DiLeo MF, Siu JC, Rhodes MK, López-Villalobos A, Redwine A, Ksiazek K, Dyer RJ. 2014. The gravity of pollination: integrating at-site features into spatial analysis of contemporary pollen movement. Molecular Ecology, 23, 2973-2982.

DiLeo MF, Siu JC, Rhodes MK, López-Villalobos A, Redwine A, Ksiazek K, Dyer RJ. 2014. The gravity of pollination: integrating at-site features into spatial analysis of contemporary pollen movement. Molecular Ecology, 23, 2973-2982.

Pollen-mediated gene flow is a major driver of spatial genetic structure in plant populations. Both individual plant characteristics and site-specific features of the landscape can modify the perceived attractiveness of plants to their pollinators and thus play an important role in shaping spatial genetic variation. Most studies of landscape-level genetic connectivity in plants have focused on the effects of interindividual distance using spatial and increasingly ecological separation, yet have not incorporated individual plant characteristics or other at-site ecological variables. Using spatially explicit simulations, we first tested the extent to which the inclusion of at-site variables influencing local pollination success improved the statistical characterization of genetic connectivity based upon examination of pollen pool genetic structure. The addition of at-site characteristics provided better models than those that only considered interindividual spatial distance (e.g. IBD). Models parameterized using conditional genetic covariance (e.g. population graphs) also outperformed those assuming panmixia. In a natural population of Cornus florida L. (Cornaceae), we showed that the addition of at-site characteristics (clumping of primary canopy opening above each maternal tree and maternal tree floral output) provided significantly better models describing gene flow than models including only between-site spatial (IBD) and ecological (isolation by resistance) variables. Overall, our results show that including interindividual and local ecological variation greatly aids in characterizing landscape-level measures of contemporary gene flow.

DOI: 10.1111/mec.12839

Dyer RJ, Chan DM, Gardiakos VA, Meadows CA. 2012. Pollination networks: quantifying pollen pool covariance networks and the influence of intervening landscape on genetic connectivity in the North American understory tree, Cornus florida L. Landscape Ecology, 27 239-251.

Dyer RJ, Chan DM, Gardiakos VA, Meadows CA. 2012. Pollination networks: quantifying pollen pool covariance networks and the influence of intervening landscape on genetic connectivity in the North American understory tree, Cornus florida L. Landscape Ecology, 27 239-251.

The manner by which pollinators move across a landscape and their resulting preferences and/or avoidances of travel through particular habitat types can have a significant impact on plant population genetic structure and population-level connectivity. We examined the spatial genetic structure of the understory tree Cornus florida (Cornaceae) adults (NAdults = 452) and offspring (NOffspring = 736) across two mating events to determine the extent to which pollen pool genetic covariance is influenced by intervening forest architecture. Resident adults showed no spatial partitioning but genotypes were positively autocorrelated up to a distance of 35 m suggesting a pattern of restricted seed dispersal. In the offspring, selfing rates were small (sm = 0.035) whereas both biparental inbreeding (sb;open canopy = 0.16, sb;closed canopy = 0.11) and correlated paternity (rp;open canopy = 0.21, rp;closed canopy = 0.07) were significantly influenced by primary canopy opening above individual mothers. The spatial distribution of genetic covariance in pollen pool composition was quantified for each reproductive event using Pollination Graphs, a network method based upon multivariate conditional genetic covariance. The georeferenced graph topology revealed a significant positive relationship between genetic covariance and pollinator movement through C. florida canopies, a negative relationship with open primary canopy (e.g., roads under open canopies and fields with no primary canopy), and no relationship with either conifer or mixed hardwood canopy species cover. These results suggest that both resident genetic structure within stands and genetic connectivity between sites in C. florida populations are influenced by spatial heterogeneity of mating individuals and quality of intervening canopy cover.

DOI: 10.1007/s10980-011-9696-x

Dyer RJ. 2007. Powers of discerning: challenges to understanding dispersal processes in natural populations. Molecular Ecology,16, 4881-4882.

Dyer RJ. 2007. Powers of discerning: challenges to understanding dispersal processes in natural populations. Molecular Ecology,16, 4881-4882.

In this issue of Molecular Ecology, authors Robledo-Arnuncio & Garcia present a compelling approach for quantifying seed dispersal in plant populations. Building upon methods previously used for quantification of pollen dispersal, the authors not only examine the behavior of the model with respect to sample sizes, dispersal distance, and the kurtosis of the dispersal function but also provide an empirical example using Prunus mahaleb.

DOI: 10.1111/j.1365-294X.2007.03581.x

Gonzales E, Hamrick JL, Smouse PE, Dyer RJ. 2006. Pollen-mediated gene dispersal within continuous and fragmented populations of a forest understory species, Trillium cuneatum. Molecular Ecology, 15 2047-2058.

