High levels of gene flow in bur oak revealed by paternity analysis using microsatellites

Abstract

Microsatellite analysis was used to characterize pollen dispersal in a stand of 62 adult bur oaks (Quercus macrocarpa) in northern Illinois. Using PCR amplification of four dinucleotide microsatellite loci, pollen donors of 282 acorns collected from three adult trees located in different parts of the stand were determined by exclusion. Overall at least 57% of the acorns were pollinated by trees outside of the stand, because all adult trees in the stand were excluded from paternity. Average pollination distance for within-stand pollinations was 75 m, and pollen donors for two of the three maternal trees were randomly dispersed throughout the stand. These two trees also received more pollen from 50 m away than from near neighbors. Self-pollination was rare or nonexistent. This study provides direct evidence for high levels of long-distance pollination in a wind-pollinated species, and contradicts traditional models of wind pollination which suggest that clouds of pollen dissipate from the source to ineffectively low densities over short distances. Instead, the mating system of bur oak seems to be extraordinarily efficient at producing highly outbred individuals and ensuring long-range pollen flow, perhaps through pollen competition or mate choice favoring distant pollen sources.