Thursday, July 15, 2004

Rats as Proxy

No, this is not your internet proxy. This study involved using commensals to trace early human migration of the Pacific peoples - in this case it was the Pacific rat, Rattus exulans. The tool - Molecular Phylogenetics.

The researchers believe that the Pacific rat was introduced intentionally in the past possibly as a food item, based on the numerous bones in archaeological middens?

Pacific Rat A wood carving depicts an ancestral Polynesian colonist of Oceania. Note the rats on top of the figure. Copyright Tim Mackrell/PNAS

Besides being an ideal commensal to proxy because they were canoed around with the early migrators as food, R. exulans also have several other properties that make them so ideal to "lab rats" to sequence. Like, the Pacific rats also do not swim hence cannot self-disperse. They also don't crossbreed with the European rat (Rattus rattus and Rattus norvegicus), so there are no problems with reticulate branching. Dogs, Pigs and Chickens interbreed readily with their european counterparts presenting the problem of admixture.

The phylogenies were based on the mere 250 base pairs in the D-loop of the mitochondrial DNA. (Where the heck is the equivalent for the plants!). The trees showed 3 highly supported groups with clear geographic patternings. The first group or interaction sphere consisted solely of Southeast Asian elements from the Philipines, Borneo and Sulawesi. The second had samples from both Southeast Asia and Near Oceania(e.g New Guinea, Australia and proximate islands) . The third group consisted of mainly Remote Oceanic (Vanuatu, Fiji and Samoa) rat populations. Their finding refutes the previous Express Train Theory of migrants that swept their way from Taiwan through Southeast Asia and Near Oceania and into Remote Oceania

E. Matisoo-Smith, and J. H. Robins (2004) Origins and dispersals of Pacific peoples: Evidence from mtDNA phylogenies of the Pacific rat. PNAS 101: 9167-9172

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