Effective pollen dispersal is enhanced by the genetic structure of an Aesculus turbinata population

Abstract

Summary 1 Studies using highly informative genetic markers (e.g. microsatellites) have revealed effective movement of genes through pollen grains from more distant sources into offspring than those inferred on the basis of physical pollen movement alone. In order to identify the factors that cause such effective pollen movement in tree populations, analyses of the genetic structure of reproductive trees and parentage analyses for seedlings and saplings were conducted in a 110-ha population of Aesculus turbinata, a tall deciduous tree species that is pollinated by insects and has large seeds that are dispersed by gravity and small mammals. 2 By analysing genotypes of seed coats, which are identical to those of the seed parents, and of leaves from seedlings, pollen donors and seed parents were identified for seedlings. Differences in size and dispersal mode between pollen and seeds caused a significant difference in dispersal distances. Reflecting the restricted seed dispersal (25.3 m on average), there was significant positive relatedness between reproductive trees standing within 150 m of each other. 3 The percentage of self-pollinated offspring decreased from 8.3% at the seedling stage to 0.56% at the sapling stage. The disappearance of out-crossed individuals during the stage between seedling and sapling seemed to be associated with the spatial distance separating their parents: the proportion of juvenile trees that died was higher for those that originated from out-crosses between adults standing within 100 m of each other, where positive relatedness was found between adult trees. Consequently, the deaths of individuals arising from self-pollination events and from crosses between reproductive trees that were spatially adjacent caused longer effective pollen movement for the offspring that survived. The distance between the two parents of a tree, which indicates the distance of effective pollen movement, increased from 179.9 m for seedlings to 288.9 m for saplings. 4 The genetic structure of the adult population of A. turbinata affected the survival of offspring through biparental inbreeding depression, and, then, removal of individuals resulting from crosses between related adults in turn promoted effective pollination over longer distances.