Modelling and estimating pollen movement in oilseed rape (Brassica napus) at the landscape scale using genetic markers

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.

Spatially explicit ecological modeling improves empirical characterization of plant pathogen dispersal

Dispersal is a key ecological process, but it remains difficult to measure. By recording numbers of dispersed individuals at different distances from the source, one acquires a dispersal gradient. Dispersal gradients contain information on dispersal, but they are influenced by the spatial extent of the source. How can we separate the two contributions to extract knowledge about dispersal? One could use a small, point-like source for which a dispersal gradient represents a dispersal kernel, which quantifies the probability of an individual dispersal event from a source to a destination. However, the validity of this approximation cannot be established before conducting measurements. This represents a key challenge hindering progress in characterization of dispersal. To overcome it, we formulated a theory that incorporates the spatial extent of sources to estimate dispersal kernels from dispersal gradients. Using this theory, we re-analyzed published dispersal gradients for three major plant pathogens. We demonstrated that the three pathogens disperse over substantially shorter distances compared to conventional estimates. This method will allow the researchers to re-analyze a vast number of existing dispersal gradients to improve our knowledge about dispersal. The improved knowledge has potential to advance our understanding of species' range expansions and shifts, and inform management of weeds and diseases in crops.

Eco-Geography of Feral Cotton: A Missing Piece in the Puzzle of Gene Flow Dynamics Among Members of Gossypium hirsutum Primary Gene Pool

Mexico is the center of origin and genetic diversity of upland cotton (Gossypium hirsutum L.), the most important source of natural fiber in the world. Currently, wild and domesticated populations (including genetically modified [GM] varieties) occur in this country and gene flow among them has shaped the species’ genetic diversity and structure, setting a complex and challenging scenario for its conservation. Moreover, recent gene flow from GM cultivars to wild Mexican cotton populations has been reported since 2011. In situ conservation of G. hirsutum requires knowledge about the extent of its geographic distribution, both wild and domesticated, as well as the possible routes and mechanisms that contribute to gene flow between the members of the species wild-to-domesticated continuum (i.e., the primary gene pool). However, little is known about the distribution of feral populations that could facilitate gene flow by acting as bridges. In this study, we analyzed the potential distribution of feral cotton based on an ecological niche modeling approach and discussed its implications in the light of the distribution of wild and domesticated cotton. Then, we examined the processes that could be leading to the escape of seeds from the cultivated fields. Our results indicate that the climatic suitability of feral plants in the environmental and geographic space is broad and overlaps with areas of wild cotton habitat and crop fields, suggesting a region that could bridge cultivated cotton and its wild relatives by allowing gene flow between them. This study provides information for management efforts focused on the conservation of wild populations, native landraces, and non-GM domesticated cotton at its center of origin and genetic diversity.

Neighbor GWAS: incorporating neighbor genotypic identity into genome-wide association studies of field herbivory

An increasing number of field studies have shown that the phenotype of an individual plant depends not only on its genotype but also on those of neighboring plants; however, this fact is not taken into consideration in genome-wide association studies (GWAS). Based on the Ising model of ferromagnetism, we incorporated neighbor genotypic identity into a regression model, named "Neighbor GWAS". Our simulations showed that the effective range of neighbor effects could be estimated using an observed phenotype when the proportion of phenotypic variation explained (PVE) by neighbor effects peaked. The spatial scale of the first nearest neighbors gave the maximum power to detect the causal variants responsible for neighbor effects, unless their effective range was too broad. However, if the effective range of the neighbor effects was broad and minor allele frequencies were low, there was collinearity between the self and neighbor effects. To suppress the false positive detection of neighbor effects, the fixed effect and variance components involved in the neighbor effects should be tested in comparison with a standard GWAS model. We applied neighbor GWAS to field herbivory data from 199 accessions of Arabidopsis thaliana and found that neighbor effects explained 8% more of the PVE of the observed damage than standard GWAS. The neighbor GWAS method provides a novel tool that could facilitate the analysis of complex traits in spatially structured environments and is available as an R package at CRAN ( https://cran.rproject.org/package=rNeighborGWAS ).

