SPATIAL MODELS OF POLLEN DISPERSAL IN THE FORAGE GRASS MEADOW FESCUE

Several bivariate probability distributions, generated by different un derlying dispersal mechanisms, are fitted to the observed frequencies of an isozyme marker gene using a maximum likelihood approach. The pol len dispersal data were generated using two experimental populations o f meadow fescue (Festuca pratensis Huds.), homozygous for different al lozymes at the (Pgi-2) locus, arranged in a circular donor-acceptor fi eld design. The contribution of a plant depends on plant position, fec undity and flowering time, factors which are taken into account when f itting the different models. Several approximate likelihood-ratio test s are done between alternative nested models, and a wind threshold mod el with bimodality in the wind direction is selected. The evolutionari ly important variances and expectations of gene displacement under the selected model are calculated. It is also shown that the underlying p robability distribution is significantly more than exponentially lepto kurtic. By fitting a distribution of deposition in all three dimension s to the data, taking into account differences in plant height, separa te estimates of additional physical parameters are obtained, showing t hat gravity and vertical random movements are more important than inte rvening vegetation in limiting pollen dispersal in meadow fescue. Acco rding to the model, plants with a high seed yield contribute pollen ov er-proportionally to neighbouring plants.