Yl. Qiu et Cr. Parks, DISPARITY OF ALLOZYME VARIATION LEVELS IN 3 MAGNOLIA (MAGNOLIACEAE) SPECIES FROM THE SOUTHEASTERN UNITED-STATES, American journal of botany, 81(10), 1994, pp. 1300-1308
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 Un
ited States. All three species have regional distributions, and are in
sect-pollinated outcrossing deciduous trees with seeds dispersed by bi
rds, yet strikingly different levels of genetic variability were obser
ved among them. In comparison with other woody angiosperm species, hi.
fraseri possesses a moderate amount of variation at the population le
vel (A = 1.4, P = 39.2, H-o, = 0.111, and H-e = 0.111), whereas M. mac
rophylla and M. tripetala are genetically depauperate in their populat
ions (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 lat
ter 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 establ
ishment of the populations might have also played significant roles in
loss of genetic diversity in M. macrophylla and M. tripetala. We sugg
est that historical factors are important determinants of genetic vari
ation profile of a species, in addition to life history and ecological
characteristics as generally recognized.