Role of Transversional Substitution on DNA Barcode Based Species Differentiation:Catfish as a Case Study

Cytochrome c oxidase (COI) DNA barcode is considered to be universal species identifier for animals based on many superlative features, but none has considered the pattern of characteristic nucleotide variation for their role if any. In this study, 50 COI barcode sequences of 10 species of catfishes were analyzed in two steps. In the first step, accurate delineation of species was confirmed while the second step involves analysis of the nucleotide variability in terms of transition and transversion at the different taxon levels and their subsequent effect on genetic divergence. The analysis revealed that the individuals within all the species exhibit distinct barcode cluster and a straight forward divergence at the congeneric (17.4% ± 1.8%) and conspecific level (0.7% ± 0.3%) allowing their unambiguous identification. Transition was found to be significantly higher (t= 4.85 - 41.12, p<0.0001) than transversion at all the three taxon levels. However, as transversion showed a significant increase (0.47 ± 0.38, 29.65 ± 5.12 and 34.21 ± 5.50) with the increase in taxon comparison (conspecific, congeneric and intergeneric), transition only showed a significant rise (t= 119.6, p<0.00001) between the conspecific (1.64 ± 0.76) and congeneric level (54.96 ± 6.75) and then transition became saturated at the intergeneric level (54.83 ± 6.68). The study importantly revealed that the genetic divergence between the congeners at the partial fragment of COI was highly correlated (Pearson correlation, R= 0.935, P<0.001) with transversional substitution than with transition (R= 0.334, P<0.05). Hence, although transition is more frequent, still transversional substitution within the COI barcode sequence plays the key role in species delineation.