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The complete mitochondrial genome of Pelodiscus axenaria (Testudines: Trionychidae).

ABSTRACT: The complete mitochondrial genome sequence of Pelodiscus axenaria was determined by shotgun sequencing. The total length of mitogenome is 16,593 bp, and contains 13 protein-coding genes, 22 tRNA genes, 2 ribosome RNA genes, and 1 control region. Most of the genes of P. axenaria were distributed on the H-strand, except for the ND6 subunit gene and eight tRNA genes which were encoded on the L-strand. The phylogenetic tree of P. axenaria and 11 other closely related species was reconstructed. The phylogenetic analyses based on these mitogenomes presented here will be useful for further insights on the evolutionary relationships of Trionychidae.

SUBMITTER: Yu XM

PROVIDER: S-EPMC7687537 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The characterization of mitochondrial genome has been evidenced as an efficient field of study for phylogenetic and evolutionary analysis in vertebrates including turtles. The aim of this study was to distinguish the structure and variability of the Trionychidae species mitogenomes through comparative analysis. The complete mitogenome (16796 bp) of an endangered freshwater turtle, Nilssonia nigricans was sequenced and annotated. The mitogenome encoded for 37 genes and a major non-coding control region (CR). The mitogenome was A + T biased (62.16%) and included six overlapping and 19 intergenic spacer regions. The Relative synonymous codon usage (RSCU) value was consistent among all the Trionychidae species; with the exception of significant reduction of Serine (TCG) frequency in N. nigricans, N. formosa, and R. swinhoei. In N. nigricans, most of the transfer RNAs (tRNAs) were folded into classic clover-leaf secondary structures with Watson-Crick base pairing except for trnS1 (GCT). The comparative analysis revealed that most of the tRNAs were structurally different, except for trnE (TTC), trnQ (TTG), and trnM (CAT). The structural features of tRNAs resulted ≥ 10 mismatched or wobble base pairings in 12 tRNAs, which reflects the nucleotide composition in both H- and L-strands. The mitogenome of N. nigricans also revealed two unique tandem repeats (ATTAT)8, and (TATTA)20 in the CR. Further, the conserved motif 5'-GACATA-3' and stable stem-loop structure was detected in the CRs of all Trionychidae species, which play an significant role in regulating transcription and replication in the mitochondrial genome. Further, the comparative analysis of Ka/Ks indicated negative selection in most of the protein coding genes (PCGs). The constructed Maximum Likelihood (ML) phylogeny using all PCGs showed clustering of N. nigricans with N. formosa. The resulting phylogeny illustrated the similar topology as described previously and consistent with the taxonomic classification. However, more sampling from different taxonomic groups of Testudines and studies on their mitogenomics are desirable for better understanding of the phylogenetic and evolutionary relationships.

Project description:The characterization of a complete mitogenome is widely used in genomics studies for systematics and evolutionary research. However, the sequences and structural motifs contained within the mitogenome of Testudines taxa have rarely been examined. The present study decodes the first complete mitochondrial genome of the Indian Tent Turtle, Pangshura tentoria (16,657 bp) by using next-generation sequencing. This denovo assembly encodes 37 genes: 13 protein-coding genes (PCGs), 22 transfer RNA (tRNAs), two ribosomal RNA, and one control region (CR). Most of the genes were encoded on majority strand, except for one PCG (NADH dehydrogenase subunit 6) and eight tRNAs. Most of the PCGs were started with an ATG initiation codon, except for Cytochrome oxidase subunit 1 with "GTG" and NADH dehydrogenase subunit 5 with "ATA." The termination codons, "TAA" and "AGA" were observed in two subunits of NADH dehydrogenase gene. The relative synonymous codon usage analysis revealed the maximum abundance of alanine, isoleucine, leucine, and threonine. The nonsynonymous/synonymous ratios were <1 in all PCGs, which indicates strong negative selection among all Geoemydid species. The study also found the typical cloverleaf secondary structure in most of the tRNA genes, except for serine with the lack of the conventional DHU arm. The comparative study of Geoemydid mitogenomes revealed the occurrence of tandem repeats was frequent in the 3' end of CR. Further, two copies of a unique tandem repeat "TTCTCTTT" were identified in P. tentoria. The Bayesian and maximum-likelihood phylogenetic trees using concatenation of 13 PCGs revealed the close relationships of P. tentoria with Batagur trivittata in the studied dataset. All the Geoemydid species showed distinct clustering with high bootstrap support congruent with previous evolutionary hypotheses. We suggest that the generations of more mitogenomes of Geoemydid species are required, to improve our understanding of their in-depth phylogenetic and evolutionary relationships.

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The complete mitochondrial genome of Pelodiscus axenaria (Testudines: Trionychidae).

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