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Research Article
Complete mitochondrial genomes of Boiga kraepelini and Hebius craspedogaster (Reptilia, Squamata, Colubridae) and their phylogenetic implications
expand article infoShuangshuang Shan‡, Yu Wang‡
‡ Zhejiang Normal University, Jinhua, China

Corresponding author: Yu Wang ( yuwang@zjnu.edu.cn )

Academic editor: Robert Jadin
© 2022 Shuangshuang Shan, Yu Wang.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation: Shan S, Wang Y (2022) Complete mitochondrial genomes of Boiga kraepelini and Hebius craspedogaster (Reptilia, Squamata, Colubridae) and their phylogenetic implications. ZooKeys 1124: 191-206. https://doi.org/10.3897/zookeys.1124.87861
ZooBank: urn:lsid:zoobank.org:pub:45E005D9-2093-4ABD-B21F-6EC4256EA3D1
Open Access

Abstract

The complete sequence of the mitochondrial genome is a powerful tool for studying phylogenetic relationships and molecular evolution in various species. In this work, the mitogenomes of Boiga kraepelini and Hebius craspedogaster were sequenced and characterized for the first time. The lengths of the B. kraepelini and H. craspedogaster mitogenomes were 17,124 bp and 17,120 bp, respectively, and both included 13 protein-coding genes, 22 tRNAs, two rRNAs and two control regions. The arrangements of these mitochondrial genes were the same in B. kraepelini and H. craspedogaster. In addition, both genome compositions showed A+T bias (59.03%, 60.93%) and had positive AT skews (0.179, 0.117) and negative GC skews (-0.397, -0.348). The phylogenetic results illustrated a close relationship between B. kraepelini and the genus Lycodon. Moreover, H. craspedogaster was clustered with other Hebius snakes and closely related to other Natricinae species. These results will provide references for further research on the phylogeny of Colubridae.

Keywords

Colubrinae, mitogenomes, Natricinae, phylogenetic analysis, protein-coding genes

Introduction

Colubridae is a family with high species diversity in the suborder Serpentes, which is distributed on almost all continents (Pough et al. 2004). The hierarchical classification of Colubridae can be divided into eight subfamilies (Ahaetuliinae, Calimariinae, Colubrinae, Dipsadinae, Grayiinae, Natricinae, Pseudoxenodontinae, and Sibynophiinae) based on molecular markers and morphological characters (Figueroa et al. 2016; Zaher et al. 2019). However, the relationships among these subfamilies and the relationships among genera in a specific subfamily are still unclear since varied genes have been applied in phylogenetic statistics (Lawson et al. 2005; Pyron et al. 2013a, b; Figueroa et al. 2016; Zheng and Wiens 2016; Zaher et al. 2019). Boiga kraepelini Stejneger, 1902 and other Boiga species are arboreal snakes distributed in Asia, Australia and Pacific islands (Weinell et al. 2021). As a genus belonging to Colubridae, Boiga species share the characteristics of rapid movement with other colubrid species, with the exception of posterior groove teeth and low toxicity. Species listed in the genus Hebius are mainly distributed in the eastern, southern and southeastern regions of Asia (Guo et al. 2012). They are usually small- to medium-sized snakes and considered innocuous (Zhao 2006). More evidence should be obtained to understand their phylogenetic position since Hebius is a relatively new genus split from the genus Amphiesma in recent years (Guo et al. 2014).

The mitochondrial genomes of snakes are circular molecules that contain 13 protein-coding genes (PCGs), 22 transfer RNA (tRNA) genes, 2 ribosomal RNA (rRNA) genes, and one or two duplicate control regions. Due to the advantages of small size, matrilineal inheritance, relatively stable genetic structure, easy amplification and high evolutionary rate, partial or full sequences of the mitogenome have been extensively used in molecular evolution, comparative and evolutionary genomics, phylogenetics and population genetics research in various animal species (Kim et al. 2018; Huang et al. 2019). With the development of sequencing technology, a large number of animal mitochondrial genomes have been sequenced and sequences are becoming more accessible (Zhou et al. 2016; Wang et al. 2019). As an informative molecular marker, phylogenetic relationships based on the mitogenome often result in better resolution, reliability and robustness than those of other molecular markers (Madsen et al. 2001). A previous study showed that B. kraepelini was the sister lineage to all 23 other Boiga species (Weinell et al. 2021) and that Hebius is a monophyletic genus (Guo et al. 2014) based on a few gene fragments. Here, the complete mitogenomes of B. kraepelini and H. craspedogaster Boulenger, 1899 were sequenced, annotated and characterized for the first time. To better understand the relationships among Colubridae, complete sequences of 13 mitochondrial PCGs from 38 species of Colubridae and two outgroup species were used to construct a comprehensive phylogenetic tree.

