The complete mitochondrial genome of Schizothorax eurystomus Kessler, 1872 (Cypriniformes, Cyprinidae)

Abstract Schizothorax eurystomus, Kessler 1872 is a unique economic fish in Xinjiang, China that is rarely seen in the market. Next-generation sequencing (NGS) was used to determine the complete mitochondrial genome of S. eurystomus collected from the Yarkand River in Xinjiang. The results showed that the mitochondrial genome is a circular, 16,488-bp-long nucleotide with the typical vertebrate genome structure of 13 protein-coding genes, 2 ribosomal RNA genes, 22 transfer RNA genes, and a control region. The termination-associated sequence (TAS), central conserved sequence block (CSB), and conserved sequence block were detected in the control region. Phylogenetic analysis placed S. eurystomus in a fully supported clade with S. biddulphi, and that clade was sister to S. yunnanensis. To our knowledge, this is the first study on the complete mitochondrial genome of S. eurystomus from the Yarkand River in Xinjiang, and it provides baseline genetic information for future studies.

Schizothorax eurystomus, Kessler 1872, also known as the wide-mouth hip scale fish, is a cold-water white fish that belongs to order Cypriniformes, family Cyprinidae, and subfamily Schizothoracinae (Cao et al. 2019). The S. eurystomus body is lengthened and slightly laterally flattened, and its head is tapered; S. eurystomus characteristics are shown in Figure 1. S. eurystomus used to be the main economic fish in the Tarim River and its tributaries and affiliated lakes (Cao et al. 2019). Recently, with the impact of hydropower development and human activities, the S. eurystomus resources are decreasing, and the Xinjiang autonomous region plans to list it as a protected fish (Nie et al. 2014).
Next-generation sequencing (NGS) has revolutionized the field of molecular biology because it is rapid and can generate large amounts of genomic data (Schuster 2008;Koboldt et al. 2013). Therefore, in this study, the complete mitochondrial genome of S. eurystomus was determined using NGS technology to provide insight into the population processes and evolutionary history of this species (Zhang and Xian 2015).
DNA was extracted from muscle tissue of S. eurystomus collected from Altash Station (76.29E, 37.59 N) of the Xinjiang Yarkand River in September 2021. A specimen was deposited at the CAS Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences (Hui Jia, jiahui@qdio.ac.cn) under voucher number L212. The Illumina NovaSeq sequencing platform (Illumina, San Diego, CA) was used to process sequences.
The high-quality second-generation genome sequence was assembled and analyzed using A5-miseq v20150522 (Coil et al. 2015) and SPAdes version 3.9.0 (Bankevich et al. 2012). Pilon version 1.18 (Walker et al. 2014) was used to correct the results and obtain the final mitochondrial sequence. Annotation of the complete mitochondrial genome sequence was performed on the MITOS web server (http://mitos2.bioinf.uni-leipzig.de/index.py) (Bernt et al. 2013). After sequencing, the maximum likelihood (ML) method with K2P distances was used to construct the ML tree in MEGA10 with 1000 bootstrap replicates (Kumar et al. 2018).
Similar to the size of other teleost mitogenomes, the complete mitochondrial genome of S. eurystomus was 16,488-bp long (GenBank accession ON920824). The circular map of the complete S. eurystomus mitochondrial genome is shown in Figure 1. The complete mitochondrial genome contained 13 protein-coding genes, 2 ribosomal RNA (rRNA) genes (12S rRNA and 16S rRNA), 22 transfer RNA (tRNA) genes, and a control region, similar to other vertebrates (Miya et al. 2001).
The 12S and 16S rRNA genes of S. eurystomus were 956and 1630-bp long, respectively. They were located between tRNA Phe and tRNA Leu , and were separated by tRNA Val , similar to other vertebrates (Zhang et al. 2016). The 22 tRNA genes were scattered throughout the genome, ranging 67-76 bp and folded into a cloverleaf secondary structure with normal base shedding. The major non-coding region in S. eurystomus was located between tRNA Pro and tRNA Phe , and was 821-bp long. The termination-associated sequence (TAS), central conserved sequence block (CSB), and CSB were detected in the control region and were similar to those of most bony fishes (Jia et al. 2021).
One complete mitogenome of S. eurystomus has been deposited in GenBank (https://www.ncbi.nlm.nih.gov/nuccore/KY436758.1), however, there is no paper published for the description in details on it. In this study, we collected and identified five specimens, and then used NGS to obtain their mitochondrial genome. The complete sequences were used for phylogenetic analysis and the phylogenetic results revealed that a previously collected specimen (KY436758.1) was genetically distant from other related species; therefore, the complete mitochondrial genome of S. eurystomus needed to be updated. Phylogenetic analysis of the complete mitochondrial genome of S. eurystomus revealed that it belonged to a clade with S. biddulphi, and they are sister to S. yunnanensis ( Figure 3). This study is the first to phylogenetically analyze S. eurystomus in detail and provides a scientific basis for future molecular systematic and phylogenetic studies of bony fishes in Cyprinidae.

Ethical approval
This study did not involve ethical approval, and the field investigation was supported by the Xinjiang Fisheries Research Institute, Urumqi 830000, China.

Disclosure statement
No potential conflict of interest was reported by the author(s).