Disparity of allozyme variation levels in three Magnolia (Magnoliaceae) species from the southeastern United States

Allozyme variation at 17 loci encoding ten enzyme systems was examined in 21, 22, and 15 populations across the ranges of Magnolia fraseri, M. macrophylla, and M. tripetala, respectively, in the southeastern United States. All three species have regional distributions, and are insect-pollinated outcrossing deciduous trees with seeds dispersed by birds, yet strikingly different levels of genetic variability were observed among them. In comparison with other woody angiosperm species, M. fraseri possesses a moderate amount of variation at the population level (A = 1.4, P = 39.2, H_o = 0.111, and H_e = 0.111), whereas M. macrophylla and M. tripetala are genetically depauperate in their populations (A = 1.2, P = 18.7, H_o = 0.47, and H_e = 0.055; A = 1.1, P = 11.0, H_o = 0.032, and H_e = 0.033). Examination of population structure revealed a small amount of inbreeding within populations and extensive intra- and interregional differentiation among populations of the latter two species. These two factors are perhaps partly responsible for the low genetic variability in populations of the two Magnolia species. Furthermore, the bottleneck effect caused by extinctions during cold periods of the Quaternary glaciations and human deforestation in the last two centuries as well as the founder effect in postglacial establishment of the populations might have also played significant roles in loss of genetic diversity in M. macrophylla and M. tripetala. We suggest that historical factors are important determinants of genetic variation profile of a species, in addition to life history and ecological characteristics as generally recognized.