SIMPLE SEQUENCE REPEATS PROVIDE A DIRECT ESTIMATE OF POLLEN-MEDIATED GENE DISPERSAL IN THE TROPICAL TREE GLIRICIDIA-SEPIUM

An understanding of the processes that determine the observed patterns of genetic variation in natural plant populations is an important fac tor in the management of biodiversity. Pollen-mediated gene dispersal is recognized as a major determinant of population genetic structure. Here, the utility of simple sequence repeat (SSR) analysis was investi gated for the measurement of pollen-mediated gene transfer by paternit y exclusion in a restricted, fragmented and endangered population of t he insect-pollinated tropical leguminous tree Gliricidia sepium locate d in Guatemala. Data at a single SSR locus, which revealed six allelic variants, were employed to generate minimum distance curves of pollen dispersal. Combined data from all six alleles indicated that a minimu m of 1.8% of transfer events occurred over a distance of greater than 75 m. However, this value represents an underestimate because of the e xclusion approach employed for analysis. Considering the four rarest a lleles in the population only (combined frequency = 0.196), which prov ides a less biased indicator of gene transfer, a minimum of 6.1% of po llen movements could be attributed to greater than 75 m. One extreme e xample of gene transfer of over 275 m was recorded. Estimates of polle n transfer suggest a homogenizing effect on genetic structure over the spatial scale of the study population and provide an important indica tor for the genetic management of natural and exotic stands of G. sepi um. This study provides the first example of SSR analysis being employ ed to estimate directly pollen movement in a natural stand of any tree species.