Abstract
BACKGROUND: Cavefish exhibit significant morphological changes that result in trade-offs in metabolic requirements and energy utilization in perpetual darkness. As cellular "powerhouses", mitochondria play crucial roles in energy metabolism, suggesting that mitochondrial genes have likely experienced selective pressures during cavefish evolution. RESULTS: This study presents the first assembly of the complete mitogenome of Triplophysa yangi, a typical cavefish species in China. The mitogenome is 17,068 bp long, marking the longest recorded for the genus Triplophysa, and includes 13 protein-coding genes (PCGs), 2 rRNAs, 25 tRNAs, and a noncoding control region. An ~ 500 bp insertion between ND2 and WANCY regions was observed, comprising a large intact tandem repeat unit (A'-N'-OL'-C') flanked by two unannotated sequences (U1/U2). The evolutionary origin of this repeat unit may involve either in situ duplication events with subsequent functional divergence-where neofunctionalization, subfunctionalization, or pseudogenization drove differential mutation rates between paralogs-or alternatively, horizontal acquisition from exogenous genetic material that became functionally integrated into the ancestral T. yangi mitogenome through co-option mechanisms. Phylogenetic analyses revealed two major clades within Triplophysa-epigean and hypogean lineages-consistent with previous classifications, while cave-restricted species exhibited signs of parallel evolution within the hypogean lineage. Selective pressure analysis indicated that the hypogean lineage (cave-dwelling groups, II & III) have a significantly increased ratio of nonsynonymous to synonymous substitution rates (ω) compared to the epigean lineage (surface-dwelling group, I), suggesting a combination of adaptive selection and relaxed functional constraints in cave-dwelling species. CONCLUSIONS: The duplication of tRNAs in T. yangi and the potential positive selection sites identified in Triplophysa cavefish further indicated adaptive evolution in mitochondrial PCGs in response to extreme subterranean conditions.