Giving a talk up at Temple University, last seminar of the year but one I’ve been looking forward to giving for a while.
Here is a map of the dogwood we’ve sampled in the Fan region of Richmond Virginia.
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.
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.
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.