A new method of estimating the pollen dispersal curve independently of effective density

Isolation by distance in populations with power-law dispersal

Limited dispersal of individuals between generations results in isolation by distance, in which individuals further apart in space tend to be less related. Classic models of isolation by distance assume that dispersal distances are drawn from a thin-tailed distribution and predict that the proportion of the genome that is identical by descent between a pair of individuals should decrease exponentially with the spatial separation between them. However, in many natural populations, individuals occasionally disperse over very long distances. In this work, we use mathematical analysis and coalescent simulations to study the effect of long-range (power-law) dispersal on patterns of isolation by distance. We find that it leads to power-law decay of identity-by-descent at large distances with the same exponent as dispersal. We also find that broad power-law dispersal produces another, shallow power-law decay of identity-by-descent at short distances. These results suggest that the distribution of long-range dispersal events could be estimated from sequencing large population samples taken from a wide range of spatial scales.

Influence of Pollen Dispersal and Mating Pattern in Domestication of Intermediate Wheatgrass, a Novel Perennial Food Crop

Intermediate wheatgrass (IWG) is a perennial forage grass that is currently being domesticated as a grain crop. It is a primarily wind-pollinated outcrossing species and expresses severe inbreeding depression when self-pollinated. Characterization of pollen dispersal, mating parameters, and change in genetic diversity due to pollen movement is currently lacking in IWG. In this study, we examined pollen dispersal in an IWG selection nursery by evaluating 846 progeny from 15 mother plants and traced their parentage to 374 fathers. A set of 2,500 genomic loci was used to characterize the population. We assigned paternity to 769 (91%) progeny and the average number of fathers per mother plant was 37, from an average of 56 progeny examined per mother. An extensive number (80%) of pollination events occurred within 10 m of the mother plants. Pollination success was not correlated with trait attributes of the paternal genotypes. Mating system analysis confirmed that IWG is highly outcrossing and inbreeding was virtually absent. Neither genetic diversity nor the genome-estimated trait values of progeny were significantly affected by pollinator distance. The distance of pollinator in an IWG breeding nursery therefore was not found to be a major contributor in maintaining genetic diversity. These findings reveal the pollen dispersal model in IWG for the first time and its effect on genetic diversity, which will be valuable in designing future IWG breeding populations. Information generated and discussed in this study could be applied in understanding gene flow and genetic diversity of other open-pollinated species.

Spatial variation in bird pollination and its mitigating effects on the genetic diversity of pollen pools accepted by Camellia japonica trees within a population at a landscape level

Bird pollination can vary spatially in response to spatial fluctuations in flowering even within plant populations. In this study, we examined the hypothesis that the spatial variation in bird pollination may induce mitigating effects, which maintains or increases genetic diversity of pollen pools at local sites with low flowering densities. To test this hypothesis, we analyzed the landscape-level genetic effects within a population of Camellia japonica on the pollen pools accepted by individuals in two reproductive years by using genotypes at eight microsatellite loci of 1323 seeds from 19 seed parents. Regression analyses using the quadratic models of correlated paternity between pollen pools against spatial distances between the seed-parent pairs revealed not only local pollination but also some amount of long-distance pollen dispersal. The genetic diversity of pollen pools accepted by seed parents tended to be negatively related to the densities of flowering individuals near the seed parents during winter (when the effective pollination of C. japonica is mediated mostly by Zosterops japonica). We show that the low density of flowering individuals may induce the expansion of the foraging areas of Z. japonica and consequently increase the genetic diversity of pollen pools. This spatial variation in bird pollination may induce the mitigating effects on the C. japonica population. The comparisons between the two study years indicate that the overall pattern of bird pollination and the genetic effects described here, including the mitigating effects, may be stable over time.

Limited pollen dispersal, small genetic neighborhoods, and biparental inbreeding in Vallisneria americana

PREMISE OF THE STUDY

Pollen dispersal is a key process that influences ecological and evolutionary dynamics of plant populations by facilitating sexual reproduction and gene flow. Habitat loss and fragmentation have the potential to reduce pollen dispersal within and among habitat patches. We assessed aquatic pollen dispersal and mating system characteristics in Vallisneria americana-a water-pollinated plant with a distribution that has been reduced from historic levels.