A new isolation device for shortening gene flow distance in small-scale transgenic maize breeding

The transmission of pollen is the main cause of maize gene flow. Under the compulsory labeling system for genetically modified (GM) products in China, isolation measures are crucial. At present, there is no effective isolation device for preventing and controlling the short-range flow of GM maize pollen. The purposes of the present experiments were to overcome the deficiencies of existing technology and to demonstrate a new isolation device for decreasing the gene flow distance of GM maize. The isolation device we invented was shown to be more robust than traditional isolation methods, and it can be disassembled and repeatedly reused. The most important point was that the frequency of gene flow could be greatly reduced using this device. When the distance from the isolation device was more than 1 m, the gene flow rate could be decreased to less than 1%, and when the distance from the isolation device was more than 10 m, the gene flow rate could be reduced to less than 0.1%. When the isolation device was adopted to isolate GM maize in conjunction with bagging the tassels of GM maize at the pollination stage, the gene flow could be controlled to less than 0.1% when the distance from the isolation device was more than 1 m. This device was, however, only applicable for small plots and can shorten the isolation distance of GM maize planting and improve the purity of seeds, all while meeting the needs of close isolation breeding. The use of this device represents a feasible method for risk prevention and control of GM crops.

Dispersal Kernels may be Scalable: Implications from a Plant Pathogen

AIM

Understanding how spatial scale of study affects observed dispersal patterns can provide insights into spatiotemporal population dynamics, particularly in systems with significant long-distance dispersal (LDD). We aimed to investigate the dispersal gradients of two rusts of wheat with spores of similar size, mass, and shape, over multiple spatial scales. We hypothesized that a single dispersal kernel could fit the dispersal from all spatial scales well, and that it would be possible to obtain similar results in spatiotemporal increase of disease when modeling based on differing scales.

LOCATION

Central Oregon and St. Croix Island.

TAXA

Puccinia striiformis f. sp. tritici, Puccinia graminis f. sp. tritici, Triticum aestivum.

METHODS

We compared empirically-derived primary disease gradients of cereal rust across three spatial scales: local (inoculum source and sampling unit = 0.0254 m, spatial extent = 1.52m) field-wide (inoculum source = 1.52 m, sampling unit = 0.305 m, and spatial extent = 91.44 m), and regional (inoculum source and sampling unit = 152 m, spatial extent = 10.7 km). We then examined whether disease spread in spatially explicit simulations depended upon the scale at which data were collected by constructing a compartmental time-step model.

RESULTS

The three data sets could be fit well by a single inverse-power law dispersal kernel. Simulating epidemic spread at different spatial resolutions resulted in similar patterns of spatiotemporal spread. Dispersal kernel data obtained at one spatial scale can be used to represent spatiotemporal disease spread at a larger spatial scale.

MAIN CONCLUSIONS

Organisms spread by aerially dispersed small propagules that exhibit LDD may follow similar dispersal patterns over a several hundred- or thousand-fold expanse of spatial scale. Given that the primary mechanisms driving aerial dispersal remain constant, it may be possible to extrapolate across scales when empirical data are unavailable at a scale of interest.

