Estimating the relative rates of pollen and seed migration among plant populations

Interpopulation gene flow in plants is mediated by a combination of pollen and seed dispersal. The effectiveness of pollen and seeds in bringing about gene flow depends upon the mode of inheritance of the genetic marker. For nuclear and paternally inherited markers, gene flow occurs in both pollen and seed. For maternally inherited markers, genes are only dispersed in seeds. As a result, levels of population differentiation under drift-migration equilibrium are expected to differ for markers with contrasting modes of inheritance, and the extent of such differences should be related to the relative levels of pollen and seed migration among populations. A model is developed that relates expected values of population differentiation (F st ) for nuclear, paternally and maternally inherited markers, to pollen and seed migration rates. The model is used to estimate the relative rates of seed and pollen flow in six species of plants where F st values are available for both nuclear and maternally inherited markers. Estimates of (pollen flow/seed flow) range from four in wild barley to 200 in oaks, and this pattern of variation is consistent with the reproductive characteristics of the species concerned.