Comparisons of genetic diversity among disjunct populations of Magnolia tripetala

Background/Question/Methods

The geographic ranges of plants are often comprised of a core area with high population density surrounded by disjunct populations at the margins of the core. Pleistocene refugial core populations are often sources of high genetic diversity with genetic diversity decreasing as distance from the refugium increases. In contrast, genetically isolated disjunct populations are hypothesized to have low genetic diversity due to a presumed lack of gene flow with populations in the core of the species’ range. This study used microsatellites to evaluate patterns of genetic diversity and gene flow between core populations, disjunct Pleistocene refugial populations, and northern disjunct populations (representing the most recent northern expansion) of Magnolia tripetala. In addition, this study identified the phylogeographic history of disjunct populations and quantified the relationship between population isolation and population genetic diversity. 

Results/Conclusions

There was no significant difference observed between the level of genetic diversity in the different population types (core, disjunct Pleistocene refugial, and northern disjunct). The disjunct Pleistocene refugial populations had significantly more inbreeding (P=0.01275) than the other two populations types, although all sampled populations of M. tripetala had notably high inbreeding levels. Low gene flow was observed among all populations. This pattern of population structure suggests that the main core of M. tripetala is experiencing less gene flow than previously thought. It also suggests that all the populations of M. tripetala sampled in this study are genetically isolated from each other. Possible explanations for observations may be the slow seed dispersal and pollination mechanisms that generally lead to inbreeding in several Magnolia species.