Tracking the invasion: dispersal of Hymenoscyphus fraxineus airborne inoculum at different scales

Ash dieback is caused by an invasive pathogen Hymenoscyphus fraxineus, which emerged in Europe in the 1990s and jeopardizes the management of ash stands. Although the biological cycle of the pathogen is well understood, its dispersal patterns via airborne spores remain poorly described. We investigated the seasonal and spatial patterns of dispersal in France using both a passive spore-trapping method coupled with a real-time PCR assay and reports of ash dieback based on symptom observations. Spores detection varies from year to year with a detection ability of 30-47% depending on meteorological conditions, which affect both production of inoculum and efficiency of the trapping. Nevertheless, our results are consistent and we showed that the sporulation peak occurred from June to August and that spores were detected up to 50-100 km ahead of the disease front, proving the presence of the pathogen before any observation of symptoms. The spore dispersal gradient was steep, most of inoculum remaining within 50 m of infected ashes. Two dispersal kernels were fitted using Bayesian methods to estimate the mean dispersal distance of H. fraxineus from inoculum sources. The estimated mean distances of dispersal, either local or regional scale, were 1.4 km and 2.6 km, respectively, the best fitting kernel being the inverse power-law. This information may help to design disease management strategies.

Defensive chemicals of neighboring plants limit visits of herbivorous insects: Associational resistance within a plant population

Despite our understanding of chemical defenses and their consequences for plant performance and herbivores, we know little about whether defensive chemicals in plant tissues, such as alkaloids, and their spatial variation within a population play unappreciated and critical roles in plant-herbivore interactions. Neighboring plants can decrease or increase attractiveness of a plant to herbivores, an example of a neighborhood effect. Chemical defensive traits may contribute to neighborhood effects in plant-herbivore interactions. We examined the effects of nicotine in leaves (a non-emitted defense chemical) on plant-herbivore interactions in a spatial context, using two varieties of Nicotiana tabacum with different nicotine levels. A common garden experiment demonstrated that visits by grasshoppers decreased with increasing density of neighboring plants with a greater nicotine level. In contrast, visits of leaf caterpillars were not affected by neighbors, irrespective of nicotine levels. Thus, our results clearly highlighted that the neighborhood effect caused by the nicotine in leaves depended on the insect identity, and it was mediated by plant-herbivore interactions, rather than plant-plant interactions. This study demonstrates that understanding of effects of plant defensive traits on plant-herbivore interactions requires careful consideration of the spatial distribution of plant defenses, and provides support for the importance of spatial context to accurately capture the ecological and evolutionary consequences of plant-herbivore interactions.

Evaluation of maximum potential gene flow from herbicide resistant Brassica napus to its male sterile relatives under open and wind pollination conditions

Pollen-mediated gene flow (PMGF) from genetically modified (GM) Brassica napus to its wild relatives by wind and insects is a major ecological concern in agricultural ecosystems. This study conducted is to estimate maximum potential gene flow and differentiate between wind- and bee-mediated gene flows from herbicide resistant (HR) B. napus to its closely-related male sterile (MS) relatives, B. napus, B. juncea and Raphanus sativus. Various markers, including pods formation in MS plants, herbicide resistance, and SSR markers, were used to identify the hybrids. Our results revealed the following: 1) maximum potential gene flow (a maximum % of the progeny of pollen recipient confirmed hybrid) to MS B. napus ranged from 32.48 to 0.30% and from 14.69 to 0.26% at 2-128 m from HR B. napus under open and wind pollination conditions, respectively, and to MS B. juncea ranged from 21.95 to 0.24% and from 6.16 to 0.16%, respectively; 2) estimates of honeybee-mediated gene flow decreased with increasing distance from HR B. napus and ranged from 17.78 to 0.03% at 2-128 m for MS B. napus and from 15.33 to 0.08% for MS B. juncea; 3) a small-scale donor plots would strongly favour insect over wind pollination; 4) no gene flow occurred from HR B. napus to MS R. sativus. Our approach and findings are helpful in understanding the relative contribution of wind and bees to gene flow and useful for estimating maximum potential gene flow and managing environmental risks associated with gene flow.

