Mitochondrial genome of Scutiger boulengeri (Anura, Megophryidae) reveals an exceptionally expanded non-coding region and its phylogenetic position within Megophryidae

Scutiger boulengeri is an amphibian distributed in high-altitude mountainous environments on the Qinghai-Xizang Plateau and its surrounding regions. However, its mitochondrial genomic characteristics and phylogenetic position remain to be further investigated. In this study, the complete mitochondrial genome of S. boulengeri was sequenced and assembled and a preliminary analysis conducted. The results revealed that the mitochondrial genome of S. boulengeri is 49,991 bp in length and comprises 13 protein-coding genes (PCGs), 22 tRNA genes, 2 rRNA genes, and an expanded non-coding region encompassing the putative mitochondrial control region. The overall organization is largely consistent with basic characteristics of vertebrate mitochondrial genomes. The overall A+T content of this mitochondrial genome is approximately 69.77%, indicating AT bias. The expanded non-coding region encompassing the putative control region reaches 22,418 bp in length, and large non-coding intergenic spacers were identified in the regions adjacent to trnW - trnF and trnP - trnQ . The Ka/Ks values of all 13 PCGs were lower than 1, suggesting that these protein-coding genes as a whole may be subject to purifying selection. Phylogenetic analyses based on cytb and concatenated sequences of the 13 PCGs revealed a close relationship between S. boulengeri and Scutiger liupanensis . Further analysis based on the 13 PCGs dataset suggested that these two species may form a sister group and, together with Scutiger ningshanensis , constitute a closely related clade. Divergence time estimation indicated that the divergence between S. ningshanensis and the clade comprising S. boulengeri and S. liupanensis occurred approximately 5.96 MYA, whereas the recent divergence between S. boulengeri and S. liupanensis occurred approximately 2.10 MYA. These divergence time estimates correspond to the timescale of regional environmental changes during the Pliocene/Pleistocene. In summary, the mitochondrial genome of S. boulengeri exhibits characteristics of structural expansion coexisting with functional conservation. This study provides complete mitochondrial genome data for the Tibetan spiny toad and offers fundamental molecular evidence for investigating molecular evolution and conservation genetics of amphibians in the southeastern Qinghai-Xizang Plateau region.