Scientists have found that the most accurate way of determining where migratory fish come from is to analyse the DNA of their parasites, rather than the genetics of the fish themselves.
In a study by scientists from Oregon State University, genetic fingerprinting of trout parasites was up to four times more effective in identifying the origin of the fish compared to analyses of the DNA of the trout themselves.
The groundbreaking study, which was published in the latest issue of the journal Ecology, describes how the biologists studied small fragments of DNA called microsatellites in a trematode parasite of the Rainbow trout, Oncorhynchus mykiss, and claims that by analysing the genetics of the parasite which had infected the fish, they were able to tell where the fish came from with a much greater degree of accuracy than the old method of analysing the DNA of the fish itself.
Charles Criscione, Becky Cooper and Michael Blouin sampled Rainbow trout and their parasite, Plagioporus shawi, in each of five rivers in the coastal mountains of Oregon in the United States of America and then analysed their genetic makeup.
The genetic differences between the different geographic populations of trout weren't marked enough to allow them to determine which population the fish belonged to, but the differences in the genetics of the parasites were much greater, and the scientists were able to accurately predict which river the fish came from by the genetics of their trematode parasites.
Michael Blouin told the Medford News: "Sometimes, unfortunately, there just is not that much genetic difference between animals from different geographic locations over a wide area. Thus, you can't use an animal's DNA to accurately identify its population of origin. On the other hand, it appears that parasites from different locations tend to be much more genetically distinct than their hosts from the same locations."
"On one level, this is just basic DNA fingerprinting, the type used in everything from human paternity testing to crime scenes and wildlife biology. It's existing technology that's fairly efficient and cost effective. But no one had ever considered the wide genetic variation found in parasites and the much more accurate information that might give us."
The study is believed to be the first to show that parasite genotypes can assign hosts back to their source population more accurately than the host's own genotype.
The study claims that the phenomena is not likely to be limited to fish either: "Arthropod, nematode and trematode parasites typically have equal or higher genetic structure than their dispersing hosts. Thus, the use of parasites to assign hosts back to source populations may have broad application."
For more details on the study see the paper: Criscione CD, Cooper B and MS Blouin (2006) - Parasite genotypes identify source populations of migratory fish more accurately than fish genotypes. Ecology: Vol. 87, No. 4, pp. 823-828.