Analysis of population genetic structure and gene flow in an annual plant before and after a rapid evolutionary response to drought

The impact of environmental change on population structure is not well understood. This study aimed to examine the effect of a climate change event on gene flow over space and time in two populations of Brassica rapa that evolved more synchronous flowering times over 5 years of drought in southern California. Using plants grown from seeds collected before and after the drought, we estimated genetic parameters within and between populations and across generations. We expected that with greater temporal opportunity to cross-pollinate, due to reduced phenological isolation, these populations would exhibit an increase in gene flow following the drought. We found low but significant FST, but no change in FST or Nm across the drought, in contrast to predictions. Bayesian analysis of these data indicates minor differentiation between the two populations but no noticeable change in structure before and after the shift in flowering times. However, we found high and significant levels of FIS, indicating that inbreeding likely occurred in these populations despite self-incompatibility in B. rapa. In this system, we did not find an impact of climate change on gene flow or population structuring. The contribution of gene flow to adaptive evolution may vary by system, however, and is thus an important parameter to consider in further studies of natural responses to environmental change.

Evaluating contemporary pollen dispersal in two common grassland species Ranunculus bulbosus L. (Ranunculaceae) and Trifolium montanum L. (Fabaceae) using an experimental approach

Pollen flow is a key biological process that connects plant populations, preventing genetic impoverishment and inbreeding. Pollen-mediated long-distance dispersal (LDD) events are especially important for plant species in increasingly fragmented landscapes. Patterns of pollen dispersal were directly estimated and dispersal kernels modelled in an experimental population of Ranunculus bulbosus and Trifolium montanum to determine the potential for LDD. Eight and 11 microsatellite markers were used for R. bulbosus and T. montanum, respectively, to run a likelihood-based paternity analysis on randomly chosen offspring (Ntotal  = 180 per species) from five maternal plants. High rates of selfing were found in R. bulbosus (average 45.7%), while no selfing was observed in T. montanum. The majority (60%) of mating events occurred at very short distances: the median of the observed dispersal distances was 0.8 m in both species, and the average distances were 15.9 and 10.3 m in R. bulbosus and T. montanum, respectively. Modelling the pollen dispersal kernel with four different distribution functions (exponential-power, geometric, 2Dt and Weibull) indicated that the best fit for both species was given by a Weibull function. Yet, the tail of the T. montanum pollen dispersal kernel was thinner than in R. bulbosus, suggesting that the probability for LDD is higher in the latter species. Even though the majority of pollen dispersal occurred across short distances, the detection of several mating events up to 362 m (R. bulbosus) and 324 m (T. montanum) suggests that pollen flow may be sufficient to ensure population connectivity in these herb species across fragmented grasslands in Swiss agricultural landscapes.

Feral genetically modified herbicide tolerant oilseed rape from seed import spills: are concerns scientifically justified?

One of the concerns surrounding the import (for food and feed uses or processing) of genetically modified herbicide tolerant (GMHT) oilseed rape is that, through seed spillage, the herbicide tolerance (HT) trait will escape into agricultural or semi-natural habitats, causing environmental or economic problems. Based on these concerns, three EU countries have invoked national safeguard clauses to ban the marketing of specific GMHT oilseed rape events on their territory. However, the scientific basis for the environmental and economic concerns posed by feral GMHT oilseed rape resulting from seed import spills is debatable. While oilseed rape has characteristics such as secondary dormancy and small seed size that enable it to persist and be redistributed in the landscape, the presence of ferals is not in itself an environmental or economic problem. Crucially, feral oilseed rape has not become invasive outside cultivated and ruderal habitats, and HT traits are not likely to result in increased invasiveness. Feral GMHT oilseed rape has the potential to introduce HT traits to volunteer weeds in agricultural fields, but would only be amplified if the herbicides to which HT volunteers are tolerant were used routinely in the field. However, this worst-case scenario is most unlikely, as seed import spills are mostly confined to port areas. Economic concerns revolve around the potential for feral GMHT oilseed rape to contribute to GM admixtures in non-GM crops. Since feral plants derived from cultivation (as distinct from import) occur at too low a frequency to affect the coexistence threshold of 0.9% in the EU, it can be concluded that feral GMHT plants resulting from seed import spills will have little relevance as a potential source of pollen or seed for GM admixture. This paper concludes that feral oilseed rape in Europe should not be routinely managed, and certainly not in semi-natural habitats, as the benefits of such action would not outweigh the negative effects of management.

