Information of mating system and pollination is important for management and conservation of tree populations. The estimations of outcrossing rate and distances of effective pollen dispersal allow us to know critical limits for remnant tree density and/or necessary area of forests for their regeneration. In this study, we studied genetic structure and gene flow by pollination of a Dryobalanops aromatica (Dipterocarpaceae) population in a troipocal rainforest in Southeast Asia.

The aims of this study are: (i) to elucidate genetic structure of a D. aromatica population, (ii) to estimate outcrossing rate and distances of effective pollen dispersal, and (iii) to evaluate the effects of observed pollen dispersal on genetic traits of the population.

The study was conducted in a 52 ha plot established at the Lambir Hills National Park in Sarawak, Malaysia. Fresh leaves were picked up from adult trees larger than 30 cm in stem diameter and one-year old seedlings under 4 mother trees. From the leaves, DNA was extracted and their genotypes were discriminated by microsatellite analysis. Based on the genotype data, relatedness between adults was calculated and the paternity of each seedling was determined by comparing genotypes of adults and seedlings.

Results/Conclusions

The observed heterozygosity and expected heterozygosity were similar values. This result suggests that mating occurs randomly in this population. The genetic relatedness between adults was positive only at short distance scales. The F_st values between patches within the population were significantly larger than zero but their absolute values were small. Thus, spatial genetic structure exists, but its magnitude is small. Outcrossing rates varied among mother trees and it ranged from 0.72 to 1.00. These values are similar to those reported for other dipterocarp species. The frequency distribution of pollen dispersal distances for each mother tree has the mode at a distance class < 200 m. On the other hand, long-distance pollen dispersal was also observed, although its frequency was low.

The large outcrossing rate may be owing to high density of adlut D. aromatica trees in the plot and/or to widespread gene flow among trees by pollination. Such pollination might be realized because pollen grains of D. aromatica are dispersed by bees that can fly long distances. Mating with distant trees may lead to the weak genetic structure observed in the D. aromatica population.