METHODS

We examined pollen neighborhood size, biparental inbreeding, and pollen dispersal, based on seed paternity using the indirect paternity method KinDist, from samples of 18-39 mothers and 14-20 progeny per mother from three sites across 2 years.

KEY RESULTS

On average, fruits contained seeds sired by seven fathers. We found significant biparental inbreeding and limited pollen dispersal distances (0.8-4.34 m). However, in a number of cases, correlated paternity did not decline with distance, and dispersal could not be reliably estimated.

CONCLUSIONS

Frequent pollen dispersal is not expected among patches, and even within patches, gene flow via pollen will be limited. Limited pollen dispersal establishes genetic neighborhoods, which, unless overcome by seed and propagule dispersal, will lead to genetic differentiation even in a continuous population. Unless loss and fragmentation drive populations to extreme sex bias, local pollen dispersal is likely to be unaffected by habitat loss and fragmentation per se because the spatial scale of patch isolation already exceeds pollen dispersal distances. Therefore, managing specifically for pollen connectivity is only relevant over very short distances.

Extensive seed and pollen dispersal and assortative mating in the rain forest tree Entandrophragma cylindricum (Meliaceae) inferred from indirect and direct analyses

Pollen and seed dispersal are key processes affecting the demographic and evolutionary dynamics of plant species and are also important considerations for the sustainable management of timber trees. Through direct and indirect genetic analyses, we studied the mating system and the extent of pollen and seed dispersal in an economically important timber species, Entandrophragma cylindricum (Meliaceae). We genotyped adult trees, seeds and saplings from a 400-ha study plot in a natural forest from East Cameroon using eight nuclear microsatellite markers. The species is mainly outcrossed (t = 0.92), but seeds from the same fruit are often pollinated by the same father (correlated paternity, r_p  = 0.77). An average of 4.76 effective pollen donors (N_ep ) per seed tree contributes to the pollination. Seed dispersal was as extensive as pollen dispersal, with a mean dispersal distance in the study plot approaching 600 m, and immigration rates from outside the plot to the central part of the plot reaching 40% for both pollen and seeds. Extensive pollen- and seed-mediated gene flow is further supported by the weak, fine-scale spatial genetic structure (Sp statistic = 0.0058), corresponding to historical gene dispersal distances (σ_g ) reaching approximately 1,500 m. Using an original approach, we showed that the relatedness between mating individuals (F_ij  = 0.06) was higher than expected by chance, given the extent of pollen dispersal distances (expected F_ij  = 0.02 according to simulations). This remarkable pattern of assortative mating could be a phenomenon of potentially consequential evolutionary and management significance that deserves to be studied in other plant populations.

Gene flow and natural selection shape spatial patterns of genes in tree populations: implications for evolutionary processes and applications

A central question in evolutionary biology is how gene flow and natural selection shape geographic patterns of genotypic and phenotypic variation. My overall research program has pursued this question in tree populations through complementary lines of inquiry. First, through studies of contemporary pollen and seed movement, I have studied how limited gene movement creates fine-scale genetic structure, while long-distance gene flow promotes connectivity. My collaborators and I have provided new tools to study these processes at a landscape scale as well as statistical tests to determine whether changes in landscape conditions or dispersal vectors affect gene movement. Second, my research on spatial patterns of genetic variation has investigated the interacting impacts of geography and climate on gene flow and selection. Third, using next-generation genomic tools, I am now studying genetic variation on the landscape to find initial evidence of climate-associated local adaptation and epigenetic variation to explore its role in plant response to the climate. By integrating these separate lines of inquiry, this research provides specific insight into real-world mechanisms shaping evolution in tree populations and potential impacts of landscape transformation and climate change on these populations, with the prospective goal of contributing to their management and conservation.

