Bacles CFE, Ennos RA. Paternity analysis of pollen-mediated gene flow for Fraxinus excelsior L. in a chronically fragmented landscape. Heredity 101: 368-380

Institute of Evolutionary Biology, Ashworth Laboratories, School of Biological Sciences, The University of Edinburgh, Edinburgh, UK.
Heredity (Impact Factor: 3.81). 10/2008; 101(4):368-80. DOI: 10.1038/hdy.2008.66
Source: PubMed


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.

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    • "The procedure holds true for seed genotypes with the exception of non-amplifying seed genotypes that were not transformed if their mother trees were heterozygous. Previous studies demonstrated that this procedure significantly increases exclusion probabilities and decreases genotyping error rates when compared with other transformations (e.g. the binning procedure) (Bacles and Ennos, 2008; Piotti et al., 2012). "
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    ABSTRACT: Background and aims Ulmus minor has undergone a severe demographic reduction because of Dutch Elm Disease (DED). The introduction in Europe of the exotic Ulmus pumila, highly tolerant to DED, has widely replaced native U. minor populations. Morphological and genetic evidence of hybridization has been reported, and thus the need for assessing inter-specific gene flow patterns in natural populations. Therefore, this work is aimed at studying pollen gene flow in a remnant U. minor stand surrounded by trees of both species scattered in an agricultural landscape. Methods All trees from a small natural stand (350) and the surrounding agricultural area within a 5 km radius (89) were genotyped at six microsatellite loci. Trees were morphologically characterized as U. minor, U. pumila, or intermediate phenotypes, and morphological identification was compared to Bayesian clustering of genotypes. For paternity analysis, seeds were collected in two consecutive years from 20 and 28 mother trees, respectively. Maximum likelihood paternity assignment was used to elucidate intra- and inter-specific gene flow patterns. Key Results Genetic structure analyses indicated the presence of two genetic clusters only partially matching the morphological identification. Paternity analysis results were consistent between the two consecutive years investigated and showed high pollen immigration rates (~ 0.80) and mean pollination distances (~ 3 km), and a skewed distribution of reproductive success. Finally, few inter-cluster pollinations and putative hybrid individuals were found. Conclusions Pollen gene flow is not impeded in the fragmented agricultural landscape investigated. High pollen immigration and extensive pollen dispersal distances are likely counteracting the potential loss of genetic variation caused by isolation. We also found some evidence that U. minor and U. pumila can hybridise when in sympatry. Although hybridisation might have beneficial effects on both species, remnant U. minor populations represent a valuable source of genetic diversity to be preserved.
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    • "Genotyping error can arise from several sources: null alleles due to mutation in priming sites or poor DNA template quality, stutter bands caused by polymerase slippage, human introduced error due to allele miscalling , and mutations generating new alleles (Dakin & Avise 2004; Hoffman & Amos 2005). Genotyping error caused primarily by inconsistently amplifying (null and stutter) alleles has been encountered in many studies of pollen flow (Dow & Ashley 1996; Bacles & Ennos 2008; Piotti et al. 2012), leading to observed mismatches between seeds and maternal parent trees. Null alleles, allelic dropout, and stutter bands have all been observed in other studies using microsatellite loci from this marker set (Krutovsky et al. 2009; Fussi et al. 2013). "
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    • "Wind-pollinated tree species are capable of longdistance dispersal and deforestation can potentially disrupt panmictic mating pattern among populations (e.g. Jump & Penuelas 2006, but see Bittencourt & Sebbenn 2007; Bacles & Ennos 2008). We therefore examined isolation-by-distance (IBD) patterns between primary and secondary forests by calculating pairwise Nei's genetic distance and the corresponding linear pairwise geographic distance using the Mantel Test implemented in IDBWS (Jensen et al. 2005). "
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