MOLECULAR DIVERGENCE IN THE EASTERN ASIA EASTERN NORTH-AMERICA DISJUNCT SECTION RYTIDOSPERMUM OF MAGNOLIA (MAGNOLIACEAE)

Molecular divergence in the eastern Asia-eastern North American disjun ct section Rytidospermum of Magnolia was investigated by allozyme elec trophoresis, chloroplast DNA (cpDNA) restriction site analysis, and ge ne sequencing. We calculated Nei's genetic identities between two Asia n species, M. officinalis var. biloba and M. hypoleuca, and three Amer ican species, M. tripetala, M. fraseri var. fraseri, and M. macrophyll a var. macrophylla, by using gene frequency data from 17 nuclear-encod ed allozyme loci in 67 populations. We then estimated cpDNA sequence d ivergence between the five species by examining restriction site varia tion for ten endonucleases over the entire genome. Finally, nucleotide sequences of the chloroplast gene rbcL were compared between M. hypol euca, M. tripetala, and M. macrophylla var. macrophylla. All three met hods consistently yielded low divergence values between the American s pecies M. tripetala and its Asian sister taxa, M. officinalis var. bil oba 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 an d M. macrophylla var. macrophylla, neither of which is sister to the A sian taxa, exhibited much higher divergence form the Asian taxa. We in terpreted the low divergence between M. tripetala adn its Asian sister taxa as a result of recent separation (the late Miocene to the early Pliocene), based on time estimates from molecular data as well as geol ogical and paleoclimatic evidence. A comparison of our results with th ose of the earlier studies revealed a diverse array of levels of diver gence between several eastern Asian and eastern North American species pairs. Though different extinction patterns and variation in molecula r evolutionary rates may be partly responsible, this heterogeneous pat tern of divergence is best explained by different times of disjunction in different taxa, which in turn suggests that the floristic similari ty between the two continents was most likely attained by multiple mig rations via both Bering and North Atlantic land bridges, or possibly e ven with involvement of dispersal.