Pollen Vectors Research Articles

Mating strategies and pollen limitation in a globally threatened perennial Polemonium vanbruntiae

Knowledge of the factors that limit reproduction is critical to an understanding of plant ecology, and is particularly important for predicting population viability for threatened species. Here, we investigated the pollination biology of a globally threatened plant, Polemonium vanbruntiae, using hand-pollination experiments in four natural populations to determine the degree of pollen limitation. In addition, we investigated the mating system and extent to which plants can self-fertilize by comparing geitonogamously and autonomously self-fertilized plants with purely outcrossed and open-pollinated plants. In contrast to several of the more common species of Polemonium, we found no pollen limitation in any of the four populations of P. vanbruntiae over two years. The lack of pollen limitation was best explained by the capacity for P. vanbruntiae to both geitonogamously and autonomously self-fertilize, unlike some of its more common congeners. Geitonogamously selfed flowers set equivalent numbers of seeds when compared to purely outcrossed and open-pollinated flowers. However, autonomously selfed flowers produced significantly fewer seeds, demonstrating that pollinators play an important role as inter- and intra-plant pollen vectors in this system. Our results support the reproductive assurance hypothesis, whereby the ability to self assures fertilization for plants in small populations. Self-compatibility in Polemonium vanbruntiae may decrease extinction risk of isolated populations experiencing a stochastic pollinator pool within a restricted geographic range. In addition, a mixedmating strategy, including the ability for clonal reproduction, may explain the ability for this rare species to persist in small, fragmented populations.

THE GENETIC ARCHITECTURE OF REPRODUCTIVE ISOLATION IN LOUISIANA IRISES: POLLINATION SYNDROMES AND POLLINATOR PREFERENCES

In animal-pollinated plants, pollinator preferences for divergent floral forms can lead to partial reproductive isolation. We describe regions of plant genomes that affect pollinator preferences for two species of Louisiana Irises, Iris brevicaulis and Iris fulva, and their artificial hybrids. Iris brevicaulis and I. fulva possess bee and bird-pollination syndromes, respectively. Hummingbirds preferred I. fulva and under-visited both I. brevicaulis and backcrosses toward this species. Lepidopterans preferred I. fulva and backcrosses toward I. fulva, but also under-visited I. brevicaulis and I. brevicaulis backcrosses. Bumblebees preferred I. brevicaulis and F1 hybrids and rarely visited I. fulva. Although all three pollen vectors preferred one or the other species, these preferences did not prevent visitation to other hybrid/parental classes. Quantitative trait locus (QTL) mapping, in reciprocal BC1 mapping populations, defined the genetic architecture of loci that affected pollinator behavior. We detected six and nine QTLs that affected pollinator visitation rates in the BCIb and BCIf mapping populations, respectively, with as many as three QTLs detected for each trait. Overall, this study reflects the possible role of quantitative genetic factors in determining (1) reproductive isolation, (2) the pattern of pollinator-mediated genetic exchange, and thus (3) hybrid zone evolution.

High paternal diversity in the self‐incompatible herbArabidopsis halleridespite clonal reproduction and spatially restricted pollen dispersal

The number of sires fertilizing a given dam is a key parameter of the mating system in species with spatially restricted offspring dispersal, since genetic relatedness among maternal sibs determines the intensity of sib competition. In flowering plants, the extent of multiple paternity is determined by factors such as floral biology, properties of the pollen vector, selfing rate, spatial organization of the population, and genetic compatibility between neighbours. To assess the extent of multiple paternity and identify ecological factors involved, we performed a detailed study of mating patterns in a small population of a self-incompatible clonal herb, Arabidopsis halleri. We mapped and genotyped 364 individuals and 256 of their offspring at 12 microsatellite loci and jointly analysed the level of multiple paternity, pollen and seed dispersal, and spatial genetic structure. We found very low levels of correlated paternity among sibs (P(full-sib) = 3.8%) indicating high multiple paternity. Our estimate of the outcrossing rate was 98.7%, suggesting functional self-incompatibility. The pollen dispersal distribution was significantly restricted (mean effective pollen dispersal distance: 4.42 m) but long-distance successful pollination occurred and immigrating pollen was at most 10% of all pollination events. Patterns of genetic structure indicated little extent of clonal reproduction, and a low but significant spatial genetic structure typical for a self-incompatible species. Overall, in spite of restricted pollen dispersal, the multiple paternity in this self-incompatible species was very high, a result that we interpret as a consequence of high plant density and high pollinator service in this population.

