Résumé
Pollen and seed dispersal are key processes affecting the demographic and
evolutionary dynamics of plant species and are also important
considerations for the sustainable management of timber trees. Through
direct and indirect genetic analyses, we studied the mating system and the
extent of pollen and seed dispersal in an economically important timber
species, Entandrophragma cylindricum (Meliaceae). We genotyped adult
trees, seeds and saplings from a 400-ha study plot in a natural forest
from East Cameroon using eight nuclear microsatellite markers. The species
is mainly outcrossed (t = 0.92), but seeds from the same fruit are often
pollinated by the same father (correlated paternity, rp = 0.77). An
average of 4.76 effective pollen donors (Nep) per seed tree contributes to
the pollination. Seed dispersal was as extensive as pollen dispersal, with
a mean dispersal distance in the study plot approaching 600 m, and
immigration rates from outside the plot to the central part of the plot
reaching 40% for both pollen and seeds. Extensive pollen- and
seed-mediated gene flow is further supported by the weak, fine-scale
spatial genetic structure (Sp statistic = 0.0058), corresponding to
historical gene dispersal distances (σg) reaching approximately 1,500 m.
Using an original approach, we showed that the relatedness between mating
individuals (Fij = 0.06) was higher than expected by chance, given the
extent of pollen dispersal distances (expected Fij = 0.02 according to
simulations). This remarkable pattern of assortative mating could be a
phenomenon of potentially consequential evolutionary and management
significance that deserves to be studied in other plant populations.