Comparative analysis and phylogenetic and evolutionary implications of mitogenomes of Chinese Sinocyclocheilus cavefish(Cypriniformes:Cyprinidae)

DEAR EDITOR,Cavefish are of considerable interest due to the remarkable morphological changes that occurred during their adaptation to cave or subterranean river habitats.These changes are thought to involve trade-offs in metabolic requirements and energy utilization.Mitochondria play a vital role in oxygen use and energy metabolism;thus,mitochondrial genes are likely to have experienced specific selective pressures during cavefish evolution.In this study,we assembled and annotated the mitogenome of Sinocyclocheilus sanxiaensis,a typical cavefish species in China.Using this sequence and other available mitogenomes from the NCBI database,we reconstructed an updated phylogeny of Sinocyclocheilus based on 29 species.Five species groups were revealed,with the recently erected S.microphthalmus species group supported.To determine whether divergent selective pressures have acted on the protein-coding genes(PCGs)of the 29 Sinocyclocheilus species exhibiting different morphological features(including eye shape and body color),the fish were classified into three groups according to their cave-related traits.The branch model test revealed higher nonsynonymous/synonymous substitution ratios(ω)in the cave-dwelling groups,potentially resulting from a balance between adaptive selection and relaxed functional constraints.Sophisticated analysis using site and branch-site models identified 24 highly credible positive selection sites located on nine PCGs,with most sites also showing radical changes in amino acid properties.This study sheds light on the phylogeny and mitogenomic evolution of Sinocyclocheilus cavefish,highlighting their successful adaptation and survival in subterranean environments.