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

Abstract

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 300 ha 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 fertilizing one seed or more, but only three trees fertilizing more than ten seeds. Spatial analysis suggests a fat-tailed pollen dispersal curve with 85% of detected pollination occurring within 100 m, and 15% spreading between 300 and 1900 m from the source.Identification of immigrating pollen sourced from two neighbouring remnants indicates further effective dispersal at 2900 m. 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 realized 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.

Comparison of pollen gene flow among four European beech (Fagus sylvatica L.) populations characterized by different management regimes

“…Therefore, it can be hypothesized that the reduction of pollen-mediated gene flow from paternity analysis (performed on non-dispersed seeds) to parentage analysis (on seedlings) might result from a slight advantage in the seedling establishment of offspring from local adults in the Table 4 Number of genotyped seeds (N seed ); number of seeds assigned to local fathers (N loc ); number of seeds not assigned to local fathers (N ext ); proportion of external pollination events per mother (N ext /N seed ); number of seeds assigned to local fathers with only one father above the LOD threshold (N single ); number of seeds assigned to local fathers with more than one father above the LOD threshold (N multiple ); number of seeds assigned to local fathers with two or more most likely fathers (same highest LOD scores) (unresolved assignments, N unres ); number of seeds whose paternity was resolved (N res ) and ratio between resolved paternity assignments and genotyped seeds (N res /N seed ) reported for each maternal tree in the four plots colonization process. In the same way Bacles and Ennos (2008) detected a limitation in the recruitment of genes carried by immigrant pollen by comparing gene flow rates from non-dispersed seeds and newly established seedlings in fragmented Fraxinus excelsior populations. High gene flow through pollen seems a general characteristic of Fagaceae.…”

“…To overcome methodological uncertainties related to different gene flow pattern analyses, we used two different approaches among the most common in dispersal studies on plants (Gerber et al, 2000;Burczyk et al, 2002;Bacles et al, 2006). In both approaches we employed recently developed techniques to take into account and correct possible genotyping errors that can inflate gene flow rates (Bacles and Ennos, 2008;Chybicki and Burczyk, 2010a). The two methods provided almost identical estimates of gene flow rates.…”

“…An estimate of null allele frequencies in the data sets from French stands based on different approaches was also conducted by Oddou-Muratorio et al (2009). To take into account possible genotyping errors during microsatellite scoring, we accommodated loci for which the presence of null alleles was suspected, and performed parentage analysis following different approaches, as suggested in recent literature (Bacles and Ennos, 2008;Oddou-Muratorio et al, 2009).…”

“…As by Bacles and Ennos (2008), instead of introducing in the analysis a global stochastic error rate, we performed two subsequent transformations of the original data sets (referred to hereafter as RAW data sets): a binning procedure for allele miscalls (obtaining the BIN data sets) and a subsequent genotype substitution procedure for allele dropouts (BINNULL data sets). In the binning procedure all alleles with frequency below 0.01 were binned with alleles 40.01 of the nearest size.…”

“…Recent methodological improvements allow to estimate the statistical precision of a parentage/paternity assignment for a given sample of a reproductive population (Gerber et al, 2003;Burczyk and Chybicki, 2004). In any case, both Oddou-Muratorio et al (2003) and Bacles and Ennos (2008) claimed that it is better to use several approaches together to estimate genetic exchange among populations.…”