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Sequencing and characterization of the complete mitochondrial genome of Japanese Swellshark (Cephalloscyllium umbratile)

To further comprehend the genome features of Cephalloscyllium umbratile (Carcharhiniformes), an endangered species, the complete mitochondrial DNA (mtDNA) was firstly sequenced and annotated. The full-length mtDNA of C. umbratile was 16,697 bp and contained ribosomal RNA (rRNA) genes, 13 protein-coding genes (PCGs), 23 transfer RNA (tRNA) genes, and a major non-coding control region. Each PCG was initiated by an authoritative ATN codon, except for COX1 initiated by a GTG codon. Seven of 13 PCGs had a typical TAA termination codon, while others terminated with a single T or TA. Moreover, the relative synonymous codon usage of the 13 PCGs was consistent with that of other published Carcharhiniformes. All tRNA genes had typical clover-leaf secondary structures, except for tRNA-Ser (GCT), which lacked the dihydrouridine ‘DHU’ arm. Furthermore, the analysis of the average Ka/Ks in the 13 PCGs of three Carcharhiniformes species indicated a strong purifying selection within this group. In addition, phylogenetic analysis revealed that C. umbratile was closely related to Glyphis glyphis and Glyphis garricki. Our data supply a useful resource for further studies on genetic diversity and population structure of C. umbratile.

a number of shared unique mitochondrial gene features in Chondrichthyes, towards a better understanding of the functions and evolution of Chondrichthyes [24][25][26][27] . So far, there was still a notably lack of mtDNA information in Carcharhiniformes. In order to provide a theoretical foundation for the conservation strategy of C. umbratile within Scyliorhinidae and new sight for further studies of phylogenetically-informative sequence data, in the current study the complete mtDNA of C. umbratile was sequenced, assembled and annotated, and compared with other members of Carcharhiniformes.

Results and Discussion
Genome size and organization. About 1.5 G raw data is generated with reads length 125 bp. Sequencing coverage and depth (X) of mtDNA data is 100% and approximately 394.23, respectively. Reads number is 52,660 and total bases (bp) is 6,582,500. The mtDNA of C. umbratile was a closed-circular DNA molecule of 16,697 bp in length (GenBank: KX354996; Fig. 1, Table 1), which was comparable to other Carcharhiniformes mtDNA ranging from 16,697 bp in Scyliorhinus canicula 25 to 16,719 bp in Carcharhinus acronotus 28 . Nucleotide BLAST (blastn) of the whole C. umbratile mtDNA against other Carcharhiniformes revealed sequence identities with closely related species of 88% (S. canicula), 84% (Proscyllium habereri), and 84% (Pseudotriakis microdon) and with distantly related species of 82% (Scoliodon laticaudus), 82% (Hemigaleus microstoma), 82% (Hemipristis elongata) (Supplementary Table 1). The mtDNA of C. umbratile contained 2 rRNA genes, 13 PCGs, 22 tRNA genes and D-loop region. The arrangement of the genes was identical to that of other Scyliorhinidae mtDNA (Table 1) 29,30 . Among these genes, 29 genes (12 PCGs, 2 rRNA genes and 15 tRNA genes) are located on the heavy strand (H-strand) and the others (1 PCGs and 8 tRNA genes) are located on the light strand (L-strand) ( Table 1). These obvious features have also been reported in other Carcharhiniformes species 31,32 and could be regarded as effective markers for authentication at genus and species level.
The nucleotide composition of the mtDNA is biased toward A + T nucleotides (52.9%), which made up of 61.8%, 61.4%, 61.5% and 68.7% in the PCGs, tRNA, rRNA and D-loop region, respectively (Table 2). However, the A + T nucleotide composition in C. umbratile was the lowest among Carcharhiniformes. The positive AT skew (0.025) observed here with the presence of more As than Ts, was similar to that only in Sphyrna tiburo (0.031), nevertheless, mtDNA in majority of Carcharhiniformes showed negative AT skew ( Table 2). The GC skew ranged  Table 2). The C. umbratile mtDNA was negative (−0.245), indicating the presence of more Cs than Gs.
