Characterization of the complete mitochondrial genomes of two Critically Endangered wedgefishes: Rhynchobatus djiddensis and Rhynchobatus australiae

We present the complete mitochondrial genomes of the Critically Endangered whitespotted wedgefish, Rhynchobatus djiddensis (Forsskål, 1775), and bottlenose wedgefish, Rhynchobatus australiae (Whitley, 1939), with the R. djiddensis mitogenome documented for the first time. The genomes for R. djiddensis and R. australiae are 16,799 and 16,805 bp in length, respectively. Both comprise 13 protein-coding regions, 22 tRNA genes, two rRNA genes, and a non-coding control region. All protein-coding regions consistently start with the ATG start codon; however, the alternative start codon GTG is observed at the start of the COX1 gene. NADH2, COX2, and NADH4 have incomplete stop codons: T or TA, and tRNALeu and tRNASer, have atypical codons: UAA, UGA, GCU, and UAG. The phylogenetic analysis places R. djiddensis and R. australiae within the Rhynchobatus genus, separate from other families in the order Rhinopristiformes. We also highlight the most variable gene regions to expedite future primer design, of which NADH2 was the most variable (4.5%) when taking gene length into account. These molecular resources could promote the taxonomic resolution of the whitespotted wedgefish species complex and aid in the genetic characterization of populations of these and related species.本研究报道了极危物种白点犁头鳐（Rhynchobatus djiddensis，Forsskål, 1775）与瓶鼻犁头鳐（Rhynchobatus australiae，Whitley, 1939）的完整线粒体基因组，其中R. djiddensis的线粒体基因组为首次报道。二者的线粒体基因组全长分别为16799 bp与16805 bp，均包含13个蛋白质编码区、22个转运RNA（tRNA）基因、2个核糖体RNA（rRNA）基因以及1个非编码控制区。所有蛋白质编码区均以ATG作为起始密码子，但细胞色素c氧化酶亚基1（COX1）基因的起始位点使用了替代起始密码子GTG。烟酰胺腺嘌呤二核苷酸脱氢酶亚基2（NADH2）、细胞色素c氧化酶亚基2（COX2）以及烟酰胺腺嘌呤二核苷酸脱氢酶亚基4（NADH4）的终止密码子不完整，仅为T或TA；而亮氨酸转运RNA（tRNALeu）与丝氨酸转运RNA（tRNASer）存在非典型密码子：UAA、UGA、GCU及UAG。系统发育分析结果显示，R. djiddensis与R. australiae均隶属于犁头鳐属（Rhynchobatus），与犁头鳐目（Rhinopristiformes）内的其他类群分属不同的科。本研究同时筛选出变异度最高的基因区域以助力后续引物设计：综合基因长度考量，烟酰胺腺嘌呤二核苷酸脱氢酶亚基2（NADH2）的变异率最高，达4.5%。本研究提供的分子数据资源，可提升白点犁头鳐物种复合群的分类分辨率，并为该类群及近缘物种种群的遗传特征分析提供支持。