Materials and methods

Sampling and DNA extraction

Specimens of B. kraepelini and H. craspedogaster were collected from Jinhua, China (29°12'N, 119°37'E). Total genomic DNA (gDNA) was extracted from tail muscle using a Rapid Animal Genomic DNA Isolation Kit (Sangon Biotech, China) according to the manufacturer’s instructions.

PCR amplification and sequencing

Conventional polymerase chain reaction (PCR) assays were conducted to amplify the complete mitogenomes of B. kraepelini and H. craspedogaster. The specific primers were designed based on the known nucleotide sequences (Suppl. material 1: Table S1) (Guo et al. 2012; Li et al. 2020; Weinell et al. 2021). Amplification was performed in a total volume of 50 μL, which contained 25 μL of 2× Es Taq MasterMix (CWBIO, China) of 3.0 mM MgCl2, each dNTP at 0.40 mM and 1.0 U of Taq DNA polymerase per μL, 2 μL each of forward and reverse primers (10 μM), 2 μL template DNA and 19 μL of sterilized water. The thermal cycling procedure was applied as follows: an initial pre-denaturation step at 95 °C for 3 min, followed by 35 cycles at 95 °C for 30 s, 60 °C for 45 s, and 72 °C elongation for 1–4 min (depending on the size of fragments), with a final extension at 72 °C for 10 min. The PCR products were recycled and purified using 1.5% agarose gel electrophoresis and genotyped using Sanger sequencing by Sangon Biotech (Shanghai) Co., Ltd., China.

Sequence assembly and gene annotation

The obtained sequences were identified using the Basic Local Alignment Search Tool (BLAST) from NCBI and were assembled using SeqMan software (DNAStar Inc., USA). The complete mitochondrial sequences were annotated by the MITOS web server (http://mitos.bioinf.uni-leipzig.de/index.py) (Bernt et al. 2013) and corrected manually. Transfer RNA (tRNA) genes were identified and predicted in the tRNAscan-SE search server (http://lowelab.ucsc.edu/tRNAscan-SE/) (Lowe and Chan 2016) using the vertebrate genetic code, and their secondary structures were visualized in the Forna web server (http://rna.tbi.univie.ac.at/forna/forna.html) (Kerpedjiev et al. 2015). The base composition of the mitogenome and the relative synonymous codon usage (RSCU) of PCGs were determined using MEGA X (Kumar et al. 2018). The skewness of nucleotide composition was measured according to the following formulas: AT-skew = [A – T] / [A + T] and GC-skew = [G – C] / [G + C] (Perra and Kocher 1995). Graphical maps of the complete mitochondrial genomes were drawn using the online visualization tool mtviz (http://pacosy.informatik.uni-leipzig.de/mtviz).

Phylogenetic analyses

To understand the phylogenetic positions of B. kraepelini and H. craspedogaster, the complete mitochondrial sequences of 13 PCGs in 38 previously available species of Colubridae and two outgroups (Naja atra and Hypsiscopus plumbea) were obtained from GenBank (Table 1). Since nucleotide sequences with substitution saturation has previously plagued phylogenetic analyses, the suitability for phylogenetic tree construction from the dataset was tested first using DAMBE7 software (Xia 2018). The nucleotide sequences were aligned through the MAFFT v.7.475 program with default settings (Katoh et al. 2002). Sequence gaps and poorly aligned regions were removed using Gblocks v.0.91 (Castresana 2000). The best-fit substitution model for the dataset was GTR + I + G by jModelTest v.2.1.10 (Darriba et al. 2012) based on Akaike Information Criterion (AIC). Phylogenetic analyses were performed using Bayesian inference (BI) and maximum likelihood (ML) methods by MrBayes v.3.2.7 (Ronquist and Huelsenbeck 2003) and IQ-TREE v.2.1.2 (Minh et al. 2020), respectively. Four independent runs were conducted using the default settings for 5,000,000 generations with a sampling frequency of 1000 and a burn-in of 25% of samples with Bayesian analyses. Only when the average standard deviation of the split frequencies was less than 0.01 and the effective sampling size greater than 200 were the Markov chain Monte Carlo (MCMC) chains considered convergent. All parameters were assessed by Tracer v.1.7.1 (Rambaut et al. 2018). In the ML analyses, branch support was estimated by 1000 ultrafast bootstrap replicates. The resultant trees were visualized using FigTree v.1.4.4 (http://tree.bio.ed.ac.uk/software/figtree/).

