Melanesian origin of Polynesian Y chromosomes

Background: Two competing hypotheses for the origins of Polynesians are the 'express-train' model, which supposes a recent and rapid expansion of Poly nesian ancestors from Asia/Taiwan via coastal and island Melanesia, and the 'entangled-bank' model, which supposes a long history of cultural and gene tic interactions among Southeast Asians, Melanesians and Polynesians. Most genetic data, especially analyses of mitochondrial DNA (mtDNA) variation, s upport the express-train model, as does linguistic and archaeological evide nce. Here, we used Y-chromosome polymorphisms to investigate the origins of Polynesians. Results: We analysed eight single nucleotide polymorphisms (SNPs) and seven short tandem repeat (STR) loci on the Y chromosome in 28 Cook Islanders fr om Polynesia and 583 males from 17 Melanesian, Asian and Australian populat ions. We found that all Polynesians belong to just three Y-chromosome haplo types, as defined by unique event polymorphisms. The major Y haplotype in P olynesians (82% frequency) was restricted to Melanesia and eastern Indonesi a and most probably arose in Melanesia. Coalescence analysis of associated Y-STR haplotypes showed evidence of a population expansion in Polynesians, beginning about 2,200 years ago. The other two Polynesian Y haplotypes were widespread in Asia but were also found in Melanesia. Conclusions: All Polynesian Y chromosomes can be traced back to Melanesia, although some of these Y-chromosome types originated in Asia. Together with other genetic and cultural evidence, we propose a new model of Polynesian origins that we call the 'slow-boat' model: Polynesian ancestors did origin ate from Asia/Taiwan but did not move rapidly through Melanesia; rather, th ey interacted with and mixed extensively with Melanesians, leaving behind t heir genes and incorporating many Melanesian genes before colonising the Pa cific.