Molecular divergence in the eastern Asia– eastern North America disjunct section Rytidospermum of Magnolia (Magnoliaceae)

Molecular divergence in the eastern Asia—eastern North American disjunct section Rytidospermum of Magnolia was investigated by allozyme electrophoresis, chloroplast DNA (cpDNA) restriction site analysis, and gene sequencing. We calculated Nei's genetic identities between two Asian species, M. officinalis var. biloba and M. hypoleuca, and three American species, M. tripetala, M. fraseri var. fraseri, and M. macrophylla var. macrophylla, by using gene frequency data from 17 nuclear-encoded allozyme loci in 67 populations. We then estimated cpDNA sequence divergence between the five species by examining restriction site variation for ten endonucleases over the entire genome. Finally, nucleotide sequences of the chloroplast gene rbcL were compared between M. hypoleuca, M. tripetala, and M. macrophylla var. macrophylla. All three methods consistently yielded low divergence values between the American species M. tripetala and its Asian sister taxa, M. officinalis var. biloba and M. hypoleuca (Nei's I = 0.712 and 0.809, respectively; D-cpDNA = 0.083% for both pairs; D-rbcL = 0.000% between M. tripetala and M. hypoleuca). The other two American species, M. fraseri var. fraseri and M. macrophylla var. macrophylla, neither of which is sister to the Asian taxa, exhibited much higher divergence from the Asian taxa. We interpreted the low divergence between M. tripetala and its Asian sister taxa as a result of recent separation (the late Miocene to early Pliocene), based on time estimates from molecular data as well as geological and paleoclimatic evidence. A comparison of our results with those of the earlier studies revealed a diverse array of levels of divergence between several eastern Asian and eastern North American species pairs. Though different extinction patterns and variation in molecular evolutionary rates may be partly responsible, this heterogeneous pattern of divergence is best explained by different times of disjunction in different taxa, which in turn suggests that the floristic similarity between the two continents was most likely attained by multiple migrations via both Bering and North Atlantic land bridges, or possibly even with involvement of dispersal.