Table 1.
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Mitochondrial genome sequences with GenBank accession numbers used in this study.

Family Species Accession No.
Colubridae Boiga kraepelini This study
Elaphe anomala KP900218
Elaphe bimaculata KM065513
Elaphe dione MH460961
Elaphe carinata KU180459
Elaphe davidi KM401547
Elaphe poryphyracea GQ181130
Elaphe quadrivirgata AB738958
Elaphe quatuorlineata MK334307
Elaphe sauromates MK070315
Elaphe schrenckii KP888955
Elaphe taeniurus KC990021
Euprepiophis perlacea KF750656
Gonyosoma frenatum MW413812
Lycodon flavozonatus KR911720
Lycodon rufozonatum KF148622
Lycodon ruhstrati MK867843
Lycodon semicarinatus AB008539
Oligodon chinensis MK347418
Oocatochus rufodorsatus KC990020
Orientocoluber spinalis MT304473
Pantherophis slowinskii DQ523162
Pituophis catenifer sayi KU833245
Ptyas dhumnades KF148621
Ptyas major KF148620
Ptyas mucosa KT982276
Thermophis baileyi MF326642
Thermophis shangrila MF066951
Thermophis zhaoermii GQ166168
Hebius craspedogaster This study
Hebius optatum MN427890
Hebius vibakari ruthveni KP684155
Nerodia sipedon JF964960
Opisthotropis guangxiensis MT571495
Opisthotropis latouchii MK570292
Pseudagkistrodon rudis MW327508
Rhabdophis tigrinus KU641019
Pseudoxenodon stejnegeri MW018358
Sibynophis chinensis KF360246
Sibynophis collaris JN211315
Elapidae Naja atra EU913475
Homalopsidae Hypsiscopus plumbea DQ343650

Results and discussion

Genome content and organization

The complete mitogenomes of B. kraepelini and H. craspedogaster (GenBank accession numbers: MW699848 and MW699847, respectively) were closed double stranded DNA molecules 17,124 bp and 17,120 bp in length, respectively (Fig. 1). Both contained 37 typical mitochondrial genes, including 13 PCGs, 22 tRNA genes, two rRNA genes (rrnS and rrnL), two putative control regions (CRs) and one origin of light-strand replication (OL). Among these genes, 28 were encoded on the heavy strand, while the remaining nine genes, including one PCG (nad6) and eight tRNAs (trnQ, trnA, trnN, trnC, trnY, trnS2, trnE and trnP), were located on the light strand (Fig. 1, Table 2). The arrangement of genes in these two species was consistent with other species of snakes (Dong and Kumazawa 2005; Li 2014; Qian 2018). The nucleotide composition of B. kraepelini was 34.81% A, 24.22% T, 28.61% C and 12.36% G, and that of H. craspedogaster was 34.04% A, 26.89% T, 26.34% C and 12.73% G. Both species showed a significant bias toward A + T (59.03% for B. kraepelini and 60.93% for H. craspedogaster). In addition, the positive AT skew (0.179 and 0.117) and negative GC skew (-0.397 and -0.348) for B. kraepelini and H. craspedogaster, respectively, indicated higher frequencies of A and C than of T and G present in the whole mitogenome (Table 3). The biased A+T content and skewness in nucleotide composition of B. kraepelini and H. craspedogaster were highly similar to those of other Colubridae species (He et al. 2010; Sun et al. 2017; Wang et al. 2019).

Figure 1. 

Graphical maps of Boiga kraepelini and Hebius craspedogaster mitogenomes. Thirteen protein-coding genes (PCGs) and two ribosomal RNA genes (rrnS and rrnL) are shown with standard abbreviation. Twenty-two transfer RNA (tRNA) are abbreviated by a single letter. CR1 and CR2 are two putative control regions.