Mating system and early viability resistance to habitat fragmentation in a bird-pollinated eucalypt

Habitat fragmentation has been shown to disrupt ecosystem processes such as plant-pollinator mutualisms. Consequently, mating patterns in remnant tree populations are expected to shift towards increased inbreeding and reduced pollen diversity, with fitness consequences for future generations. However, mating patterns and phenotypic assessments of open-pollinated progeny have rarely been combined in a single study. Here, we collected seeds from 37 Eucalyptus incrassata trees from contrasting stand densities following recent clearance in a single South Australian population (intact woodland=12.6 trees ha(-1); isolated pasture=1.7 trees ha(-1); population area=10 km(2)). 649 progeny from these trees were genotyped at eight microsatellite loci. We estimated genetic diversity, spatial genetic structure, indirect contemporary pollen flow and mating patterns for adults older than the clearance events and open-pollinated progeny sired post-clearance. A proxy of early stage progeny viability was assessed in a common garden experiment. Density had no impact on mating patterns, adult and progeny genetic diversity or progeny growth, but was associated with increased mean pollen dispersal. Weak spatial genetic structure among adults suggests high historical gene flow. We observed preliminary evidence for inbreeding depression related to stress caused by fungal infection, but which was not associated with density. Higher observed heterozygosities in adults compared with progeny may relate to weak selection on progeny and lifetime-accumulated mortality of inbred adults. E. incrassata appears to be resistant to the negative mating pattern and fitness changes expected within fragmented landscapes. This pattern is likely explained by strong outcrossing and regular long-distance pollen flow.

Long-distance wind-dispersal of spores in a fungal plant pathogen: estimation of anisotropic dispersal kernels from an extensive field experiment

Given its biological significance, determining the dispersal kernel (i.e., the distribution of dispersal distances) of spore-producing pathogens is essential. Here, we report two field experiments designed to measure disease gradients caused by sexually- and asexually-produced spores of the wind-dispersed banana plant fungus Mycosphaerella fijiensis. Gradients were measured during a single generation and over 272 traps installed up to 1000 m along eight directions radiating from a traceable source of inoculum composed of fungicide-resistant strains. We adjusted several kernels differing in the shape of their tail and tested for two types of anisotropy. Contrasting dispersal kernels were observed between the two types of spores. For sexual spores (ascospores), we characterized both a steep gradient in the first few metres in all directions and rare long-distance dispersal (LDD) events up to 1000 m from the source in two directions. A heavy-tailed kernel best fitted the disease gradient. Although ascospores distributed evenly in all directions, average dispersal distance was greater in two different directions without obvious correlation with wind patterns. For asexual spores (conidia), few dispersal events occurred outside of the source plot. A gradient up to 12.5 m from the source was observed in one direction only. Accordingly, a thin-tailed kernel best fitted the disease gradient, and anisotropy in both density and distance was correlated with averaged daily wind gust. We discuss the validity of our results as well as their implications in terms of disease diffusion and management strategy.

Patterns of pollen dispersal in a small population of the Canarian endemic palm (Phoenix canariensis)

The genetic diversity of small populations is greatly influenced by local dispersal patterns and genetic connectivity among populations, with pollen dispersal being the major component of gene flow in many plants species. Patterns of pollen dispersal, mating system parameters and spatial genetic structure were investigated in a small isolated population of the emblematic palm Phoenix canariensis in Gran Canaria island (Canary Islands). All adult palms present in the study population (n=182), as well as 616 seeds collected from 22 female palms, were mapped and genotyped at 8 microsatellite loci. Mating system analysis revealed an average of 5.8 effective pollen donors (Nep) per female. There was strong variation in correlated paternity rates across maternal progenies (ranging from null to 0.9) that could not be explained by the location and density of local males around focal females. Paternity analysis revealed a mean effective pollen dispersal distance of ∼71 m, with ∼70% of effective pollen originating from a distance of <75 m, and 90% from <200 m. A spatially explicit mating model indicated a leptokurtic pollen dispersal kernel, significant pollen immigration (12%) from external palm groves and a directional pollen dispersal pattern that seems consistent with local altitudinal air movement. No evidence of inbreeding or genetic diversity erosion was found, but spatial genetic structure was detected in the small palm population. Overall, the results suggest substantial pollen dispersal over the studied population, genetic connectivity among different palm groves and some resilience to neutral genetic erosion and subsequently to fragmentation.