Proximity to forest edge does not affect crop production despite pollen limitation

A decline in pollination function has been linked to agriculture expansion and intensification. In northwest Argentina, pollinator visits to grapefruit, a self-compatible but pollinator-dependent crop, decline by approximately 50% at 1km from forest edges. We evaluated whether this decrease in visitation also reduces the pollination service in this crop. We analysed the quantity and quality of pollen deposited on stigmas, and associated limitation of fruit production at increasing distances (edge: 10, 100, 500 and 1000m) from the remnants of Yungas forest. We also examined the quantitative and qualitative efficiency of honeybees as pollen vectors. Pollen receipt and pollen tubes in styles decreased with increasing distance from forest edge; however, this decline did not affect fruit production. Supplementation of natural pollen with self- and cross-pollen revealed that both pollen quantity and quality limited fruit production. Despite pollen limitation, honeybees cannot raise fruit production because they often do not deposit sufficient high-quality pollen per visit to elicit fruit development. However, declines in visitation frequency well below seven visits during a flower's lifespan could decrease production beyond current yields. In this context, the preservation of forest remnants, which act as pollinator sources, could contribute to resilience in crop production. Like wild plants, pollen limitation of the yield among animal-pollinated crops may be common and indicative not only of pollinator scarcity, but also of poor pollination quality, whereby pollinator efficiency, rather than just abundance, can play a broader role than previously appreciated.

The effect on Citrus fruit of excluding pollinating insects at flowering and implications for breeding new seedless cultivars

SummaryExperiments were conducted with orchard and container-grown trees of 33 Citrus cultivars in which pollinator insects were prevented from visiting flowers. Seventeen cultivars set fruit under orchard conditions when insects were excluded from flowers by mesh bags, while 16 others failed to develop fruit. Of the cultivars that set fruit in bagged limbs, four gave seedless and 13 gave seeded fruit. In addition, two pollen-sterile hybrids of ‘Imperial’ mandarin ‘Hamlin’ sweet orange set seedless fruit when insects were excluded at flowering, but developed seeded fruit in open-pollinated situations, or when controlled cross-pollinations were made. One of these hybrids was used to demonstrate that wind was not a pollen vector. Glasshouse experiments confirmed the results obtained for some of the cultivars tested in the orchard. Variation in anther position relative to the stigma among five cultivars suggested that self- or auto-pollination can occur without vector intervention. ‘Ellendale’ tangor, as well as ‘Imperial’ and ‘Kara’ mandarins were shown to be capable of autonomic, parthenocarpic fruit development. The results are discussed in terms of screening for parthenocarpy when breeding new seedless Citrus cultivars.

The impact of pollination syndrome and habitat on gene flow: a comparative study of two Streptocarpus (Gesneriaceae) species

Gene flow through pollen and seed dispersal is important in terms of population differentiation and eventually speciation. Seed and pollen flow are affected in turn by habitats and pollen vectors. We examined the effect of different pollinators and habitats on gene flow by comparing two species of Streptocarpus, using microsatellite and chloroplast RFLP markers. Populations of the forest-dwelling S. primulifolius were highly differentiated according to nuclear microsatellite data and had mutually exclusive chloroplast haplotypes. This result is congruent with infrequent seed dispersal and limited between-population foraging by the long-tongued fly pollinator Stenobasipteron wiedemanni. In contrast, populations of S. dunnii growing in exposed crags had lower levels of population differentiation according to both nuclear and chloroplast data, congruent with a hypothesis of more effective between population seed dispersal and greater pollen-mediated gene flow due to the sunbird pollinator Nectarinia famosa. The population genetic behavior of these species is reflected in their taxonomy and phylogenetic position; S. primulifolius belongs to a taxonomically complex clade in which recent speciation is evident, while the clade containing S. dunnii is characterized by taxonomically well-defined species on longer phylogenetic branches. Our study shows that pollinator movements and seed dispersal patterns are a major determinant of the evolutionary trajectories of these species.