Table 2.
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Summary of the mitogenomes of Boiga kraepelini and Hebius craspedogaster.

Gene Strand Boiga kraepelini Hebius craspedogaster Anti-codon
Location Size (bp) Start / Stop codon Location Size (bp) Start / Stop codon
trnF H 1–61 61 – 1–63 63 – GAA
rrnS H 62–978 917 – 64–988 925 – –
trnV H 979–1042 64 – 989–1052 64 – TAC
rrnL H 1043–2498 1456 – 1053–2497 1445 – –
nad1 H 2515–3478 964 ATA/T 2519–3482 964 ATA/T –
trnI H 3479–3544 66 – 3483–3546 64 – GAT
CR2 – 3545–4556 1012 – 3547–4537 991 – –
trnL2 H 4557–4629 73 – 4538–4610 73 – TAA
trnQ L 4631–4701 71 – 4611–4681 71 – TTG
trnM H 4703–4764 62 – 4682–4744 63 – CAT
nad2 H 4765–5794 1030 ATT/T 4745–5771 1027 ATG/T –
trnW H 5795–5859 65 – 5772–5838 67 – TCA
trnA L 5860–5922 63 – 5841–5905 65 – TGC
trnN L 5923–5994 72 – 5906–5978 73 – GTT
OL – 5997–6031 35 – 5981–6015 35 – –
trnC L 6030–6089 60 – 6014–6072 59 – GCA
trnY L 6090–6151 62 – 6074–6135 62 – GTA
cox1 H 6144–7754 1611 ATA/AGG 6128–7738 1611 ATA/AGG –
trnS2 L 7745–7811 67 – 7729–7795 67 – TGA
trnD H 7812–7875 64 – 7796–7860 65 – GTC
cox2 H 7876–8560 685 ATG/T 7862–8546 685 ATG/T –
trnK H 8561–8624 64 – 8547–8609 63 – TTT
atp8 H 8626–8784 159 ATG/TAA 8610–8774 165 ATG/TAA –
atp6 H 8775–9455 681 ATG/TAA 8765–9445 681 ATG/TAA –
cox3 H 9455–10238 784 ATG/T 9445–10228 784 ATG/T –
trnG H 10239–10299 61 – 10229–10289 61 – TCC
nad3 H 10300–10642 343 ATT/T 10290–10632 343 ATA/T –
trnR H 10643–10707 65 – 10633–10696 64 – TCG
nad4L H 10708–10998 291 ATG/TAA 10697–10987 291 ATG/TAA –
nad4 H 10998–12335 1338 ATG/TAA 10987–12321 1335 ATG/TAA –
trnH H 12336–12400 65 – 12322–12386 65 – GTG
trnS1 H 12402–12458 57 – 12388–12444 57 – GCT
trnL1 H 12456–12526 71 – 12442–12512 71 – TAG
nad5 H 12527–14290 1764 ATG/TAA 12514–14295 1782 ATG/TAA –
nad6 L 14286–14798 513 ATG/AGG 14291–14809 519 ATG/AGA –
trnE L 14799–14860 62 – 14810–14872 63 – TTC
cob H 14861–15977 1117 ATG/T 14873–15989 1117 ATG/T –
trnT H 15978–16043 66 – 15990–16053 64 – TGT
trnP L 16044–16105 62 – 16054–16115 62 – TGG
CR1 – 16106–17124 1019 – 16116–17120 1005 – –
Table 3.
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Nucleotide composition of Boiga kraepelini and Hebius craspedogaster mitogenomes; the values for B. kraepelini are shown before the slash (/) and of H. craspedogaster are listed after the slash.