A flexible quantitative methodology for the analysis of gene-flow between conventionally bred maize populations using microsatellite markers

Previous studies of gene-flow in agriculture have used a range of physical and biochemical markers, including transgenes. However, physical and biochemical markers are not available for all commercial varieties, and transgenes are difficult to use when trying to estimate gene flow in the field where the use of transgenes is often restricted. Here, we demonstrate the use of simple sequence repeat microsatellite markers (SSRs) to study gene flow in maize. Developing the first quantitative analysis of pooled SSR samples resulted in a high sampling efficiency which minimised the use of resources and greatly enhanced the possibility of hybrid detection. We were able to quantitatively distinguish hybrids in pools of ten samples from non-hybrid parental lines in all of the 24 pair-wise combinations of commercial varieties tested. The technique was used to determine gene flow in field studies, from which a simple model describing gene flow in maize was developed.

Astylus atromaculatus (Coleoptera: Melyridae): abundance and role in pollen dispersal in Bt and non-Bt cotton in South Africa

In South Africa, modified Bt (Cry1 Ac) cotton cultivars and organic ones coexist. This raises the question of the risk of dissemination of genetically modified (GM) pollen to non-GM crops by visiting insects. We inventoried the flower-visiting insects in Bt and non-Bt cotton fields of the South African Highveld region and investigated their role in pollen dispersal. Their diversity and abundance varied slightly among sites, with Astylus atromaculatus as the predominant insect on both Bt and non-Bt cotton flowers. The other major flower-visiting species were Apis mellifera and solitary Apidae. No differences were found in the abundance of each taxum between Bt and non-Bt cotton except for Scoliidae and Nitidulidae, which were scarce overall (<0.5%) but more abundant on the non-Bt flowers in the central area of the field at one site. The pollen load on A. atromaculatus was as high as on Apis mellifera. Cage tests showed that A. atromaculatus can pollinate female cotton plants by transferring pollen from male donor plants. In the field, the flight range of this insect was generally short (25 m), but it can occasionally reach up to 200 m or even more. This study therefore highlights that A. atromaculatus, commonly regarded as a pest, could be an unexpected but efficient pollinator. Because its population density can be high, this species could mediate unwanted cotton pollen flow when distances between coexiting fields are not sufficient.

Using linked markers to estimate the genetic age of a volunteer population: a theoretical and empirical approach

Volunteers deriving from unharvested seeds of a crop can lead to persistent feral populations and participate in genetic exchanges across the agro-ecosystem, both between crop varieties and between crops and their wild relatives. A first step to understand the importance of volunteers is to characterize their capacity to reproduce autonomously for several generations. For that purpose, we constructed and evaluated a maximum-likelihood method to estimate the genetic age of a population deriving from one of the most common field crop type: an F1-hybrid variety. The method estimates the number of reproduction cycles that occurred since the cultivation of that variety. It makes use of genotypic data at a number of linked microsatellite loci pairs, thus exploiting the recombination of parental haplotypes, which is expected to occur as the population is reproducing. Estimates with moderate bias and variance were found for a broad range of parameter values in simulations, and the method revealed robust to some deviations from the assumptions of the underlying model. We propose a specific procedure to test the hypothesis of persistence, that is has a given volunteer population experienced more than one cycle of reproduction since the F1-hybrid state? The method was applied to both an experimental and a natural sunflower volunteer population and revealed promising, considering these ideal case studies. Possible further developments toward more complex natural systems are discussed.