Protein-coding gene features. The PCG region formed 68.5% of the C. umbratile mitogenome, and was 11,440 bp long. Furthermore, a contrast of nucleotide composition, AT-skew, and GC-skew of Carcharhiniformes PCGs were exhibited in Table 2. A + T content of the rRNA genes was 61.75%. The AT skew value (−0.282) of the PCG region in the C. umbratile mtDNA was higher than that of several reported mtDNA, nevertheless the negative GC skew (−0.069) was similar to that observed in other fish 33,34 .
A total of 3,803 amino acids of PCGs are encoded in C. umbratile. In addition, the codon usage is shown in Table 3. The most frequent amino acids in the PCGs of C. umbratile were Leucine (17.3%), Isoleucine (9.02%) and Alanine (7.45%) ( Continued that the codons encoding Leu, Thr, Ala, Arg, Gln, Gly, Pro and Ser were the most frequently present, nevertheless those encoding Asn, Asp, Cys and Lys were rare (Fig. 2). In the PCGs of the eight species examined, codon distributions and amino acid content were corresponding among species (Fig. 3). It was declared that conserved amino acid sequences were present among those fish 28,32,40 . Moreover, codons with A or T in the third position were overused in comparison to other synonymous codons, for example, the codons for glutamine CAG and GAG were rare, while the synonymous codons CAA and GAA were prevalent (Fig. 4), which is consistent with previous observations of Carcharhiniformes 36 .
Transfer RNAs and ribosomal RNAs. The representative complement structures of 22 tRNAs were identified in the C. umbratile mtDNA, ranging from 62 bp (tRNA Thr ) to 76 bp (tRNA Lys ) 35,36 for 1,538 bp in total (Table 1). Of those, the highest A + T content of tRNAs was S. canicula and the lowest was C. sorrah. Fifteen tRNA genes were encoded on the H strand while the remains were located in the L strand ( Table 1). The overall A+T content of tRNAs was 61.38% which was approximate to that observed in Loxodon macrorhinus (61.4%). The negative AT skew (−0.098) and positive GC skew (0.059) showed in the C. umbratile mtDNA were also analogous with several sequenced Carcharhiniformes (Table 2).     Table 3. Codon usage of Cephalloscyllium umbratile mitochondrial protein-coding genes. The forecasted tRNAs were shown in Fig. 5. All of the tRNAs could be folded into classic clover-leaf secondary structures in C. umbratile, except for tRNA-Ser (GCT), which lacked the dihydrouridine 'DHU' arm (Fig. 5). The 'DHU' arm of this tRNA was a large loop instead of the conserved stem-and-loop structure. Due to a representative characteristics 41 , it was also observed in other Chondrichthyes mtDNA, including Chiloscyllium griseum 42 T. obesus 37 and so on. Fifteen of the tRNA genes were each observed to have at least one G-T mismatches in their respective secondary structures, which forming a weak bond. Five T-T mismatches were present in the respective amino acid acceptor stems of tRNA Asp(GTC) , tRNA Cys(GCA) , tRNA His(GTG) , tRNA Ile(GAT) and tRNA Met(CAT) (Fig. 5). Interestingly, A-G mismatch was also present in tRNA-Leu (TAA). Unmatched base pairs perceived in tRNA sequences can be amended by RNA-editing mechanisms that were well known for vertebrate mtDNA 43 . The A + T content of the rRNA genes was 61.46%, indicating an A+C-rich trend as in other Scyliorhinidae fish 25 . AT and GC skews were negative (−0.082) and positive (0.132), respectively ( Table 2). The 12S rRNA and 16S rRNA subunit gene of C. umbratile was 954 bp and 1,668 bp in length, respectively. As in other vertebrates 44 , both two genes are separated by the tRNA Val gene, and located between tRNA Phe and tRNA Leu(UUR) (Fig. 1, Table 1). The overall content of the rRNA was analogous to that observed for other Carcharhiniformes. The control region. The length of D-loop region of C. umbratile was 1,059 bp, which was less long than majority of Carcharhiniformes. The A + T content was 68.65%, and equal with other Carcharhiniformes (Table 2), which was consistent with the findings of previous reports on other teleosts 33,45,46 . Moreover, both of the AT-skew and GC-skew were strongly negative ( Table 2).
