freshwater. The existence of mucus cells and dense junctions, which are associated with increased gene expression, might be related to the adaptation of G. przewalskii to high salinity-alkalinity.