Thank you for not flowering: conservation genetics and gene flow analysis of native and non-native populations of Fraxinus (Oleaceae) in Ireland

The risks of gene flow between interfertile native and introduced plant populations are greatest when there is no spatial isolation of pollen clouds and phenological patterns overlap completely. Moreover, invasion probabilities are further increased if introduced populations are capable of producing seeds by selfing. Here we investigated the mating system and patterns of pollen-mediated gene flow among populations of native ash (Fraxinus excelsior) and mixed plantations of non-native ash (F. angustifolia and F. excelsior) as well as hybrid ash (F. excelsior × F. angustifolia) in Ireland. We analysed the flowering phenology of the mother trees and genotyped with six microsatellite loci in progeny arrays from 132 native and plantation trees (1493 seeds) and 444 potential parents. Paternity analyses suggested that plantation and native trees were pollinated by both native and introduced trees. No signs of significant selfing in the introduced trees were observed and no evidence of higher male reproductive success was found for introduced trees compared with native ones either. A small but significant genetic structure was found (φft=0.05) and did not correspond to an isolation-by-distance pattern. However, we observed a significant temporal genetic structure related to the different phenological groups, especially with early and late flowering native trees; each phenological group was pollinated with distinctive pollen sources. Implications of these results are discussed in relation to the conservation and invasiveness of ash and the spread of resistance genes against pathogens such as the fungus Chalara fraxinea that is destroying common ash forests in Europe.

The power to detect recent fragmentation events using genetic differentiation methods

Habitat loss and fragmentation are imminent threats to biological diversity worldwide and thus are fundamental issues in conservation biology. Increased isolation alone has been implicated as a driver of negative impacts in populations associated with fragmented landscapes. Genetic monitoring and the use of measures of genetic divergence have been proposed as means to detect changes in landscape connectivity. Our goal was to evaluate the sensitivity of Wright's F st, Hedrick' G'st , Sherwin's MI, and Jost's D to recent fragmentation events across a range of population sizes and sampling regimes. We constructed an individual-based model, which used a factorial design to compare effects of varying population size, presence or absence of overlapping generations, and presence or absence of population sub-structuring. Increases in population size, overlapping generations, and population sub-structuring each reduced F st, G'st , MI, and D. The signal of fragmentation was detected within two generations for all metrics. However, the magnitude of the change in each was small in all cases, and when N e was >100 individuals it was extremely small. Multi-generational sampling and population estimates are required to differentiate the signal of background divergence from changes in Fst , G'st , MI, and D associated with fragmentation. Finally, the window during which rapid change in Fst , G'st , MI, and D between generations occurs can be small, and if missed would lead to inconclusive results. For these reasons, use of F st, G'st , MI, or D for detecting and monitoring changes in connectivity is likely to prove difficult in real-world scenarios. We advocate use of genetic monitoring only in conjunction with estimates of actual movement among patches such that one could compare current movement with the genetic signature of past movement to determine there has been a change.

Extensive contemporary pollen-mediated gene flow in two herb species, Ranunculus bulbosus and Trifolium montanum, along an altitudinal gradient in a meadow landscape

BACKGROUND AND AIMS

Genetic connectivity between plant populations allows for exchange and dispersal of adaptive genes, which can facilitate plant population persistence particularly in rapidly changing environments.

METHODS

Patterns of historic gene flow, flowering phenology and contemporary pollen flow were investigated in two common herbs, Ranunculus bulbosus and Trifolium montanum, along an altitudinal gradient of 1200-1800 m a.s.l. over a distance of 1 km among five alpine meadows in Switzerland.

KEY RESULTS

Historic gene flow was extensive, as revealed by Fst values of 0·01 and 0·007 in R. bulbosus and T. montanum, respectively, by similar levels of allelic richness among meadows and by the grouping of all individuals into one genetic cluster. Our data suggest contemporary pollen flow is not limited across altitudes in either species but is more pronounced in T. montanum, as indicated by the differential decay of among-sibships correlated paternity with increasing spatial distance. Flowering phenology among meadows was not a barrier to pollen flow in T. montanum, as the large overlap between meadow pairs was consistent with the extensive pollen flow. The smaller flowering overlap among R. bulbosus meadows might explain the slightly more limited pollen flow detected.

