A chloroplast DNA Phylogenetic study of the eastern Asia-eastern North America disjunct section Rytidospermum of Magnolia (Magnoliaceae)

Chloroplast DNA restriction site variation of ten endonucleases was examined among all ten species or varieties of the eastern Asia–eastern North America disjunct section Rytidospermum of Magnolia. Representatives from seven of the other ten sections of Magnolia and four related genera (Liriodendron, Manglietia, Michelia, and Talauma) were also included in the survey. A cladistic analysis of 200 variable sites using Wagner parsimony yielded 11 equally most parsimonious trees with a consistency index of 0.793 and a retention index of 0.870. The section Rytidospermum is polyphyletic in these trees. Magnolia tripetala from the southeastern U.S. is the only American species that has a sister relationship to the Asian taxa, M. hypoleuca, M. officinalis var. officinalis, M. officinalis var. biloba, and M. rostrata. Other American taxa in the section fall into two lineages, with M. macrophylla var. macrophylla, M. macrophylla var. ashei, and M. macrophylla var. dealbata in one, and M. fraseri var. fraseri and M. fraseri var. pyramidata in the other. They are not related to the Asian species as previously believed. The relationships revealed here are in agreement with morphological, allozymic, and cross compatibility data. These results demonstrate that a robust phylogenetic hypothesis is an important prerequisite for understanding biogeographic patterns.