Implication of Nucleotide Substitution at Third Codon Position of the DNA Barcode Sequences

DNA markers (barcode) differentiate species based on their nucleotide sequence diversity among various species. In this study we analyzed the rate and pattern of nucleotide substitution and their consequent influence on the amino acid substitution patterns of the sequences used as barcode mitochondrial COI, cyt b and the exon 1 of nuclear IRBP gene for animals from 15 different species of vertebrates. The analysis shows unlike other codon positions, nucleotide substitution at the third codon position does not show strong correlation with the amino acid substitution, for the three gene sequences. Furthermore, COI gene shows a very low percentage of amino acid variability (15.38%) inspite of high percentage of variation in its nucleotide sequence (40.76%) as well as a significantly (p<0.0001) low level of amino acid sequence divergence than the other gene fragments under study. Interestingly among the compared sequences, a significantly conserved amino acid substitution pattern seems to be a unique feature of barcode region of the COI gene making it a more efficient marker for species identification. Hence, it was concluded that the property of species identification of these sequences is based upon the variable nature of third codon position.