CONCLUSIONS

High levels of pollen flow among altitudes in both R. bulbosus and T. montanum should facilitate exchange of genes which may enhance adaptive responses to rapid climate change.

Extensive pollen flow but few pollen donors and high reproductive variance in an extremely fragmented landscape

Analysing pollen movement is a key to understanding the reproductive system of plant species and how it is influenced by the spatial distribution of potential mating partners in fragmented populations. Here we infer parameters related to levels of pollen movement and diversity of the effective pollen cloud for the wind-pollinated shrub Pistacia lentiscus across a highly disturbed landscape using microsatellite loci. Paternity analysis and the indirect KinDist and Mixed Effect Mating models were used to assess mating patterns, the pollen dispersal kernel, the effective number of males (N_ep) and their relative individual fertility, as well as the existence of fine-scale spatial genetic structure in adult plants. All methods showed extensive pollen movement, with high rates of pollen flow from outside the study site (up to 73–93%), fat-tailed dispersal kernels and large average pollination distances (δ = 229–412 m). However, they also agreed in detecting very few pollen donors (N_ep = 4.3–10.2) and a large variance in their reproductive success: 70% of males did not sire any offspring among the studied female plants and 5.5% of males were responsible for 50% of pollinations. Although we did not find reduced levels of genetic diversity, the adult population showed high levels of biparental inbreeding (14%) and strong spatial genetic structure (S_p = 0.012), probably due to restricted seed dispersal and scarce safe sites for recruitment. Overall, limited seed dispersal and the scarcity of successful pollen donors can be contributing to generate local pedigrees and to increase inbreeding, the prelude of genetic impoverishment.

The importance of pre-mating barriers and the local demographic context for contemporary mating patterns in hybrid zones of Eucalyptus aggregata and Eucalyptus rubida

The frequency of hybridization in plants is context dependent and can be influenced by the local mating environment. We used progeny arrays and admixture and pollen dispersal analyses to assess the relative importance of pre-mating reproductive barriers and the local demographic environment as explanations of variation in hybrid frequency in three mapped hybrid zones of Eucalyptus aggregata and E. rubida. A total of 731 open-pollinated progeny from 36 E. aggregata maternal parents were genotyped using six microsatellite markers. Admixture analysis identified substantial variation in hybrid frequency among progeny arrays (0-76.9%). In one hybrid zone, hybrid frequency was related to pre-mating barriers (degree of flowering synchrony) and demographic components of the local mating environment (decreasing population size, closer proximity to E. rubida and hybrid trees). At this site, average pollen dispersal distance was less and almost half (46%) of the hybrid progeny were sired by local E. rubida and hybrid trees. In contrast, at the other two sites, pre-mating and demographic factors were not related to hybrid frequency. Compared to the first hybrid zone where most of the E. rubida (76%) and all hybrids flowered, in the remaining sites fewer E. rubida (22-41%) and hybrid trees (0-50%) flowered and their reproductive success was lower (sired 0-23% of hybrids). As a result, most hybrids were sired by external E. rubida/hybrids located at least 2-3 km away. These results indicate that although pre-mating barriers and local demography can influence patterns of hybridization, their importance can depend upon the scale of pollen dispersal.

Population history and gene dispersal inferred from spatial genetic structure of a Central African timber tree, Distemonanthus benthamianus (Caesalpinioideae)

