Spatial Scales of Pollen and Seed-Mediated Gene Flow in Tropical Rain Forest Trees

Abstract

Recent methodological advances permit refined inferences of evolutionary processes from the fine-scale spatial genetic structure of plant populations. In this issue of Molecular Ecology, Born et al. (2008) exploit the full power of these methods by examining effects of ancient and recent landscape histories in an African rainforest tree species. The authors first detected admixture of distinct gene pools that may have formed in Pleistocene forest refuges. Then, comparing across six study populations in Gabon, the authors found similar patterns of fine-scale spatial genetic structure despite natural and anthropogenic variation in population density. The latter results suggest that enhanced gene dispersal may compensate for low population densities in fragmented landscapes.Keywords: assignment methods, forest history, Gabon, spatial genetic structure, tropical tree Received 31 December 2007; revision received 19 January 2008; accepted 3 February 2008Limited dispersal in plants can generate clusters of closely related individuals, which manifests as fine-scale spatial genetic structure (SGS). SGS develops at landscape levels over ecological timescales (a few generations) and may be represented as a regression of kinship over the logarithm of distance (kinship-distance curve). Development of SGS is the first stage in Wright's (1943) model of isolation by distance (IBD) for continuous populations. The recently developed Sp statistic, which quantifies the strength of SGS, can be used to compare SGS among populations and species (Vekemans & Hardy 2004). Recent meta-analyses have shown that species with self-compatibility, low population densities or poorly dispersed seeds tend to exhibit strong SGS (Vekemans, Hardy 2004;

Population history and gene dispersal inferred from spatial genetic structure of a Central African timber tree, Distemonanthus benthamianus (Caesalpinioideae)

“…Sp values in D. benthamianus are typically of the same order of magnitude as those found in other tropical trees whose seeds are heavy gliders dispersed by the wind (for example, Dicorynia (Fabaceae), Aucoumea (Burseraceae); see Hardy et al, 2006;Dick et al, 2008). This is coherent with some field observations suggesting seed dispersal to distances up to 50 m (Hecketsweiler, 1992;Cordiez, 2000).…”

“…Considering only distances o10 km gave us doubled values of Sp (except for the population 3), with a maximum Sp statistic of 0.027 (Table 3), a value in the last quartile of SGS estimates for tropical trees in the literature (Dick et al, 2008). Sp values in D. benthamianus are typically of the same order of magnitude as those found in other tropical trees whose seeds are heavy gliders dispersed by the wind (for example, Dicorynia (Fabaceae), Aucoumea (Burseraceae); see Hardy et al, 2006;Dick et al, 2008).…”

“…We then calculated Sp, defined as Sp ¼ Àb Ld60 /(1-F (1) ), where F (1) is the mean F ij between neighbouring individuals, which was approximated by F(d) for the first distance interval (d ij o1 km). Sp was also computed for d ij o10 km to allow comparisons with other tropical tree species at comparable spatial scales (Dick et al, 2008), but a serious drop of precision was consequently achieved, as on average only 18% of all pairwise comparisons were considered.…”

“…Comparing SGS among sites may provide insights into the range of variation of gene flow within species. For instance, forest fragmentation can increase SGS by breaking up gene flow and increasing local drift because of the reduction of the number of cohorts of many species (Dick et al, 2008). In contrast, long-distance seed dispersal and landscape changes that increase dispersal capacities of species may reduce the stochastic action of drift and lead to a decrease in SGS.…”

Low levels of realized seed and pollen gene flow and strong spatial genetic structure in a small, isolated and fragmented population of the tropical tree Copaifera langsdorffii Desf

“…Pollen and seed dispersal (gene flow) are the principal determinants of genetic structure and diversity in tree species (Dick et al, 2008). Estimates of both pollen and seed gene flow are required to predict the effects of forest fragmentation on the genetic structure of a tree species, and special attention should be given to established seedlings and saplings (Aguilar et al, 2008).…”