Genetic diversity and realized dispersal in the dioecious neotropical tree, Simarouba amara
Hardesty, Britta Denise
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Patterns of genetic diversity at local and large scales can provide insight into current and historical processes of gene flow, whether by pollen or seed. I used molecular markers to measure local patterns of genetic relatedness (i.e. fine scale genetic structure) in three size classes at the scale of 50 ha for Simarouba amara, in a large natural tree population on Barro Colorado Island, Panama. Simarouba amara is a widespread, dioecious, vertebrate-dispersed tropical tree species. Using multilocus genotype matching and parent-pair analyses with five microsatellite loci, I assessed the frequency of successful long-distance seedling establishment and evaluated the relative contribution of gene movement via pollen versus seed. Finally, I sampled from 14 populations of S. amara across Panama, Ecuador and French Guiana to assess levels of genetic diversity within and among a geographic expanse spanning > 3000 km. This portion of the study provides insights to the historical and evolutionary forces that may have structured current S. amara populations. My research revealed that S. amara exhibits little fine scale genetic structure at the scale of 50 ha. Only seedlings demonstrate significant spatial autocorrelation exceeding 20 m. For juveniles and adults, individuals located close to one another are not significantly related. On Barro Colorado Island, long distance seed dispersal is commonplace, as indicated by a high frequency of seedling establishment greater than 200 m from both maternal and paternal parents. Significant directional movement was evident in pollen though not in seedling establishment and gene movement was comparable via both pollen and seed. Finally, the 14 populations of S. amara sampled across Central and South America, populations differed significantly from one another using F and R-statistics, and follow and isolation by distance model. Notably, coastal populations in Panama and Ecuador differed significantly from all other populations suggesting different evolutionary histories influenced by geographic boundaries in addition to distance.