African rainforests have undergone major distribution range shifts during the Quaternary, but few studies have investigated their impact on the genetic diversity of plant species and we lack knowledge on the extent of gene flow to predict how plant species can cope with such environmental changes. Analysis of the spatial genetic structure (SGS) of a species is an effective method to determine major directions of the demographic history of its populations and to estimate the extent of gene dispersal. This study characterises the SGS of an African tropical timber tree species, Distemonanthus benthamianus, at various spatial scales in Cameroon and Gabon. Displaying a large continuous distribution in the Lower Guinea domain, this is a model species to detect signs of past population fragmentation and recolonization, and to estimate the extent of gene dispersal. Ten microsatellite loci were used to genotype 295 adult trees sampled from eight populations. Three clearly differentiated gene pools were resolved at this regional scale and could be linked to the biogeographical history of the region, rather than to physical barriers to gene flow. A comparison with the distribution of gene pools observed for two other tree species living in the same region invalidates the basic assumption that all species share the same Quaternary refuges and recolonization pathways. In four populations, significant and similar patterns of SGS were detected. Indirect estimates of gene dispersal distances (sigma) obtained for three populations ranged from 400 to 1200 m, whereas neighbourhood size estimates ranged from 50 to 110.

Pollen-mediated gene flow in isolated and continuous stands of bur oak, Quercus macrocarpa (Fagaceae)

PREMISE OF THE STUDY

Pollination patterns determine the reproductive neighborhood size of plants, the connectivity of populations, and the impacts of habitat fragmentation. We characterized pollination in three populations of Quercus macrocarpa occurring in a highly altered landscape in northeastern Illinois to determine whether isolated remnant stands were reproductively isolated. •

METHODS

We used microsatellites to genotype all adults and 787 acorns from two isolated savanna remnants and a stand in an old-growth forest. One isolated remnant occurred in a highly urbanized/industrialized landscape, and one occurred in an agricultural landscape. Parentage assignment was used to assess pollen-mediated gene flow. •

KEY RESULTS

Pollen donors from outside the study sites accounted for between 46% and 53% of paternities and did not differ significantly among sites, indicating that similar high levels of gene flow occurred at all three sites. Within stands, the mean pollination distance ranged from 42 to 70 meters, and when accounting for outside pollinations, mean pollination distances were well over 100 meters. Genetic diversity of incoming pollen was extremely high in all three stands. The number of effective pollen donors, N(ep), calculated from paternity assignment was higher than that estimated by an indirect correlated paternity approach. •

CONCLUSIONS

Our findings indicate that extremely isolated stands of oaks are unlikely to be genetically and reproductively isolated, and remnant stands may contribute to maintaining genetic connectivity in highly modified landscapes.

Influence of cryptic population structure on observed mating patterns in the wild progenitor of maize (Zea mays ssp. parviglumis)

Indirect two-generation analysis of pollen flow has proven to be an effective alternative to exhaustive paternity analysis in numerous plant populations. In this investigation, the method is extended to an annual wild maize species, Zea mays ssp. parviglumis (Poaceae). Our analysis of mating system in parviglumis revealed high levels of outcrossing and higher biparental inbreeding than typically observed in grasses. Pollen dispersal analysis suggested low levels of long-distance dispersal. Given previous evidence for intrapopulation genetic structure in parviglumis populations, we explored the impact of cryptic population structure on estimates of mating system and pollen flow. Subpopulations inferred through spatially explicit Bayesian assignment showed markedly different values for both mating system parameters and pollen flow than the entire population. Finally, a novel method of pollen haplotype assignment revealed nonrandom mating consistent with intrapopulation structure. These results indicate parviglumis could be particularly susceptible to habitat fragmentation currently occurring throughout Mexico due to recent changes in land use.

Historical and contemporary gene dispersal in wild carrot (Daucus carota ssp. carota) populations

BACKGROUND AND AIMS

Wild carrot is the ancestor of cultivated carrot and is the most important gene pool for carrot breeding. Transgenic carrot may be released into the environment in the future. The aim of the present study was to determine how far a gene can disperse in wild carrot populations, facilitating risk assessment and management of transgene introgression from cultivated to wild carrots and helping to design sampling strategies for germplasm collections.

METHODS

Wild carrots were sampled from Meijendel and Alkmaar in The Netherlands and genotyped with 12 microsatellite markers. Spatial autocorrelation analyses were used to detect spatial genetic structures (SGSs). Historical gene dispersal estimates were based on an isolation by distance model. Mating system and contemporary pollen dispersal were estimated using 437 offspring of 20 mothers with different spatial distances and a correlated paternity analysis in the Meijendel population.