Overlapping and intergenic spacer regions. There were three gene boundaries where bases overlapped between adjacent genes, ranging from 4-22 bp in size. The longest overlapping region was 22 bp between ATP8 and ATP6 (Table 1) which has been documented in several other Chondrichthyes mtDNA 4,25,32 . Moreover, intergenic spacers of C. umbratile were spread over 12 locations and ranged from 1-36 bp, making up 60 bp in total, and the longest intergenic spacer region (36 bp) was between tRNA Asn and tRNA Cys (Table 1).
Synonymous and nonsynonymous substitutions. The ratio of Ka/Ks is generally regarded as a pointer of selective pressure and evolutionary relations at the molecular level among homogenous or heterogeneous species 47,48 . It is reported that Ka/Ks > 1, Ka/Ks = 1, and Ka/Ks < 1 popularly declared positive selection, neutral mutation and negative selection, respectively 49 . To investigate the evolutionary rate differences in three Carcharhiniformes mtDNA (C. umbratile, S. canicula and P. habereri), sequence divergences by counting Ka and Ks substitution rates were next calculated. The Ka/Ks values of 13 PCGs varied from 0.0198 (COXI) to 0.5322 (ATP8) and were less than 0.6 (Ka was lower than Ks) for all other genes which indicated a strong purifying and negative selection in those fishes (Fig. 6). Our result of the Ka/Ks ratio illustrated that the multitudinous genes evolved under strong negative selection which meant natural selection against profitless mutations with negative selective coefficients 50 . The percentages of variable sites of SC/PH were the highest in COXIII and ND1 among the groups, while the percentages was the least in COXI gene, which indicated that COXIII and ND1 were under the least selective pressure, and COXI was under the most selective pressure among all mitochondrial proteins. In C. umbratile and S. canicula, the ratio of Ka/Ks was the least in all 13 protein-coding genes compared to P. habereri, implying that these two Scyliorhinidae fish had the closer phylogenetic relationship than P. habereri, which was consistent with their rozmieszczenie naturalne and ecological habit 25 . Phylogeny. To understand the phylogenetic relationships among Carcharhiniformes, base on Maximum Likelihood (ML), Neighbor Joining (NJ) and Bayesian Inference (BI) methods, a dataset of 25 species containing the concatenated nucleic acid and amino acid sequences of 13 PCGs was used to generate phylogenetic relationships (Fig. 7). The topologies of the 6 phylogenetic trees were analogical in our study. The results implied that strong statistics supported for the following relationship among the 5 Superfamily (Scyliorhinidae, Carcharhinidae, Hemigaleidae, Proscylliidae, Pseudotriakidae) (Fig. 7A,B). This clustered pattern of 5 Superfamily was broadly consistent with previous studies 32,42,[51][52][53] . Furthermore, based on all of ML, NJ and BI methods, 5 superfamily divided into 13 closely genera, and C. umbratile (Cephaloscyllium) was most closely related to S. canicula (Scyliorhinus) in Scyliorhinidae, which was accord with the tendency of nucleotide sequence identity and a recent study 51,[54][55][56][57] . Scyliorhinidae was most closely related to Proscylliidae. Additionally, further taxon sampling within Scyliorhinidae and related superfamilies is required to resolve the location of Scyliorhinidae in Carcharhiniformes.

