DNA barcode

DNA barcoding on a large scale was proposed by Paul D. N. Hebert at the University of Guelph, Ontario, Canada, and his colleagues in 2003 as a new system for classifying species. Its use is a special case of molecular systematics.

It is based on a simple notion: all species, at least all eukaryotes, contain in the cytoplasm of their cells mitochondria, the generators of ATP, the energy source of the cell.

Mitochondrial DNA contains a particular set of genes, called cytochrome c oxidase I (abbreviated COI), By concentrating on a mere part COI, Dr. Hebert has found a way of symplifying the research in molecular systematics. This part consists of only 648 base pairs ( = genetic “letters”). This sequence is referred to by Dr. Hebert as the “barcode”.
Molecular markers (e.g., allozymes, rDNA, and mtDNA) have been successfully used in molecular systematics for quite some time. But the novelty of the DNA barcoding resides in its increased scale and proposed standardization.

This barcode should be the same in all members of the same species, but be different from species to species, because mitochondrial genes have a tendency to mutate rather rapidly (speaking on a biological timescale).

A first test on discriminating bird species has shown an almost complete agreement with traditional taxonomy. Two hundred and sixty North American species were examined and differences between closely related species were, on average, found to be 18 times greater than the differences within species. However, the analysis suggested that four traditional species should be split, each into two new species. At the same time, the American Pied Oystercatcher (Haemotopus palliatus) and the Western Black Oystercatcher (Haematopus bachmani) were shown to be color morphs and should be considered as one species.

Another test on the butterfly Astraptes fulgerator has astonishing results. This species was known as a “cryptic species”, i.e. to include populations that are morphologically indistinguishable, but are reproductively isolated. Indeed the adults look similar, but the caterpillars can be quite different and use different food plants. Therefore there were doubts whether this was a single species. A small sample from north-western Costa Rica was examined. DNA barcoding showed that there were at least ten species involved. Since this species can be found from southern USA to northern Argentina, one can wonder how more species are being categorized as “Astraptes fulgerator”.

However there are already some critics of DNA barcoding. Species that diverged recently might not be distinguished in the results of DNA barcoding. Studies so far have shown that about 96% of eukaryotes can be detected with this method. DNA barcoding therefore needs to be used alongside traditional taxonomic tools and alternative forms of molecular systematics so that problem cases can be identified and errors detected.

An international effort has been set up by the Consortium for the Barcode of Life to build a comprehensive public library of species’ DNA barcodes. A global survey of all species will undoubtedly show that the total estimated number of species is currently substantially underestimated.
The impact of this library will grow with increased taxon coverage and may change the field of taxonomy. It could even give a way to identify pathogens, or give a head start in detecting bioterror agents.

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