KEY RESULTS

Significant SGSs are found in both populations and they are not significantly different from each other. Combined SGS analysis indicated significant positive genetic correlations up to 27 m. Historical gene dispersal sigma(g) and neighbourhood size N(b) were estimated to be 4-12 m [95 % confidence interval (CI): 3-25] and 42-73 plants (95 % CI: 28-322) in Meijendel and 10-31 m (95 % CI: 7-infinity) and 57-198 plants (95 % CI: 28-infinity) in Alkmaar with longer gene dispersal in lower density populations. Contemporary pollen dispersal follows a fat-tailed exponential-power distribution, implying pollen of wild carrots could be dispersed by insects over long distance. The estimated outcrossing rate was 96 %.

CONCLUSIONS

SGSs in wild carrots may be the result of high outcrossing, restricted seed dispersal and long-distance pollen dispersal. High outcrossing and long-distance pollen dispersal suggest high frequency of transgene flow might occur from cultivated to wild carrots and that they could easily spread within and between populations.

Spatiotemporal mating pattern variation in a wind-pollinated Mediterranean shrub

Spatiotemporal variation in mating patterns is poorly known in wind-pollinated plant species. Here, we analysed mating patterns of the wind-pollinated dioecious shrub Pistacia lentiscus by genotyping 904 seeds from 30 mother plants with eight microsatellite markers in a high-density population in two consecutive flowering seasons. We found significant differences in some mating system estimates between years, particularly in the levels of correlated paternity. Overall, within-mothers correlated paternity was higher in 2007 than in 2006 (r(pWM) = 0.085 and 0.030), which translated into an effective number of fathers (N(ep)) of 11.8 and 33.6 respectively. Using a smoothing interpolation technique, we show that the effective pollen cloud was spatially structured in patches of high- and low-genetic diversity, which do not remain constant from year to year. In 2006, the among-mothers correlated paternity (r(pAM)) showed no trend with distance, suggesting no restriction of pollen dispersal. However, in 2007, r(pAM) was greater than zero at short distances, revealing the existence of small-scale patterns of pollen dispersal. The fact that the studied seasons were climatically homogeneous during the flowering time suggested that the observed differences might be ascribed to between-year phenological variation of individuals in the studied population or other (unknown) factors. Numerical simulations, based on the real data set, indicated that the clumping of males and decreasing plant density, which is related to different types of pollen limitation, greatly increase correlated mating in this wind-pollinated species, which is of relevance under the frame of the continuous anthropogenic habitat disturbance suffered by Mediterranean ecosystems.

Spatial genetic structure in continuous and fragmented populations of Pinus pinaster Aiton

Habitat fragmentation, i.e., the reduction of populations into small isolated remnants, is expected to increase spatial genetic structure (SGS) in plant populations through nonrandom mating, lower population densities and potential aggregation of reproductive individuals. We investigated the effects of population size reduction and genetic isolation on SGS in maritime pine (Pinus pinaster Aiton) using a combined experimental and simulation approach. Maritime pine is a wind-pollinated conifer which has a scattered distribution in the Iberian Peninsula as a result of forest fires and habitat fragmentation. Five highly polymorphic nuclear microsatellites were genotyped in a total of 394 individuals from two population pairs from the Iberian Peninsula, formed by one continuous and one fragmented population each. In agreement with predictions, SGS was significant and stronger in fragments (Sp = 0.020 and Sp = 0.026) than in continuous populations, where significant SGS was detected for one population only (Sp = 0.010). Simulations suggested that under fat-tailed dispersal, small population size is a stronger determinant of SGS than genetic isolation, while under normal dispersal, genetic isolation has a stronger effect. SGS was always stronger in real populations than in simulations, except if unrealistically narrow dispersal and/or high variance of reproductive success were modelled (even when accounting for potential overestimation of SGS in real populations as a result of short-distance sampling). This suggests that factors such as nonrandom mating or selection not considered in the simulations were additionally operating on SGS in Iberian maritime pine populations.