Effects of Self‐Compatibility on the Distribution Range of Invasive European Plants in North America

Many plant species have been introduced to new continents, but only a small subset of these have become invasive. It has been predicted that self-compatible species, particularly those that do not need the services of pollinators, are more likely to establish and spread after long-distance dispersal. We tested whether this hypothesis, commonly called Baker's law, applies to 361 species that have invaded the United States from Europe. Species capable of autonomous seed production occurred in significantly more states than species requiring a pollen vector. Moreover, of the species that are not capable of autonomous seed production, self-compatible species occurred in significantly more states than those that are not self-compatible. The positive effect of autonomous seed production on the range of invasion was larger for abiotically pollinated species than for biotically pollinated species and for monocarpic species than for polycarpic species. These results support Baker's law, and we recommend that screening protocols for predicting invasiveness of species considered for introduction should include assessment of their breeding system.

Metapopulation pollination in the deceptive orchidAnacamptis pyramidalis

Pollen dispersal between local plant populations within a range of 6 km in a geographically defined metapopulation of the lepidopteran‐pollinated deceptive orchidAnacamptis pyramidaliswas studied on the island Öland in the Baltic Sea. LocalA. pyramidalispopulations were examined for pollinators, flowering individuals, and fruit set. Population sizes of pollen vector species were estimated using a mark–release–recapture technique. As pollen vectors, the burnet mothZygaena minosand the butterflyAporia crataegidominated. 205 out of 745 marked lepidopterans were pollinarium carriers. The proportion carriers of the total was considerably higher inZ. minos(50.3%), than inA. crataegi(21.5%) and nymphalidae (8.2%). Furthermore,Z. minosmoved much shorter distances thanA. crataegidid, while no difference in potential pollen dispersal distances were found between males and females. The number of individual vectors recaptured in another local population ofA. pyramidaliswas low:A. crataegi(8) andZ. minos(1). The ratio of pollinaria transferred to another localA. pyramidalispopulation compared to pollinaria remaining within the same local population was 1:41. This study highlights that pollen dispersal distances vary between pollen vector species.

The problem of generalized flowers: morphological aspects

Since Delpino (1868–74) recognized that flowers sharing similar physiognomies are preferentially visited by certain groups of animals, it has been commonly inferred that these biological groups (flowers and pollinators) have passed a more or less complicated evolutionary process of mutual specialization. The question, however, as to what were the initial angiosperm floral forms remains unanswered. Whatever the form, a broad spectrum of visitors was likely. Each organism needs to be adapted to its environment, for which reason the term “generalized” instead of “non‐specialized” is to be preferred. Generalized flowers occur even in otherwise highly evolved families, suggesting that these representatives have developed secondarily from a somewhat specialized pollination type; this points to the fact that “generalized” and “specialized” are relative terms. It also means that a generalized flower is not synonymous with a “primitive flower”! In terms of morphology, the original “primitive flower” form is a subject of debate. Neither in the fossil records nor among extant, archaic angiosperms is there a model organism bearing a flower with a broad spectrum of visitors, and which exhibits all organs in their plesiomorphic state. Nor is it likely that such a plant will ever be discovered. Thus, we must be content with a relative solution; there is one—hitherto only one—magnolioid family, the Winteraceae, some members of which approach the idealized model. Representatives of this family are characterized by many plesiomorphic features and have open flowers visited by a broad spectrum of pollen vectors. An explanation of how these plants could have remained in this generalized condition over long time periods is offered.

RESTRICTION OF POLLINATOR ASSEMBLAGE THROUGH FLOWER LENGTH AND WIDTH IN THREE LONG-TONGUED HAWKMOTH–POLLINATED SPECIES OF MANDEVILLA (APOCYNACEAE, APOCYNOIDEAE)1

