Changes of effective gene dispersal distances by pollen and seeds across successive life stages in a tropical tree

Pollen and seed dispersal are the two key processes in which plant genes move in space, mostly mediated by animal dispersal vectors in tropical forests. Due to the movement patterns of pollinators and seed dispersers and subsequent complex spatial patterns in the mortality of offspring, we have little knowledge of how pollinators and seed dispersers affect effective gene dispersal distances across successive recruitment stages. Using six highly polymorphic microsatellite loci and parentage analyses, we quantified pollen dispersal, seed dispersal, and effective paternal and maternal gene dispersal distances from pollen- and seed-donors to offspring across four recruitment stages within a population of the monoecious tropical tree Prunus africana in western Kenya. In general, pollen-dispersal and paternal gene dispersal distances were much longer than seed-dispersal and maternal gene dispersal distances, with the long-distance within-population gene dispersal in P. africana being mostly mediated by pollinators. Seed dispersal, paternal and maternal gene dispersal distances increased significantly across recruitment stages, suggesting strong density- and distance-dependent mortality near the parent trees. Pollen dispersal distances also varied significantly, but inconsistently across recruitment stages. The mean dispersal distance was initially much (23-fold) farther for pollen than for seeds, yet the pollen-to-seed dispersal distance ratio diminished by an order of magnitude at later stages as maternal gene dispersal distances disproportionately increased. Our study elucidates the relative changes in the contribution of the two processes, pollen and seed dispersal, to effective gene dispersal across recruitment. Overall, complex sequential processes during recruitment contribute to the genetic make-up of tree populations. This highlights the importance of a multistage perspective for a comprehensive understanding of the impact of animal-mediated pollen and seed dispersal on small-scale spatial genetic patterns of long-lived tree species.