How to model and simulate the effects of cropping systems on population dynamics and gene flow at the landscape level: example of oilseed rape volunteers and their role for co-existence of GM and non-GM crops

BACKGROUND, AIM AND SCOPE

Agricultural landscapes comprise cultivated fields and semi-natural areas. Biological components of these compartments such as weeds, insect pests and pathogenic fungi can disperse sometimes over very large distances, colonise new habitats via insect flight, spores, pollen or seeds and are responsible for losses in crop yield (e.g. weeds, pathogens) and biodiversity (e.g. invasive weeds). The spatiotemporal dynamics of these biological components interact with crop locations, successions and management as well as the location and management of semi-natural areas such as roadverges. The objective of this investigation was to establish a modelling and simulation methodology for describing, analysing and predicting spatiotemporal dynamics and genetics of biological components of agricultural landscapes. The ultimate aim of the models was to evaluate and propose innovative cropping systems adapted to particular agricultural concerns. The method was applied to oilseed rape (OSR) volunteers playing a key role for the coexistence of genetically modified (GM) and non-GM oilseed rape crops, where the adventitious presence of GM seeds in non-GM harvests (AGMP) could result in financial losses for farmers and cooperatives.

MATERIAL AND METHODS

A multi-year, spatially explicit model was built, using field patterns, climate, cropping systems and OSR varieties as input variables, focusing on processes and cultivation techniques crucial for plant densities and pollen flow. The sensitivity of the model to input variables was analysed to identify the major cropping factors. These should be modified first when searching for solutions limiting gene flow. The sensitivity to model processes and species life-traits were analysed to facilitate the future adaptation of the model to other species. The model was evaluated by comparing its simulations to independent field observations to determine its domain of validity and prediction error.

RESULTS

The cropping system study determined contrasted farm types, simulated the current situation and tested a large range of modifications compatible with each farm to identify solutions for reducing the AGMP. The landscape study simulated gene flow in a large number of actual and virtual field patterns, four combinations of regional OSR and GM proportions and three contrasted cropping systems. The analysis of the AGMP rate at the landscape level determined a maximum acceptable GM OSR area for the different cropping systems, depending on the regional OSR volunteer infestation. The analysis at the field level determined minimum distances between GM and non-GM crops, again for different cropping systems and volunteer infestations.

DISCUSSION

The main challenge in building spatially explicit models of the effects of cropping systems and landscape patterns on species dynamics and gene flow is to determine the spatial extent, the time scale, the major processes and the degree of mechanistic description to include in the model, depending on the species characteristics and the model objective.

CONCLUSIONS

These models can be used to study the effects of cropping systems and landscape patterns over a large range of situations. The interactions between the two aspects make it impossible to extrapolate conclusions from individual studies to other cases. The advantage of the present method was to produce conclusions for several contrasted farm types and to establish recommendations valid for a large range of situations by testing numerous landscapes with contrasted cropping systems. Depending on the level of investigation (region or field), these recommendations concern different decision-makers, either farmers and technical advisors or cooperatives and public decision-makers.

RECOMMENDATIONS AND PERSPECTIVES

The present simulation study showed that gene flow between coexisting GM and non-GM varieties is inevitable. The management of OSR volunteers is crucial for containing gene flow, and the cropping system study identified solutions for reducing these volunteers and ferals in and outside fields. Only if these are controlled can additional measures such as isolation distances between GM and non-GM crops or limiting the proportion of the region grown with GM OSR be efficient. In addition, particular OSR varieties contribute to limit gene flow. The technical, organisational and financial feasibility of the proposed measures remains to be evaluated by a multi-disciplinary team.