Comparison of contemporary mating patterns in continuous and fragmented Eucalyptus globulus native forests

While habitat fragmentation is a central issue in forest conservation studies in the face of broad-scale anthropogenic changes to the environment, its effects on contemporary mating patterns remain controversial. This is partly because of the inherent variation in mating patterns which may exist within species and the fact that few studies have replication at the landscape level. To study the effect of forest fragmentation on contemporary mating patterns, including effective pollen dispersal, we compared four native populations of the Australian forest tree, Eucalyptus globulus. We used six microsatellite markers to genotype 1289 open-pollinated offspring from paired fragmented and continuous populations on the island of Tasmania and in Victoria on mainland Australia. The mating patterns in the two continuous populations were similar, despite large differences in population density. In contrast, the two fragmented populations were variable and idiosyncratic in their mating patterns, particularly in their pollen dispersal kernels. The continuous populations showed relatively high outcrossing rates (86-89%) and low correlated paternity (0.03-0.06) compared with the fragmented populations (65-79% and 0.12-0.20 respectively). A greater proportion of trees contributed to reproduction in the fragmented (de/d>or= 0.5) compared with the continuous populations (de/d = 0.03-0.04). Despite significant inbreeding in the offspring of the fragmented populations, there was little evidence of loss of genetic diversity. It is argued that enhanced medium- and long-distance dispersal in fragmented landscapes may act to partly buffer the remnant populations from the negative effects of inbreeding and drift.

Joint estimation of migration rate and effective population size using the island model

Using the island model of population demography, I report that the demographic parameters migration rate and effective population size can be jointly estimated with equilibrium probabilities of identity in state calculated using a sample of genotypes collected at a single point in time from a single generation. The method, which uses moment-type estimators, applies to dioecious populations in which females and males have identical demography and monoecious populations with no selfing and requires that offspring genotypes are sampled following reproduction and prior to migration. I illustrate the estimation procedure using the infinite-island model with no mutation and the finite-island model with three kinds of mutation models. In the infinite-island model with no mutation, the estimators can be expressed as simple functions of estimates of the F-statistic parameters F(IT) and F(ST). In the finite-island model with mutation among k alleles, mutation rate, migration rate, and effective population size can be simultaneously estimated. The estimates of migration rate and effective population size are somewhat robust to violations in assumptions that may arise in empirical applications such as different kinds of mutation models and deviations from temporal equilibrium.

Assortative mating and differential male mating success in an ash hybrid zone population

BACKGROUND

The structure and evolution of hybrid zones depend mainly on the relative importance of dispersal and local adaptation, and on the strength of assortative mating. Here, we study the influence of dispersal, temporal isolation, variability in phenotypic traits and parasite attacks on the male mating success of two parental species and hybrids by real-time pollen flow analysis. We focus on a hybrid zone population between the two closely related ash species Fraxinus excelsior L. (common ash) and F. angustifolia Vahl (narrow-leaved ash), which is composed of individuals of the two species and several hybrid types. This population is structured by flowering time: the F. excelsior individuals flower later than the F. angustifolia individuals, and the hybrid types flower in-between. Hybrids are scattered throughout the population, suggesting favorable conditions for their local adaptation. We estimate jointly the best-fitting dispersal kernel, the differences in male fecundity due to variation in phenotypic traits and level of parasite attack, and the strength of assortative mating due to differences in flowering phenology. In addition, we assess the effect of accounting for genotyping error on these estimations.

RESULTS

We detected a very high pollen immigration rate and a fat-tailed dispersal kernel, counter-balanced by slight phenological assortative mating and short-distance pollen dispersal. Early intermediate flowering hybrids, which had the highest male mating success, showed optimal sex allocation and increased selfing rates. We detected asymmetry of gene flow, with early flowering trees participating more as pollen donors than late flowering trees.

CONCLUSION

This study provides striking evidence that long-distance gene flow alone is not sufficient to counter-act the effects of assortative mating and selfing. Phenological assortative mating and short-distance dispersal can create temporal and spatial structuring that appears to maintain this hybrid population. The asymmetry of gene flow, with higher fertility and increased selfing, can potentially confer a selective advantage to early flowering hybrids in the zone. In the event of climate change, hybridization may provide a means for F. angustifolia to further extend its range at the expense of F. excelsior.