The schizothoracine fish endemic to the Qinghai-Tibetan Plateau are comparatively well adapted to aquatic environments with low oxygen partial pressures. However, few studies have used transcriptomic profiling to investigate the adaptive responses of schizothoracine fish tissues to hypoxic stress. This study compared the transcriptomes of Gymnocypris eckloni subjected to 72 h of hypoxia (Dissolved oxygen, DO = 3.0 ± 0.1 mg/L) to those of G. eckloni under normoxia (DO = 8.4 ± 0.1 mg/L). To identify the potential genes and pathways activated in response to hypoxic stress, we collected muscle, liver, brain, heart, and blood samples from normoxic and hypoxic fish for RNA-Seq analysis. We annotated 337,481 gene fragments. Of these, 462 were differentially expressed in the hypoxic fish as compared to the normoxic fish. Under hypoxia, the transcriptomic profiles of the tissues differed, with muscle the most strongly affected by hypoxia. Our data indicated that G. eckloni underwent adaptive changes in gene expression in response to hypoxia. Several strategies used by G. eckloni to cope with hypoxia were similar to those used by other fish, including a switch from aerobic oxidation to anaerobic glycolysis and the suppression of major energy-requiring processes. However, G. eckloni used an additional distinct strategy to survive hypoxic environments: a strengthening of the antioxidant system and minimization of ischemic injury. Here, we identified several pathways and related genes involved in the hypoxic response of the schizothoracine fish. This study provides insights into the mechanisms used by schizothoracine fish to adapt to hypoxic environments.

Fishes endemic to the Qinghai-Tibetan Plateau are comparatively well adapted to aquatic environments with low oxygen partial pressures (hypoxia). Here, we cloned the complete cDNA of hemoglobin (Hb) α and β from the Tibetan schizothoracine fish Schizopygopsis pylzovi, and then investigated changes in Hb mRNA and protein levels in spleen, liver and kidney in response to hypoxia. We applied severe hypoxia (4 h at PO = 0.6 kPa) and moderate hypoxia (72 h at PO = 6.0 kPa) to adult S. pylzovi. Changes of Hb expression under hypoxia, together with the investigations of spleen somatic index, kidney somatic index and Hb concentration in circulation, suggest that the kidney may not only serve as the erythropoietic organ, but also act as the major blood reservoir in S. pylzovi. From this perspective, the transcriptional activity of Hb in S. pylzovi, as reflected in the kidney, was turned down quickly after the onset of severe hypoxia, while under moderate hypoxia the transcriptional activity of Hb showed upregulation for a short time, but then the transcriptional machinery was turned down slowly on prolonged exposure. Notably, the changes in Hb protein levels in spleen, liver and kidney in response to severe and moderate hypoxia were not in line with the changes in mRNA levels, which are related with the blood reservoir in the kidney. Tibetan schizothoracine fish, at least S. pylzovi, show a particular response of the transcription regulation of Hb to moderate hypoxia, which is different from that of other fish species.

Freshwater fish belonging to the genus Schizopygopsis are widespread in drainages throughout the Qinghai‐Tibetan Plateau and, thus, a model group with which to investigate how paleo‐drainage changes linked to historical uplifting within the Qinghai‐Tibetan Plateau influence speciation. To date, the phylogenetic and taxonomic relationships within Schizopygopsis remain controversial. In this study, we constructed a comprehensive molecular phylogeny of Schizopygopsis based on six mitochondrial gene sequences. We compared the taxonomic relationships revealed by this phylogeny with those obtained from morphological data. We also used this phylogeny to assess the extent to which the evolution of Schizopygopsis has been driven by paleo‐drainage changes linked to uplifting of the Qinghai‐Tibetan Plateau. Results indicated that all Schizopygopsis taxa formed a monophyletic group comprising five major clades, which were inconsistent with the taxonomic relationships based on morphology for this group. Our results also strongly supported the validity of S. anteroventris and S. microcephalus as distinct species within Schizopygopsis. Molecular calibrations showed that species within the middle Yangtze species diverged earlier (~4.5 Mya) than species within the Indus River (~3.0 Mya), the Mekong River (~2.8 Mya) and the Tsangpo + Salween rivers (~2.5 Mya). The most recent evolutionary splits occurred among species from the upper and lower Yangtze River, the Yellow River and the Qiadam Basin at about 1.8 to 0.3 Mya. Our molecular evidence and use of the molecular clock calibration have allowed us to associate speciation events within the genus Schizopygopsis to the formation and separation of paleo‐drainage connections caused by tectonic events during the uplifting of the Qinghai‐Tibetan Plateau (~4.5 Mya). This work underlines the dominant role of vicariance in shaping the evolutionary history of the genus Schizopygopsis. Further research using multiple loci and more extensive sampling will reveal a more complete picture of the phylogenetic relationships and biogeography of Schizopygopsis fishes.

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