A % T % G % C % A+T % AT-skew GC-skew
Mitogenome 34.81 / 34.04 24.22 / 26.89 12.36 / 12.73 28.61 / 26.34 59.03 / 60.93 0.18 / 0.12 -0.40 / -0.35
PCGs 35.48 / 34.72 23.53 / 27.19 11.00 / 11.15 29.99 / 26.94 59.01 / 61.92 0.20 / 0.12 -0.46 / -0.42
tRNAs 33.38 / 32.82 24.39 / 25.32 16.80 / 17.60 25.44 / 24.26 57.77 / 58.13 0.16 / 0.13 -0.21 / -0.16
rrnS 36.75 / 36.97 18.54 / 20.11 17.78 / 17.84 26.94 / 25.08 55.29 / 57.08 0.33 / 0.30 -0.21 / -0.17
rrnL 40.80 / 39.65 20.33 / 21.25 15.38 / 16.40 23.49 / 22.70 61.13 / 60.90 0.34 / 0.30 -0.21 / -0.16
rRNAs 39.23 / 38.61 19.64 / 20.80 16.31 / 16.96 24.82 / 23.63 58.87 / 59.41 0.33 / 0.30 -0.21 / -0.16
CR1 27.67 / 26.37 33.17 / 33.43 11.68 / 12.64 27.48 / 27.56 60.84 / 59.80 -0.09 / -0.12 -0.40 / -0.37
CR2 27.17 / 26.24 33.20 / 33.00 11.86 / 12.92 27.77 / 27.85 60.38 / 59.23 -0.10 / -0.11 -0.40 / -0.37
CRs 27.42 / 26.30 33.19 / 33.22 11.77 / 12.78 27.62 / 27.71 60.61 / 59.52 -0.10 / -0.12 -0.40 / -0.37

Protein-coding genes and codon usage

The lengths of 13 PCGs of B. kraepelini and H. craspedogaster varied from 159 bp (atp8) to 1764 bp (nad5) and from 165 bp (atp8) to 1782 bp (nad5), respectively (Table 2). The A+T content, AT skew and GC skew of the 13 PCGs in B. kraepelini and H. craspedogaster were 59.01% and 61.92%, 0.203 and 0.122, and -0.463 and -0.415, respectively (Table 3). Excluding terminal codons, a total of 3751 codons were used to encode proteins of B. kraepelini, while a total of 3759 codons were used to encode proteins of H. craspedogaster. All PCGs started with a standard ATN codon (ATA, ATT or ATG) and ended with the stop codon TAA, AGG, AGA or a single T in both species (Table 2). The incomplete stop codon T was frequently found in both species and in other animal mitogenomes (Ojala et al. 1981; Ki et al. 2010; Tang et al. 2020), which might be the result of post-transcriptional polyadenylation (Donath et al. 2019). Relative synonymous codon usage (RSCU), as a key parameter, was used to evaluate the bias of the synonymous codon, and the values obtained reflecting codon usage preference directly in certain gene samples (Table 4). For B. kraepelini and H. craspedogaster, the RSCU showed bias toward AT rather than GC at the third codon position. Twenty-five out of all 60 codons were regarded as abundant since these synonymous codons had positive codon usage bias (RSCU value > 1.0). However, the remaining codons, except for the UCU codon (RSCU value = 1.0) in H. craspedogaster, had negative codon usage bias (RSCU value < 1.0), and they were considered less abundant codons (Li et al. 2018). Furthermore, threonine, leucine 1, and isoleucine were the most common amino acids, while cysteine, serine 1, and aspartic acid were the least common amino acids in these two species.

Table 4.
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Amino acid composition and relative synonymous codon usage (RSCU) in the mitogenome of Boiga kraepelini and Hebius craspedogaster; RSCU values of B. kraepelini are shown before the slash (/) and of H. craspedogaster are listed after the slash.