Materials and Methods
Sample collection and mitochondrial DNA extraction. C. umbratile juveniles were collected from South China Sea (Longitude 5°20.267′ N and latitude 109°48.435′ E) in September 2014 and directly frozen. Muscle tissues were used for DNA extraction according to the Genomic DNA Extraction Kit's instructions (TaKaRa MiniBEST Universal Genomic DNA Extraction Kit Ver.5.0, Japan). The quantity (concentration) of isolated total DNA was determined by NANODROP 2000 spectrophotometer (Thermo Scientific, USA). Furthermore, quality of extracted DNA was assessed by electrophoresis on a 1% agarose gel stained with Gel Red ™ (Biotium).
Genome sequencing. According to NEBNext DNA sample libraries kit (NEB, New England)'s instructions, the normalized DNA (4 μg) was used to structure the paired-end library. Size and quantification estimation of the library were implemented by a Bioanalyzer 2100 High Sensitivity DNA chip (Agilent, USA). Illumina HiSeq. 2500 (2 × 101 bp paired-end reads) (Illumina, USA) was used to sequence the normalized library (2 nM).
Genome assembly and annotation. A de novo assembly of the paired-end HiSeq reads was performed using SeqMan NGen (http://www.dnastar.com/t-tutorials-seqman-ngen.aspx) (DNASTAR Inc., Madison, WI, USA) 58 . Assembly parameters minimum match percentage, match spacing, match size, gap penalty, mismatch penalty, maximum gap length and expected genome length were set to 93, 10, 50, 30, 20, 6% and 16,000, respectively. Accordance sequence was exported and ends were manually edited to remove duplicated nucleotides. Subsequently, the assembled sequences were aligned to NCBI nt database with blastn method (https://blast.ncbi. nlm.nih.gov/). Sequences that mapping to Carcharhiniformes mtDNA were considered as C. umbratile mtDNA. To verify the accuracy of the assembled mtDNA sequence, the primers (Supplementary Table 2) were used to amplify the genome sequence. The procedure of PCR amplification was referred from Sun et al. 59 . To determine whether this method was accurate, the sequence segments of same genomic region obtained from Sanger sequencing and shotgun assembly were compared. If they were identical, that meaning this method was precise. Moreover, the PCGs, rRNA genes, tRNA genes and D-loop region of mtDNA were annotated by MitoAnnotator (http://mitofish.aori.u-tokyo.ac.jp/annotation/input.html) 60 with parameters of complete circular genome. The mtDNA sequence of C. umbratile has been deposited in the GenBank database under accession numbers KX354996.
Genome sequence analysis. tRNAscan-SE Search Server 1.21 program was used to primordially determine Transfer RNAs 61,62 . The gene map of C. umbratile mtDNA was built by OGDRAW1.2 and embellished manually 63 . The strand skew values were reckoned in terms of the formulae by Perna and Kocher (1995) 64 . The mode of "models-> Compute Codon Usage Bias" was chose to obtain RSCU in MEGA 6.0 65  Phylogenetic analysis. To discuss the phylogenetic position of Carcharhiniformes, a total of 25 species of 13 PCG sequences were used to perform phylogenetic analysis, including those of C. umbratile. Alignments of the 13 concatenated PCGs nucleotide and amino acid sequences were conducted using ClustalX version 2.0 with default parameters 68 . Phylogenetic analyses for each concatenated dataset was performed using ML, MP and BI methods with MEGA 6.0 and Phylobayes 3.3 f, respectively 65,69 . The methods of ML and MP analysis were  Table 1. performed with GTR+I+G model and Subtree-Purning-Regrafting (SPR) model using MEGA 6.0, respectively. The evaluation of node accuracy was done by using 1,000 bootstrap replicates in MEGA 6.0 with default parameters. Furthermore, BI analysis was selecting the CAT-GTR model, two independent Markov chain Monte Carlo (MCMC) chains were run for 10,000 cycles. The phylogenetic tree was embellished using FigTree v1.4.2 (http:// tree.bio.ed.ac.uk/software/figtree/).