Butterflies poke holes in DNA barcodes
Mixing subspecies cause problems for genetic fingerprinting scheme.
A study of butterflies has highlighted a hotly debated glitch in DNA barcoding, a scheme by which some researchers hope to quickly catalogue vast numbers of species. Biologists sampling Karner blue butterflies have found that genetic scans failed to identify the endangered animals.
"This is clearly one case where if you were using DNA barcoding to identify units for conservation, it would be failing," says co-author Chris Nice of Texas State University in San Marcos.
The Karner blue, famously first identified by the novelist Vladimir Nabokov in the 1940s, differs from related butterflies in its feeding preferences and in the patterns on its tiny wings. Over the years, the Karner blue population has dwindled to some 1% of its previous numbers, leaving just a few thousand individuals in the northeastern United States.
Nice and his colleagues had previously looked at the genetics of these butterflies and found interesting patterns in how the maternal DNA had been passed down. So they set out to determine if and how this would influence DNA barcoding's results.
Barcoding aims to identify an organism by using a minimum amount of genetic information as a quick, species-specific tag. Such a tool should help researchers to map biodiversity, identify new species and aid in conservation. Some have even envisioned a future in which border agents could scan shipping containers for endangered specimens using the technique.
A standard method for barcoding, proposed in 2003, is to identify animals by using the cytochrome oxidase I (COI) gene in the mitochondria: DNA that lives outside the cell nucleus. But Nice's team found that the COI and COII genes pegged the presumed western Karners as belonging to a more common subspecies: the Melissa blue butterfly.
To check the disparity, the biologists laboriously analysed 143 locations in the nuclear DNA of 190 butterflies. This more exhaustive method supported the biologist's old-fashioned assessment of butterfly taxonomy based on physical appearance.
The reason for the barcoding's failure, the authors believe, is that mitochondrial genes may have leaked from one subspecies to the other through a few rare interbreedings.
Whereas nuclear DNA becomes diluted over time through further breeding, mitochondrial DNA, which is inherited solely from the mother, doesn't recombine and so continues to look like that of the foreign subspecies'. This DNA can become entrenched in future generations, particularly if it offers some selective advantage.
This is a problem that biologists have talked about since the beginning of barcoding, says David Schindel, executive secretary of the Consortium for the Barcode of Life, a group of more than 100 members hosted by the Smithsonian Institution in Washington DC. It has been seen in other animals before, but no one knows how widespread the phenomenon will prove to be.
Barcoding practitioners are aware of the problems with gene flow, says Schindel. He says the technique remains viable. But when differentiating subspecies, he admits, "you probably won't be successful with a single gene region."
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- Gompert Z., et al. Mol. Ecology (in press)