Amino acid Codon RSCU Codon RSCU Codon RSCU Codon RSCU
Ala (A) GCC 1.79 / 1.68 GCA 1.59 / 1.56 GCU 0.55 / 0.63 GCG 0.07 / 0.13
Arg (R) CGA 2.69 / 2.26 CGC 0.63 / 0.52 CGU 0.44 / 0.84 CGG 0.25 / 0.39
Asn (N) AAC 1.68 / 1.15 AAU 0.32 / 0.85
Asp (D) GAC 1.72 / 0.97 GAU 0.28 / 1.03
Cys (C) UGC 1.33 / 0.96 UGU 0.67 / 1.04
Glu (E) GAA 1.74 / 1.74 GAG 0.26 / 0.26
Gln (Q) CAA 1.83 / 1.86 CAG 0.17 / 0.14
Gly (G) GGA 1.96 / 1.60 GGC 0.80 / 0.88 GGG 0.73 / 0.70 GGU 0.51 / 0.82
His (H) CAC 1.66 / 1.41 CAU 0.34 / 0.59
Ile (I) AUC 1.22 / 0.90 AUU 0.78 / 1.10
Leu1 (L1) CUA 3.16 / 2.25 CUC 0.65 / 0.61 CUU 0.55 / 0.96 CUG 0.41 / 0.23
Leu2 (L2) UUA 1.09 / 1.68 UUG 0.15 / 0.26
Lys (K) AAA 1.89 / 1.79 AAG 0.11 / 0.21
Met (M) AUA 1.80 / 1.76 AUG 0.20 / 0.24
Phe (F) UUC 1.28 / 1.02 UUU 0.72 / 0.98
Pro (P) CCA 2.54 / 2.65 CCC 1.02 / 0.75 CCU 0.28 / 0.44 CCG 0.16 / 0.16
Ser1 (S1) AGC 0.88 / 0.66 AGU 0.27 / 0.33
Ser2 (S2) UCA 2.47 / 2.55 UCC 1.58 / 1.31 UCU 0.63 / 1.00 UCG 0.16 / 0.15
Thr (T) ACA 1.96 / 1.94 ACC 1.56 / 1.29 ACU 0.41 / 0.65 ACG 0.08 / 0.11
Trp (W) UGA 1.68 / 1.76 UGG 0.32 / 0.24
Tyr (Y) UAC 1.34 / 0.96 UAU 0.66 / 1.04
Val (V) GUA 1.70 / 1.71 GUU 0.96 / 1.14 GUC 0.78 / 0.62 GUG 0.56 / 0.52

Transfer RNA, ribosomal RNA genes and the A + T-rich region

Similar to other snakes, 22 tRNA genes were recovered from the mitogenomes of B. kraepelini and H. craspedogaster. The tRNA lengths of these two species ranged from 57 bp (trnS1) to 73 bp (trnL2) (Table 2). The AT content of B. kraepelini and H. craspedogaster were between 43.94% (trnI) and 66.20% (trnQ) and 43.75% (trnI) and 66.67% (trnK), respectively (Suppl. material 1: Table S2). In addition, the tRNA genes of B. kraepelini and H. craspedogaster had a positive AT skew (0.16 and 0.13, respectively) and a negative GC skew (-0.21 and -0.16, respectively) (Table 3). All tRNA genes, except trnS1 and trnC, showed typical cloverleaf secondary structures (Figs 2, 3). The trnS1 gene lacked a dihydroxyuridine arm (D arm), and the trnC gene lacked the T Ψ C loop. Deletions of the D arm and/or T Ψ C loop in tRNA genes of the mitogenome are known to occur in other Colubridae species (Li 2014). tRNA genes may lack the D arm or the T arm may exhibit lower amounts of peptide production or lower levels of aminoacylation and EF-Tu binding abilities (Watanabe et al. 2014). No pseudogene trnP was found between mitochondrial genes trnI and CR2 in either species, although it was present in some snakes (Kumazawa et al. 1998; Dong and Kumazawa 2005; Jiang et al. 2007). Species without pseudogene trnP were considered primitive snakes (Wang et al. 2009). Different from the typical arrangement of the mitogenome in vertebrates, here trnL (UUR) translocated from its original position between rrnL and nad1 to the position between CR2 and trnQ. The rearrangement of the trnL (UUR) gene is common in Alethinophidia (Dong and Kumazawa 2005; Yan et al. 2008; Chen and Zhao 2009).

Figure 2. 

Secondary structure of tRNAs in the mitogenome of Boiga kraepelini.

Figure 3. 

Secondary structure of tRNAs in the mitochondrial genome of Hebius craspedogaster.

As shown in Table 2, the gene rrnS in B. kraepelini was 917 bp in length and located between trnF and trnV, while the gene rrnL was 1456 bp in length and located between trnV and nad1. The rrnS and rrnL genes in H. craspedogaster were 8 bp longer and 11 bp shorter, respectively, than those in B. kraepelini. These two rRNA genes were AT biased; the A+T content of rrnS genes was 55.29% in B. kraepelini and 57.08% in H. craspedogaster, and the A+T content of rrnL genes was 61.13% in B. kraepelini and 60.90% in H. craspedogaster (Table 3). Both rrnS and rrnL in the two species showed the same nucleotide composition of the mitogenome: A > C > T > G.

