Phylogeny and biogeography of the eastern Asian-North American disjunct wild-rice genus (Zizania L., Poaceae).

The wild-rice genus Zizania includes four species disjunctly distributed in eastern Asia and North America, with three species (Z. aquatica, Z. palustris, and Z. texana) in North America and one (Z. latifolia) in eastern Asia. The phylogeny of Zizania was constructed using sequences of seven DNA fragments (atpB-rbcL, matK, rps16, trnL-F, trnH-psbA, nad1, and Adh1a) from chloroplast, mitochondrial, and nuclear genomes. Zizania is shown to be monophyletic with the North American species forming a clade and the eastern Asian Z. latifolia sister to the North American clade. The divergence between the eastern Asian Z. latifolia and the North American clade was dated to be 3.74 (95% HPD: 1.04-7.23) million years ago (mya) using the Bayesian dating method with the combined atpB-rbcL, matK, rps16, trnL-F, and nad1 data. Biogeographic analyses using a likelihood method suggest the North American origin of Zizania and its migration into eastern Asia via the Bering land bridge. Among the three North American species, the organellar data and the haplotype network of the nuclear Adh1a gene show a close relationship between Z. palustris and the narrowly distributed endangered species Z. texana. Bayesian dating estimated the divergence of North American Zizania to be 0.71 (95% HPD: 0.12-1.54) mya in the Pleistocene. The non-monophyly of Z. palustris and Z. aquatica in the organellar and nuclear data is most likely caused by incomplete lineage sorting, yet low-frequency unidirectional introgression of Z. palustris into Z. aquatica is present in the nuclear data as well.