Sequencing and characterization of the complete mitochondrial genome of Critically Endangered Black Soft-shell Turtle (Nilssonia nigricans)

The complete mitochondrial genome (16796 bp) of an endangered freshwater turtle, Nilssonia nigricans was firstly sequenced and annotated. The mitogenome was encoded by 37 genes and a major non-coding Control Region (CR). The mitogenome was A+T biased (62.16%) and spread with six overlapping and 19 intergenic spacer regions. The initiation codons were exceptionally changed as ATA, ATT, and ATC in three Protein-coding genes (PCGs) and a single base (A, T, and C) incomplete termination codons in nine PCGs. The Relative synonymous codon usage (RSCU) value was consistent among all the studied species; exception with significant reduction of Serine (S) frequency in N. nigricans , N. formosa , and R. swinhoei . The secondary structure of N. nigricans showed a lack of conventional dihydrouridine (DHU) arm in trnS (GCT), as well as formed a small loop structure in the acceptor stem of both trnR (TCG) and trnH (GTG). The mitogenome of N. nigricans also revealed two unique tandem repeats (ATTAT) 8 , and (TATTA) 20 in CR. Further, the average Ka/Ks values of 13 PCGs were indicating a strong natural selection in the studied Trionychidae species. The constructed Maximum Likelihood (ML) phylogeny by PCGs shows cohesive clustering of N. nigricans with N. formosa . The resulted phylogeny illustrated the similar topology by all studied species from different taxonomic ranks and supported the previous taxonomic classification. Moreover, further taxon sampling from different taxonomic hierarchy, and their mitogenomics study is vital to reconcile the Testudines phylogeny and assure their evolutionary relationship.