genetic barcodes

Introduction To DNA Barcoding

A recently established taxonomy approach attempts to identify species solely on the basis of a genetic barcode (also called a DNA barcode) consisting of one or a few DNA sequences. For example, a 648 bp region of the mitochondrial cytochrome c oxidase I (COI) gene continues to be developed as a barcode identifier in animals.

In a study of 260 bird species, this genetic region was found to be species-specific and was also 18 times more variable between species (7,057.93 percent) than within species (0,270.43 percent). This is one of the findings that led to an international collaboration known as the Life Barcode Consortium, currently housed at the Smithsonian National Museum of Natural History in Washington, DC, which promotes the eventual acquisition of genetic barcodes for all living species.

Using genetic barcodes to identify species has two general applications: identifying previously characterized species from a comparison of documented DNA sequences, and discovering new species based on new DNA sequences.

The practice of identifying species or specimens by matching sequences is becoming more widespread. However, this approach assumes that the sequences are species-specific. Hybridization happens in all major taxonomic groups and about a quarter of all animal species have not yet reached the stage of mutual monophyly. DNA sequences sometimes cross putative species boundaries.

The second application of DNA barcoding, identifying new species, is more controversial. This is partly because the range of intraspecific sequence divergence can be quite difficult to predict.

Disagreements like this are the exception rather than the rule, although data from a wider range of taxonomic groups are needed before we can reach this conclusion. Before such data can be collected, the appropriate genetic regions in these other taxonomic groups must first be identified.

For example, microbes transfer genes between putative species so frequently that sequence data for approximately 69 genes are required before closely related species can be distinguished.

In plants, hybridization and polyploidy can obscure evolutionary relationships, although proponents of genetic barcoding hope that a region of the chloroplast genome can be found that reliably distinguishes species.

They also suggest that COI will be useful in identifying different protistic species, although anaerobic species lack mitochondria and therefore need a different marker. Meanwhile, DNA barcoding is becoming an increasingly acceptable tool for species identification and could be widely used in the literature in the years to come.

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