COI DNA Barcoding of Six Schizothoracine Fishes from the Tarim River Basin, Xinjiang, China: Implications for Species Delimitation and Phylogenetic Relationships

To test the performance of COI-based DNA barcoding in species delimitation, we amplified and sequenced the mitochondrial COI gene from six Schizothoracine fishes endemic to Xinjiang, China. We then characterized the COI gene dataset, quantified genetic divergence, and inferred phylogenetic relationships using distance-based approaches. Morphological examination supported clear phenotypic differentiation among taxa. In particular, Diptychus maculatus can be readily distinguished from the densely scaled Schizothorax species by having a single pair of barbels and relatively sparse scales. The COI gene sequences showed an evident AT bias (55.1%), consistent with patterns reported for teleost mitochondrial genomes. Across all samples, 11 haplotypes were identified. Pairwise comparisons indicated a maximum interspecific divergence of 15.296%, whereas the smallest interspecific distance (0.262%) occurred between Schizothorax barbatus and Schizothorax irregularis. Species-delimitation analyses and phylogenetic reconstruction supported D. maculatus and A. laticeps as distinct mitochondrial lineages, whereas S. biddulphi, S. eurystomus, S. irregularis, and S. barbatus were not fully resolved by COI gene. The shared haplotypes and low genetic distances among the four Schizothorax species may reflect recent divergence, incomplete lineage sorting, or historical gene flow. Overall, COI gene barcoding is effective for distinguishing the major lineages in this dataset, especially D. maculatus and A. laticeps.