| Survival at high altitudes is very challenging. Nevertheless, many native peoples and animals can thrive under the cold and hypoxic conditions associated with high altitude environments. Studying those species could improve our understanding of adaptive evolution, particularly interactions and trade-offs between genes and pathways involved in simultaneous adaptive response to multiple environmental challenges. The molecular responses to high-altitude stress have been studied for over a century, but most of the previous studies focused on a single or a few candidate genes in model systems, which has limited our understanding of the molecular basis of adaptation in more non-model systems. Here, we present deep transcriptome sequencing in two closely related lizards, the high altitude-dwelling Phrynocephalus erythrurus, which typically lives at altitudes of 4,500 m above sea level (a.s.1) or more and considered to be the most high altitude-adapted lizard in the world, and the lowland-dwelling P. putjatia, to identify the candidate genes under positive selection and to explore the convergent evolutionary in the adaptation of poikilothermic animals to high altitude.More than 70 million sequence reads were generated for each species via Illumina sequencing. De novo assembly produced 56,845 and 63,140 transcripts with the N50 size of 2,681 base pairs (bp) and 2,691 bp for P. erythrurus and P. putjatia, respectively. More than half transcripts have homologous to the known proteins in both two species. 51.89% of P. erythrurus’transcripts and 51.98% of P. putjatia’s transcripts were annotated. Combining the previous sequencing data, we have constructed a more reliable tree and estimated the divergence time within the major lizard families. We estimated the divergence time between P. erythrurus and P. putjatia to be 5.43 Mya (million years ago,2.39-10.92 Mya).The evolutionary analysis applied in both high-and low- altitude species based on our phylogeny tree and 4094 high confidence single copy genes. P. erythrurus had higher Ka/Ks ratios when compared with P. putjatia, implying accelerated evolution rate in the high altitude lizard lineage.206 gene ontology (GO) categories with accelerated evolutionary rates and 43 candidate positively selected genes (PSGs) were detected along the P. erythrurus lineage. Some GO categories and candidate PSGs have functions associated with responses to hypoxia, energy metabolism and responses to UV damage. We were tentatively inferred the mechanisms of high altitude adaptation in P. erythrurus based on those findings. In addition, we found that the high-altitude ranid frog, R. kukunoris, also had higher Ka/Ks ratios compared with the closely related low-altitude frog (R. chensinensis).255 GO categories with accelerated evolutionary rates and 49 PSGs were detected along R. kukunoris lineage, and their functions were mainly overlapped with the corresponding categories detected along the P. erythrurus lineage. Two genes (UBE2D1 and YY1) showed the significant selection signals in both P. erythrurus lineage and R. kukunoris lineage. Those results may reflect a functional convergence evolution between P. erythrurus and R. kukunoris, and there may have similar mechanisms of adaptation to high altitude in the high poikilothermic animals.In conclusion, the mechanisms of high altitude adaptation in P. erythrurus were tentatively inferred based on our findings. By comparing two pairs of low- and high-altitude poikilothermic species, we found that similar functional categories had undergone positive selection in high altitude-dwelling Phrynocephalus and Rana lineages, indicating that similar mechanisms of adaptation to high altitude might have evolved in both genera. Our sequencing data will enriching the molecular resource of the sand lizards studies, and our findings provide important guidance for future functional studies on high altitude adaptation in poikilothermic animals. |
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