Pollen movement plays a critical role in the distribution of genetic variation within and among plant populations. Direct measures of pollen movement in the large, continuous populations that characterize many herbaceous plant species are often technically difficult and biologically unreliable. Here, we studied contemporary pollen movement in four large populations of Trillium cuneatum. Three populations, located in the Georgia Piedmont, are exposed to strong anthropogenic disturbances, while the fourth population, located in the Southern Appalachian Mountains, is relatively undisturbed. Using the recently developed TwoGener analysis, we extracted estimates of the effective number of pollen donors (Nep), effective mating neighbourhood size (Aep) and the average distance of pollen movement (δ) for each population. We extended the TwoGener method by developing inference on the paternal gametic contribution to the embryo in situations where offspring genotypes are inferred from seeds and elaiosomes of species with bisporic megagametogenesis. Our estimates indicate that maternal plants do not sample pollen randomly from a global pool; rather, pollen movement in all four populations is highly restricted. Although the effective number of pollen donors per maternal plant is low (Nep = 1.22–1.66) and pollen movement is highly localized in all populations, Nep in the disturbed Piedmont populations is higher and there is more pollen movement than in the mountains. The distance pollen moves is greater in disturbed sites and fragmented populations, possibly due to edge effects in Trillium habitats.

DOI: 10.1111/j.1365-294X.2006.02913.x

Sork VL, Smouse PE, Apsit VJ, Dyer RJ, Westfall RD. 2005. A two-generation analysis of pollen pool genetic structure in flowering dogwood, Cornus florida (Cornaceae), in the Missouri Ozarks. American Journal of Botany, 92 262-271.

Anthropogenic landscape change can disrupt gene flow. As part of the Missouri Ozark Forest Ecosystem Project, this study examined whether silvicultural practices influence pollen-mediated gene movement in the insect-pollinated species, Cornus florida L., by comparing pollen pool structure (Φst) among clear-cutting, selective cutting, and uncut regimes with the expectation that pollen movement should be least in the uncut regime. Using a sample of 1500 seedlings—10 each from 150 seed parents (43 in clear-cut, 74 in selective, and 33 in control sites) from six sites (each ranging from 266 to 527 ha), eight allozyme loci were analyzed with a pollen pool structure approach known as TWOGENER (Smouse et al., 2001; Evolution 55: 260–271). This analysis revealed that pollen pool structure was less in clear-cut (&PhiC = 0.090, P < 0.001) than in uncut areas (ΦU = 0.174, P < 0.001), with selective-cut intermediate (ΦS = 0.125, P < 0.001). These estimates translate into more effective pollen donors (Nep) in clear-cut (Nep = 5.56) and selective-cut (Nep = 4.00) areas than in uncut areas (Nep = 2.87). We demonstrate that &PhiC ≤ ΦS ≤ ΦU, with ΦC significantly smaller than ΦU (P < 0.034). The findings imply that, as long as a sufficiently large number of seed parents remain to provide adequate reproduction and to avoid a genetic bottleneck in the effective number of mothers, silvicultural management may not negatively affect the effective number of pollen parents, and hence subsequent genetic diversity in Cornus florida.

DOI: 10.3732/ajb.92.2.262.

Dyer RJ, Westfall RD, Sork VL, Smouse PE. 2004. Two-generation analysis of pollen flow across a landscape V: a stepwise approach for extracting factors contributing to pollen structure. Heredity, 92 204-211.

Dyer RJ, Westfall RD, Sork VL, Smouse PE. 2004. Two-generation analysis of pollen flow across a landscape V: a stepwise approach for extracting factors contributing to pollen structure. Heredity, 92 204-211.

Patterns of pollen dispersal are central to both the ecology and evolution of plant populations. However, the mechan- isms controlling either the dispersal process itself or our estimation of that process may be influenced by site-specific factors such as local forest structure and nonuniform adult genetic structure. Here, we present an extension of the AMOVA model applied to the recently developed TWOGENER analysis of pollen pool structure. This model, dubbed the Stepwise AMOVA (StAMOVA), focuses on determining to what extent ecological, demographic, and/or environmental factors influence the observed genetic variation in spatially separated pollen pools. The analysis is verified for efficacy, using an extensive battery of simulations, illustrating: (1) how nonuniform adult genetic structure influences the differentiation of spatially separated pollen pools, and (2) how effectively the Stepwise analysis performs in carrying out the appropriate corrections. Finally, the model is applied to a Quercus alba data set, from which we have prior evidence that the adult genetic structure is nonuniformly distributed across the sampling landscape. From this data set, we show how the Stepwise model can be applied to remove the effects of spatial adult genetic structure on pollen pool differentiation and contrast these results with those derived from the original TWOGENER analysis.

