Divergent adaptation to Qinghai-Tibetan Plateau implicated from transciptome study of Gymnocypris dobula and Schizothorax nukiangensis

Hb adaptation to hypoxia in high-altitude fishes: Fresh evidence from schizothoracinae fishes in the Qinghai-Tibetan Plateau

Uncovering the genetic basis of hypoxic adaptation is one of the most active research areas in evolutionary biology. Among air-breathing vertebrates, modifications of hemoglobin (Hb) play a pivotal role in mediating an adaptive response to high-altitude hypoxia. However, the relative contributions in water-breathing organisms are still unclear. Here, we tested the Hb concentration of fish at different altitudes. All species showed species-specific Hb concentration, which has a non-positive correlation with altitude. Moreover, we investigated the expression of Hb genes by the RNA-seq and quantitative real-time PCR (qRT-PCR), and Hb composition by two-dimensional electrophoresis (2-DE). The results showed that the multiple Hb genes and isoforms are co-expressed in schizothoracinae fishes endemic to the Qinghai-Tibetan Plateau (QTP). Phylogenetic analyses of Hb genes indicated that the evolutionary relationships are not easily reconciled with the organismal phylogeny. Furthermore, evidence of positive selection was found in the Hb genes of schizothoracinae fishes through the selection pressure analysis. We demonstrated that positively selected sites likely facilitated the functional divergence of Hb isoforms. Taken together, this study indicated that the long-term maintenance of high Hb concentration may be a disadvantage for physiologically acclimating to high altitude hypoxia. Meanwhile, the genetically based modification of Hb-O₂ affinity in schizothoracinae fishes might facilitate the evolutionary adaptation to Tibetan aqueous environments.

Hypoxia-inducible factor 1α from a high-altitude fish enhances cytoprotection and elevates nitric oxide production in hypoxic environment

Hypoxia-inducible factors (HIFs) are master transcription factor regulating hypoxic responses in vertebrates. Species of Schizothoracine, a sub-family of cyprinidae, are highly endemic to the hypoxic Qinghai-Tibetan Plateau (QTP). What roles the HIFs play in hypoxic adaptation in the Schizothoracine fish is little known. In this study, the HIF-1α/B gene from Gymnocypris dobula (Gd) was characterized. The predicted protein for Gd-HIF-1α/B contains the main domains (bHLH, PAS, PAC, ODD, N-TAD, and C-TAD). Moreover, a specific mutation that the proline hydroxylation motif (LXXLAP) mutated into PxxLAP was observed in Gd-HIF-1α/B CODD domain, which may lead to changes in the function. To clarify whether HIF-1α/B of G. dobula possesses hypoxic adaptive features, Gd-HIF1α/B and Schizothorax prenanti-HIF1α/B (Sp-HIF1α/B) were cloned into an expression vector and transfected into 293T cells. Cell viability was found to be significantly higher in cells transfected with Gd-HIF-1α/B than those transfected with Sp-HIF-1α/B under hypoxic conditions. In addition, G. dobula HIF-1α/B showed stronger activity in transactivating the expression of nitric oxide (NO)-synthesizing enzyme, NOS2B under hypoxia stresses than the orthologous gene from S. prenanti, which were accompanied with upregulated expressions of NOS2B in heart of G. dobula, which may attribute to elevated NO levels detected in G. dobula than the lower land species. These results indicated that the HIF-1α plays an important role in mediating the iNOS signaling system in the process of evolutionary adaptation of the Schizothoracine to the highland environment.

Comprehensive transcriptome data for endemic Schizothoracinae fish in the Tibetan Plateau

The Schizothoracinae fishes, endemic species in the Tibetan Plateau, are considered as ideal models for highland adaptation and speciation investigation. Despite several transcriptome studies for highland fishes have been reported before, the transcriptome information of Schizothoracinae is still lacking. To obtain comprehensive transcriptome data for Schizothoracinae, the transcriptome of a total of 183 samples from 14 representative Schizothoracinae species, were sequenced and de novo assembled. As a result, about 1,363 Gb transcriptome clean data was obtained. After the assembly, we obtain 76,602-154,860 unigenes for each species with sequence N50 length of 1,564-2,143 bp. More than half of the unigenes were functionally annotated by public databases. The Schizothoracinae fishes in this work exhibited diversified ecological distributions, phenotype characters and feeding habits; therefore, the comprehensive transcriptome data of those species provided valuable information for the environmental adaptation and speciation of Schizothoracinae in the Tibetan Plateau.