Additionally, similar to some snakes, there were two control regions in both species mitogenomes, in which CR1 was located between trnP and trnF, and CR2 was located between trnI and trnL (UUR). The nucleotide composition and length of the two control regions in the same species were almost identical. The AT skews and GC skews of the two CRs in B. kraepelini and H. craspedogaster were negative, indicating that T and C were more numerous than A and G (Table 3).

Phylogenetic analyses

Phylogenetic trees were constructed based on nucleotide sequences of 13 PCGs in 38 Colubridae species and two outgroups from the families Elapidae and Homalopsidae (Fig. 4). An identical topological structure was produced using both BI and ML methods. Five monophyletic clades that represented five subfamilies, Colubrinae, Natricinae, Sibynophiinae, Dipsadinae and Pseudoxenodontinae, were identified in the family Colubridae. The tree showed a close relationship (BI posterior probabilities [PP] = 1; ML bootstrap [BP] = 67) between Natricinae and Sibynophiinae, and the subfamily Colubrinae was a sister clade of the clade containing Natricinae and Sibynophiinae. These results were consistent with the findings from previous phylogenetic studies (Figueroa et al. 2016; Zaher et al. 2019). In terms of species, B. kraepelini was well supported as most closely related to the genus Lycodon in the subfamily Colubrinae. In addition, both Figueroa et al. (2016) and Weinell et al. (2021) reported that the genus Boiga was the sister group of the genus Lycodon based on multiple mitochondrial segments and nuclear genes. Hebius craspedogaster was clustered with other Hebius species and formed a monophyletic clade. The monophyly of the genus Hebius was also supported by multilocus (Deepak et al. 2021) and morphological (Hou et al. 2021) phylogenetic analyses.

Figure 4. 

Phylogenetic tree inferred from the nucleotide sequences of 13 mitogenome protein-coding genes using the Bayesian inference (BI) and maximum likelihood (ML) methods. Values on branches separated by slash (/) indicate posterior probability (BI, left) and bootstrap (ML, right).

Both Boiga and Hebius are species-rich genera in the family Colubridae, with more than 30 species each (Uetz et al. 2022). The phylogenetic relationships within each genus are still unresolved since there are still some species with uncertain systematic positions (Pyron et al. 2013a, 2013b; Deepak et al. 2021). The first mitogenome sequence of Boiga and the complete mitochondrial sequence of H. craspedogaster from this study will provide more molecular evidence to clarify their taxonomic status and understand potential unknown evolutionary relationships.

Conclusions

In this study, we sequenced and characterized the complete mitochondrial genomes of B. kraepelini and H. craspedogaster for the first time. The mitogenomes of B. kraepelini and H. craspedogaster were 17,124 bp and 17,120 bp in size, respectively, including 13 PCGs, 22 tRNAs, two rRNAs and two control regions. Both (B. kraepelini and H. craspedogaster) genome compositions were A+T biased (59.03% and 60.93%, respectively) and showed positive AT skews (0.179 and 0.117, respectively) and negative GC skews (-0.397 and -0.348, respectively). All of the tRNA genes could be folded into typical cloverleaf secondary structures, with the exception of trnS1, which lacks the D arm, and trnC, which lacks the T Ψ C loop. Phylogenetic analyses were performed with 38 other species from the family Colubridae and two outgroup species. Five clades that represent five subfamilies, Colubrinae, Natricinae, Sibynophiinae, Dipsadinae and Pseudoxenodontinae, were identified. The genus Boiga was closely related to the genus Lycodon, and both genera belong to the subfamily Colubrinae. Hebius craspedogaster was clustered with the other two Hebius species and closely related to other Natricinae species. This work will be helpful for understanding the evolutionary relationships within the family Colubridae and will provide basic data for the molecular identification of these two species.

Acknowledgements

This research was supported by the National Natural Science Foundation of China under Grant No. 31400472 and Zhejiang Provincial Natural Science Foundation of China under Grant No. L19C030005.

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Supplementary material

Supplementary material 1 

Table S1, S2

Shuangshuang Shan, Yu Wang

Data type: docx file

Explanation note: Table S1. Primers used for mitogenome amplification of Boiga kraepelini and Hebius craspedogaster. Table S2. Nucleotide composition of each tRNA of Boiga kraepelini and Hebius craspedogaster.

This dataset is made available under the Open Database License (http://opendatacommons.org/licenses/odbl/1.0/). The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this Dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited.
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