DOI: 10.1038/sj.hdy.6800397

Sork VL, Davis FW, Smouse PE, Apsit VJ, Dyer RJ, Fernandez-M JF, Kuhn B. 2002. Pollen movement in declining populations of California Valley oak, Quercus lobata: where have all the fathers gone? Molecular Ecology, 11 1657-1668.

Sork VL, Davis FW, Smouse PE, Apsit VJ, Dyer RJ, Fernandez-M JF, Kuhn B. 2002. Pollen movement in declining populations of California Valley oak, Quercus lobata: where have all the fathers gone? Molecular Ecology, 11 1657-1668.

The fragmented populations and reduced population densities that result from human disturbance are issues of growing importance in evolutionary and conservation biology. A key issue is whether remnant individuals become reproductively isolated. California Valley oak (Quercus lobata) is a widely distributed, endemic species in California, increasingly jeopardized by anthropogenic changes in biota and land use. We studied pollen movement in a savannah population of Valley oak at Sedgwick Reserve, Santa Barbara County, to estimate effective number of pollen donors (Nep) and average distance of effective pollen movement (δ). Using TWOGENER, our recently developed hybrid model of paternity and genetic structure treatments that analyses maternal and progeny multilocus genotypes, we found that current Nep = 3.68 individuals. Based on an average adult density of d = 1.19 stems/ha, we assumed a bivariate normal distribution to model current average pollen dispersal distance (δ) and estimated δ = 64.8 m. We then deployed our parameter estimates in spatially explicit models of the Sedgwick population to evaluate the extent to which Nep may have changed, as a consequence of progressive stand thinning between 1944 and 1999. Assuming that pollen dispersal distance has not changed, we estimate Nep was 4.57 individuals in 1944, when stand density was 1.48. Both estimates indicate fewer effective fathers than one might expect for wind-pollinated species and fewer than observed elsewhere. The results presented here provide a basis for further refinements on modelling pollen movement. If the trends continue, then ongoing demographic attrition could further reduce neighbourhood size in Valley oak resulting in increased risk of reproductive failure and genetic isolation.

DOI: 10.1046/j.1365-294X.2002.01574.x

Sork VL, Davis FW, Dyer RJ, Smouse PE. 2002. Mating patterns in a savanna population of Valley Oak (Quercus lobata Nee). USDA Forest SErvice Gen. Tech. Rep, PSW-GTR-184 427-439.

Sork VL, Davis FW, Dyer RJ, Smouse PE. 2002. Mating patterns in a savanna population of Valley Oak (Quercus lobata Nee). USDA Forest SErvice Gen. Tech. Rep, PSW-GTR-184 427-439.

California valley oak is threatened by landscape alteration and failing recruitment in remnant stands. Its reproductive ecology is a key element of the seedling recruitment process. We first examine the mating system, to determine the extent of inbreeding in a population at Sedgwick Reserve, in Santa Barbara County. We then quantify variation in germination success and acorn size, evaluating their spatial patterns across the site. We collected acorns from 21 mapped focal trees in fall 1999, measured their average seed weight and germination success, and identified their multilocus genotypes. Using a mixed mating model, we observed significant, but modest selfing (outcrossing rate: tm = 0.96) and no mating among relatives (tm – ts) = 0.0. The effective pollen donor number was estimated to be between 5 and 7 individuals, depending on the inbreeding coefficient of the adults. These mating results indicate relatively little inbreeding but low numbers of pollen donors. Mothers differed significantly in seed weight (range: ~ 4 – 10 g) and germination percentage (range: 0 – 90 percent), and a bivariate analysis showed a gradient across the study site. Such a pattern suggests that environment conditions influence acorn size and germination success. Future work will address whether isolated individuals are at risk of selfing, for the expression of inbreeding depression on seed traits, or a reduction in the effective pollen donor number.

USDA Forest Service Gen. Tech. Rep. PSW-GTR-184.

Smouse PE, Dyer RJ, Westfall RD, Sork VL. 2001. Two-generation analysis of pollen flow across a landscape I. Male gamete heterogeneity among females. Evolution, 55 260-271.

Smouse PE, Dyer RJ, Westfall RD, Sork VL. 2001. Two-generation analysis of pollen flow across a landscape I. Male gamete heterogeneity among females. Evolution, 55 260-271.