Apocynaceae have a precise pollination mechanism. However, broad pollinator assemblages, including several insect orders, have often been recorded. We test how the corolla tube length and operative width required for effective release of the pollination mechanism could restrict the pollinator assemblage in putatively hawkmoth-pollinated Apocynaceae that differ in flower depth. If corolla tube length were effective in filtering pollinators, a broader pollen vector assemblage would be expected in short-flowered species. For this to occur, the operative width of the pollinating apparatus would be additionally expected not to be specific to any particular pollinator. Alternatively, if the pollination mechanism were to have any effect in the exclusion of pollinators, access to short flowers would be expected to be limited by proboscis width rather than length. Pollination of three species in the Apocynaceae was studied in seven populations through direct observations and palynological analysis of hawkmoths. Proboscis widths of hawkmoths carrying and not carrying pollen were compared with operative floral width as measured by proboscis analogues of different diameters that could successfully release the pollination mechanism. Species studied were exclusively hawkmoth pollinated. Pollen was always attached to pollinator proboscides, either near the base or on the apical half. The long-tongued hawkmoth species Manduca sexta (L.) was the major pollinator of Mandevilla longiflora (Desf.) Pichon and Mandevilla petraea (A. St.-Hil.) Pichon. Surprisingly, another long-tongued species, Manduca tucumana (Rothschild & Jordan), was the main pollinator of the short-flowered Mandevilla laxa (Ruiz & Pav.) Woodson. Here, the operative flower width was a decisive factor restricting the pollinator spectrum to hawkmoths with proboscides narrow enough to release the pollination apparatus. Short-tongued hawkmoths, which also have wider proboscides, cannot release the pollination mechanism. In M. petraea, the operative length, and not the operative width, restricts the pollinator assemblage. Thus, two different plant strategies were observed to restrict the pollinator spectrum: floral tube length and the operative width of the pollination mechanism.

Investigation of the breeding systems of four Aizoaceae species in Namaqualand, South Africa

We investigated the breeding systems of four different Aizoaceae species in Namaqualand, South Africa. Results showed that all four species are xenogamous with negligible amounts of seeds produced when flowers are left to spontaneous self-pollination (autogamy) or self-pollinated. Experiments allowing single visits of pollinators resulted in significantly higher seed numbers in relation to bagged unmanipulated flowers for two species. Pollen–ovule ratios were extremely high and supported the notion that the investigated Aizoaceae species are obligate outcrossers depending on insects as pollen vectors.

Reproductive system of Eriocnema fulva Naudin (Melastomataceae), an endemic species of Minas Gerais state, SE Brazil

Eriocnema fulva Naudin is a perennial herb, endemic to Minas Gerais state, SE Brazil, found on humid, shaded rocky riverbanks in montane semideciduous seasonal forests. The species is threatened, but information regarding its biology is still lacking, although such information is fundamental to any management plan. We aimed to evaluate the reproductive system of Eriocnema fulva in the Jambreiro Forest (19 degrees 58'-59' S and 43 degrees 55'-52' W, 800-1100 m altitude), municipality of Nova Lima, by experiments carried out in 1997 and 1998. The flowers are white, and flowering is of the steady state type, occurring once a year from November to December. Anthers are poricidal, and pollen is the only resource for visitors. The chromosome number is n = 17 during meiosis. The species is self-compatible, but does not produce fruits by spontaneous self-pollination or agamospermy; it requires pollen vectors and buzz pollination in order to produce fruits. Cross-pollination is the main reproductive strategy of E. fulva, and is accentuated by the small number of flowers (one or two in each plant) opened per day. Although the population studied was shaded by forest canopy, the seeds needed light to germinate. Germination ratio was lower in germination cabinet on filter paper (14% after 30 days) than in greenhouse on soil brought from the forest (47% after 25 days). Although the fruit is a capsule and the seeds are small, dispersion (anemochory or hydrochory) does not seem to occur at long distance, as it is the case for other Melastomataceae species with similar syndrome.

Flower-visiting behavior of male bees is triggered by nectar-feeding insects

Bees are important pollinators for many flowering plants. Female bees are thought to be more effective pollinators than male bees because they carry much more pollen than males. Males of some solitary bee species are known to patrol near flowers that females visit. Because patrolling males visit flowers to mate or defend their territories, they may function as pollinators. However, the significance of patrolling males to pollination has not been studied. We studied males of a solitary bee, Heriades fulvohispidus (Megachilidae), patrolling near flowers and visiting flowers that attracted nectar-feeding insects, including conspecifics, on the Ogasawara (Bonin) Islands. To test the hypothesis that patrolling male bees may function as pollen vectors, we compared the frequency of visits by H. fulvohispidus to flowers of an endemic plant, Schima mertensiana (Theaceae); comparisons were made among flowers with a dead H. fulvohispidus, a dead beetle, a piece of plastic, and nothing (control flowers). Patrolling H. fulvohispidus more frequently visited flowers with a dead conspecific, a dead beetle, or a piece of plastic than the control flowers. Our experiment demonstrates that nectar-feeding insects (including conspecifics and other insects) enhance the flower-visiting frequency of patrolling H. fulvohispidus males on S. mertensiana flowers. Furthermore, we observed S. mertensiana pollen on patrolling males as well as females, suggesting that male bees may also function as pollen vectors.