Climate change alters reproductive isolation and potential gene flow in an annual plant

Climate change will likely cause evolution due not only to selection but also to changes in reproductive isolation within and among populations. We examined the effects of a natural drought on the timing of flowering in two populations of Brassica rapa and the consequences for predicted reproductive isolation and potential gene flow. Seeds were collected before and after a 5-year drought in southern California from two populations varying in soil moisture. Lines derived from these seeds were raised in the greenhouse under wet and drought conditions. We found that the natural drought caused changes in reproductive timing and that the changes were greater for plants from the wet than from the dry site. This differential shift caused the populations to become more phenological similar, which should lead to less reproductive isolation and increased gene flow. We estimated a high level of assortative mating by flowering time, which potentially contributed to the rapid evolution of phenological traits following the drought. Estimates of assortative mating were higher for the wet site population, and assortative mating was reduced following the drought. This study shows that climate change can potentially alter gene flow and reproductive isolation within and among populations, strongly influencing evolution.

Pollen dispersal in sugar beet production fields

Pollen-mediated gene flow has important implications for biodiversity conservation and for breeders and farmers' activities. In sugar beet production fields, a few sugar beet bolters can produce pollen as well as be fertilized by wild and weed beet. Since the crop, the wild beets, and the weed beets are the same species and intercross freely, the question of pollen flow is an important issue to determine the potential dispersal of transgenes from field to field and to wild habitats. We report here an experiment to describe pollen dispersal from a small herbicide-resistant sugar beet source towards male sterile target plants located along radiating lines up to 1,200 m away. Individual dispersal functions were inferred from statistical analyses and compared. Pollen limitation, as expected in root-production fields, was confirmed at all the distances from the pollen source. The number of resistant seeds produced by bait plants best fitted a fat-tailed probability distribution curve of pollen grains (power-law) dependent on the distance from the pollen source. A literature survey confirmed that power-law function could fit in most cases. The b coefficient was lower than 2. The number of fertilized flowers by background (herbicide-susceptible) pollen grains was uniform across the whole field. Airborne pollen had a fertilization impact equivalent to that of one adjacent bolter. The individual dispersal function from different pollen sources can be integrated to provide the pollen cloud composition for a given target plant, thus allowing modeling of gene flow in a field, inter-fields in a small region, and also in seed-production area. Long-distance pollen flow is not negligible and could play an important role in rapid transgene dispersal from crop to wild and weed beets in the landscape. The removing of any bolting, herbicide-resistant sugar beet should be compulsory to prevent the occurrence of herbicide-resistant weed beet, thus preventing gene flow to wild populations and preserving the sustainable utility of the resistant varieties. Whether such a goal is attainable remains an open question and certainly would be worth a large scale experimental study.

Determining the viability response of pine pollen to atmospheric conditions during long-distance dispersal

Pollen of forest trees can move on the scales of tens to hundreds of kilometers, but the question of its viability during this long distance dispersal (LDD) has yet to be answered. While empirical studies of pollen viability in forest tree species are rare, controlled and scalable data to outdoor studies of the contribution of UV irradiation on pollen viability are not available. A simple protocol that allows the quantification of the viability response of pollen to UV, temperature, and humidity is developed and described here. Bench-scale conditions that approximate a wide range of atmospheric conditions including different humidity, temperature, and UV irradiation condition are used to determine the independent effects of each abiotic stress factor, and empirical functions are fitted and used to scale these bench-scale experiments to outdoor conditions. As a case study, pollen was sampled from two populations of Pinus taeda during two years and was used to quantify the decrease in viability due to atmospheric conditions during LDD. Contrary to maize pollen, P. taeda pollen viability decreased due to humid and cold conditions. The viability response of pollen to UV-A and UV-B corresponded to a viability reduction of about 10% after a full day of exposure. These laboratory findings were corroborated by an outdoor solar exposure experiment. The Fu-Liou online radiation model and a data set of radiosonde observations were used to estimate the typical conditions that would be encountered by LDD pollen. If initially caught in a strong updraft, dispersing P. taeda pollen could be carried many days and thousands of kilometers in the air. The empirical equations for P. taeda pollen viability reduction due to abiotic stresses predicted that 50% of the pollen would survive 24 hours of LDD under typical external conditions. The viable range of the pollen is, therefore, shorter than the physical dispersal distance. The methods used in our experiments are applicable for determination of dispersing pollen viability, especially when effects of different adverse conditions need to be separated. The empirical viability equations that resulted from our experiments can be used in an atmospheric dispersal model to estimate the viable range of tree pollen.