Gene flow is a key factor in the spatial genetic structure in spatially distributed species. Evolutionary biologists interested in microevolutionary processes and conservation biologists interested in the impact of landscape change require a method that measures the real time process of gene movement. We present a novel two-generation (parent-offspring) approach to the study of genetic structure (TwoGener) that allows us to quantify heterogeneity among the male gamete pools sampled by maternal trees scattered across the landscape and to estimate mean pollination distance and effective neighborhood size. First, we describe the model’s elements: genetic distance matrices to estimate intergametic distances, molecular analysis of variance to determine whether pollen profiles differ among mothers, and optimal sampling considerations. Second, we evaluate the model’s effectiveness by simulating spatially distributed populations. Spatial heterogeneity in male gametes can be estimated by ΦFT, a male gametic analogue of Wright’s FST and an inverse function of mean pollination distance. We illustrate TwoGener in cases where the male gamete can be categorically or ambiguously determined. This approach does not require the high level of genetic resolution needed by parentage analysis, but the ambiguous case is vulnerable to bias in the absence of adequate genetic resolution. Finally, we apply TwoGener to an empirical study of Quercus alba in Missouri Ozark forests. We find that ΦFT = 0.06, translating into about eight effective pollen donors per female and an effective pollination neighborhood as a circle of radius about 17 m. Effective pollen movement in Q. alba is more restricted than previously realized, even though pollen is capable of moving large distances. This case study illustrates that, with a modest investment in field survey and laboratory analysis, the TwoGener approach permits inferences about landscape-level gene movements.

DOI: 10.1111/j.0014-3820.2001.tb01291.x

Dyer RJ, Sork VL. 2001. Pollen pool heterogeneity in shortleaf pine, Pinus echinata, Mill. Molecular Ecology, 10 859-866.

Pollen is the dominant vector of gamete exchange for most temperate tree species. Because pollen movement influences the creation, maintenance and erosion of genetic structure in adult populations, it is important to understand what factors influence the process of pollen movement. Isolation by distance in pollen donor populations can create highly structured pollen polls by increased sampling of local fathers. Extrinsic factors, such as the intervening vegetative structure and local pollen donor densities, can also influence the genetic composition of local pollen pools. Using paternally inherited chloroplast microsatellite markers, we examined the structure and diversity of pollen pools in Pinus echinata Mill. in southern Missouri, USA. Our analysis is based on a multivariate AMOVA analysis of stands (~1 ha; six per region) nested within regions (~800 ha; four each). Significant multilocus structure of the pollen pool within regions (ΦSR = 0.095), but not among regions (ΦRT = 0.010), indicates that pollen movement is relatively restricted. Furthermore, the significant correlation between pairwise genetic and physical distances (Mantel correlation; ρ = 0.32) provided support for the isolation by distance hypothesis. Our results indicated that availability of pollen donors did not affect diversity of the pollen pool, measured by the number of unique multilocus genotypes at each stand. However, pollen pool diversity was negatively associated with vegetative structure, measured as total forest tree density. Our findings indicated that on-going pollen movement within continuous forest is relatively restricted as a result of both isolation by distance and vegetative structure.

DOI: 10.1046/j.1365-294X.2001.01251.x

Smouse PE, Dyer RJ, Westfall RD, Sork VL. 2001. Two-generation analysis of pollen flow across a landscape I. Male gamete heterogeneity among females. Evolution, 55 260-271.

Smouse PE, Dyer RJ, Westfall RD, Sork VL. 2001. Two-generation analysis of pollen flow across a landscape I. Male gamete heterogeneity among females. Evolution, 55 260-271.

Gene flow is a key factor in the spatial genetic structure in spatially distributed species. Evolutionary biologists interested in microevolutionary processes and conservation biologists interested in the impact of landscape change require a method that measures the real time process of gene movement. We present a novel two-generation (parent-offspring) approach to the study of genetic structure (TwoGener) that allows us to quantify heterogeneity among the male gamete pools sampled by maternal trees scattered across the landscape and to estimate mean pollination distance and effective neighborhood size. First, we describe the model’s elements: genetic distance matrices to estimate intergametic distances, molecular analysis of variance to determine whether pollen profiles differ among mothers, and optimal sampling considerations. Second, we evaluate the model’s effectiveness by simulating spatially distributed populations. Spatial heterogeneity in male gametes can be estimated by ΦFT, a male gametic analogue of Wright’s FST and an inverse function of mean pollination distance. We illustrate TwoGener in cases where the male gamete can be categorically or ambiguously determined. This approach does not require the high level of genetic resolution needed by parentage analysis, but the ambiguous case is vulnerable to bias in the absence of adequate genetic resolution. Finally, we apply TwoGener to an empirical study of Quercus alba in Missouri Ozark forests. We find that ΦFT = 0.06, translating into about eight effective pollen donors per female and an effective pollination neighborhood as a circle of radius about 17 m. Effective pollen movement in Q. alba is more restricted than previously realized, even though pollen is capable of moving large distances. This case study illustrates that, with a modest investment in field survey and laboratory analysis, the TwoGener approach permits inferences about landscape-level gene movements.

DOI: 10.1111/j.0014-3820.2001.tb01291.x