Gene flow at the leading range edge: the long-term consequences of isolation in European Beech (Fagus sylvatica L. Kuhn)

Aim Isolation is expected to lead to negative impacts on populations due to a reduction in effective population size and gene flow, exacerbating the effects of genetic drift, which might be stronger in peripheral and fragmented populations. Fagus sylvatica (European beech) in southern Sweden presents a gradient of isolation towards the leading range edge of the species. We sought to determine the impact of long‐term isolation on genetic diversity and population genetic structure within populations of this species. Location Samples were obtained from 14 sites towards the northern edge of the native range of beech in Sweden. Taxon Fagaceae. Methods Using historical sources, we obtained area‐ and distance‐based measures of isolation. We measured genetic diversity and structure by using nuclear microsatellite marker data, and performed parentage analysis to estimate external pollen‐mediated gene flow. We implemented a partial least squares regression to determine the effects of isolation on each of the genetic diversity estimators and the measures of external pollen‐mediated gene flow. Results Long‐term isolation generally had a negative impact on genetic diversity, which is exacerbated over time, further affecting progeny and suggesting that isolated populations are subject to strong genetic drift, possibly due to the combination of founder events and persistent small population sizes. Bayesian cluster analysis revealed that isolation was also acting as a barrier to gene flow in the north‐eastern distribution of beech. Main conclusions Isolation at the leading range edge of beech in Sweden has created gradients of contemporary gene flow within the species. The long‐term cumulative effects of isolation on this wind‐pollinated tree species and its negative impacts on genetic diversity and gene flow, could lead to inbreeding depression and higher extinction risk where populations remain small and isolated

Paternity analysis of pollen-mediated gene flow for Fraxinus excelsior L. in a chronically fragmented landscape

Paternity analysis based on microsatellite marker genotyping was used to infer contemporary genetic connectivity by pollen of three population remnants of the wind-pollinated, wind-dispersed tree Fraxinus excelsior, in a deforested Scottish landscape. By deterministically accounting for genotyping error and comparing a range of assignment methods, individual-based paternity assignments were used to derive population-level estimates of gene flow. Pollen immigration into a 300ha landscape represents between 43% and 68% of effective pollination, mostly depending on assignment method. Individual male reproductive success is unequal, with 31 of 48 trees fertilising one seed or more, but only three trees fertilising more than ten seeds. Spatial analysis suggests a fat-tailed pollen dispersal curve with 85% of detected pollination occurring within 100m, and 15% spreading between 300m and 1900m from the source. Identification of immigrating pollen sourced from two neighbouring remnants indicates further effective dispersal at 2900m. Pollen exchange among remnants is driven by population size rather than geographic distance, with larger remnants acting predominantly as pollen donors, and smaller remnants as pollen recipients. Enhanced wind dispersal of pollen in a barren landscape ensures that the seed produced within the catchment includes genetic material from a wide geographic area. However, gene flow estimates based on analysis of non-dispersed seeds were shown to underestimate realised gene immigration into the remnants by a factor of two suggesting that predictive landscape conservation requires integrated estimates of post-recruitment gene flow occurring via both pollen and seed

Data from: Genetic differentiation and species cohesion in two widespread Central American Begonia species

Begonia is one of the ten largest plant genera, with over 1500 species. This high species richness may in part be explained by weak species cohesion, which has allowed speciation by divergence in allopatry. In this study, we investigate species cohesion in the widespread Central American Begonia heracleifolia and Begonia nelumbiifolia, by genotyping populations at microsatellite loci. We then test for post-zygotic reproductive barriers using experimental crosses, and assess whether sterility barriers are related to intraspecific changes in genome size, indicating major genome restructuring between isolated populations. Strong population substructure was found for B. heracleifolia (FST=0.364, F′ST=0.506) and B. nelumbiifolia (FST=0.277, F′ST=0.439), and Bayesian admixture analysis supports the division of most populations into discrete genetic clusters. Moderate levels of inferred selfing (B. heracleifolia s=0.40, B. nelumbiifolia s=0.62) and dispersal limitation are likely to have contributed to significant genetic differentiation (B. heracleifolia Jost’s D=0.274; B. nelumbiifolia D=0.294). Interpopulation crosses involving a divergent B. heracleifolia population with a genome size ~10% larger than the species mean had a ~20% reduction in pollen viability compared with other outcrosses, supporting reproductive isolation being polymorphic within the species. The population genetic data suggest that Begonia populations are only weakly connected by gene flow, allowing reproductive barriers to accumulate between the most isolated populations. This supports allopatric divergence in situ being the precursor of speciation in Begonia, and may also be a common speciation mechanism in other tropical herbaceous plant groups