Quantifying the introgressive hybridisation propensity between transgenic oilseed rape and its wild/weedy relatives

In order to estimate the introgressive hybridisation propensity (IHP) between genetically modified (GM) oilseed rape (Brassica napus) and certain of its cross-compatible wild/weedy relatives at the landscape level, a conceptual approach was developed. A gene flow index was established enclosing the successive steps to successfully achieve introgressive hybridisation: wild/weedy relatives and oilseed rape should co-occur, have overlapping flowering periods, be compatible, produce viable and fertile progeny, and the transgenes should persist in natural/weedy populations. Each step was described and scored, resulting in an IHP value for each cross-compatible oilseed rape wild/weedy relative. The gene flow index revealed that Brassica rapa has the highest introgressive hybridisation propensity (IHP value = 11.5), followed by Hirschfeldia incana and Raphanus raphanistrum (IHP = 6.7), Brassica juncea (IHP = 5.1), Diplotaxis tenuifolia and Sinapis arvensis (IHP = 4.5) in Flanders. Based on the IHP values, monitoring priorities can be defined within the pool of cross-compatible wild/weedy oilseed rape relatives. Moreover, the developed approach enables to select areas where case-specific monitoring of GM oilseed rape could be done in order to detect potential adverse effects on cross-compatible wild/weedy relatives resulting from vertical gene flow. The implementation of the proposed oilseed rape-wild relative gene flow index revealed that the survey design of existing botanical survey networks does not suit general surveillance needs of GM crops in Belgium. The encountered hurdles to implement the gene flow index and proposals to acquire the missing data are discussed.

Long distance pollen-mediated gene flow at a landscape level: the weed beet as a case study

Gene flow is a crucial parameter that can affect the organization of genetic diversity in plant species. It has important implications in terms of conservation of genetic resources and of gene exchanges between crop to wild relatives and within crop species complex. In the Beta vulgaris complex, hybridization between crop and wild beets in seed production areas is well documented and the role of the ensuing hybrids, weed beets, as bridges towards wild forms in sugar beet production areas have been shown. Indeed, in contrast to cultivated beets that are bi-annual, weed beets can bolt, flower and reproduce in the same crop season. Nonetheless, the extent of pollen gene dispersal through weedy lineages remains unknown. In this study, the focus is directed towards weed-to-weed gene flow, and we report the results of a pollen-dispersal analysis within an agricultural landscape composed of five sugar beet fields with different levels of infestation by weed beets. Our results, based on paternity analysis of 3240 progenies from 135 maternal plants using 10 microsatellite loci, clearly demonstrate that even if weedy plants are mostly pollinated by individuals from the same field, some mating events occur between weed beets situated several kilometres apart (up to 9.6 km), with rates of interfield-detected paternities ranging from 11.3% to 17.5%. Moreover, we show that pollen flow appears to be more restricted when individuals are aggregated as most mating events occurred only for short-distance classes. The best-fit dispersal curves were fat-tailed geometric functions for populations exhibiting low densities of weed beets and thin-tailed Weibull function for fields with weed beet high densities. Thus, weed beet populations characterized by low density with geographically isolated individuals may be difficult to detect but are likely to act as pollen traps for pollen emitted by close and remote fields. Hence, it appears evident that interfield pollen-mediated gene flow between weed beets is almost unavoidable and could contribute to the diffusion of (trans)genes in the agricultural landscape.

Dispersal ecology: where have all the seeds gone?

How effective are different animals at dispersing seeds? A new study has traced seeds sampled in faeces to their mother of origin and concluded that carnivorous mammals can be better dispersers than birds.