Breeding system of the subantarctic plant species Pringlea antiscorbutica R. Br. and search for potential insect pollinators in the Kerguelen Islands

The reproductive biology of Pringlea antiscorbutica R. Br. (Brassicaceae), an endemic species from the southern Indian Ocean islands, is investigated here. Controlled crosses were performed between plants grown in a common garden on the Kerguelen Islands. Searching for potential insect pollinators, we investigated the flying ability of all insects known from the Kerguelen Islands. We showed evidence for self-compatibility, low pollen limitation in the absence of a biotic pollen vector and slight selfing depression in P. antiscorbutica. The insects present on the islands are mostly wingless and are not likely to be efficient pollinators of this species. Since P. antiscorbutica shows morphological adaptations to insect pollination despite the absence of pollinators in its present range, we conclude that this species recently evolved from insect pollination to wind and/or autonomous pollination.

Biologia floral de Costus spiralis (Jacq.) Roscoe (Costaceae) e mecanismos para evitar a autopolinização

The floral biology of Costus spiralis (Jacq.) Roscoe (Costaceae) was studied on swampy edges of a gallery forest in Uberlândia, Minas Gerais. Costus spiralis is an herb with 0.5 m to 2.0 m in height which blossoms from January to April (rainy season). It presents spirally twisted branches with terminal inflorescences that produce only one flower per day. The flowers are subtended by conspicuous red bracts that can attract pollinators. The flowers are hermaphroditic, red, tubular, odourless and present diurnal anthesis. Concentration of sugars in nectar was c. 20% and volume c. 9.0 µL. C. spiralis is a self-compatible, non apomictic species, which does not present spontaneous self-pollination. It presents movement herkogamy to avoid self-pollination. The pollinators of C. spiralis were the hummingbirds Phaethornis pretrei (Lesson & DeLattre) (Phaethornithinae), Eupetomena macroura (Gmelin) and Heliomaster squamosus (Temminck) (Trochilinae). Amazilia fimbriata (Gmelin) acted as nectar robber. Costus spiralis has a floral morphology adapted to pollination by Phaethornithinae, with a long and curved floral tube which conforms with the beak morphology of these birds. The strategy of trapline foraging by Phaethornithinae hummingbirds, favors the reproduction of the plant, increasing pollen flow between groups of C. spiralis. There was no difference between germination rates of seeds from self-pollination and cross-pollination, but the seeds produced from natural fruit-set presented significantly higher germination rates than those from hand pollination treatments. The results confirm the efficiency and importance of the hummingbirds as pollen vectors for C. spiralis.

Containment of regulated genetically modified cotton in the field

Genetically modified (GM) insect and herbicide resistant cottons are now the mainstay of the cotton industry based in the eastern States of Australia. However, during the early years of breeding and seed increase, there was some uncertainty among regulators about containment measures needed to prevent the movement of regulated GM traits into adjacent fields of conventional cotton and possibly into the human or animal food chain. This was particularly relevant in the more tropical north of Australia that was being used as a winter nursery. Field experiments on GM cottons in eastern Australia were commonplace, but even there, regulators required confirmation that the containment practices already in place (buffer zones and isolation distances) were effective. A series of experiments in a range of environments, designed to monitor the movement of GM pollen into adjacent cotton buffers, adjacent fields, across bare ground and over long distances are reported here. Gene flow from GM plots into adjacent conventional cotton was found to be much higher in northern Australia than in eastern Australia, most likely due to higher honey bee ( Apis mellifera) numbers, often deliberately enhanced to aid the surrounding horticultural industries. Movement of pollen over bare ground to nearby crops was observed at a number of locations indicating that small isolation zones should not be used for containment of regulated GM cotton. In general, buffers of 20 m of conventional cotton surrounding GM plots proved to be highly effective unless bee numbers were artificially high. Based on insect observations and rates of pollen flow, it was concluded that pollen beetles ( Carpophilus spp.), another potential pollen vector normally highly abundant in cotton, were not associated with movement of pollen more than 1 m away from crops.

Impact of mate availability, population size, and spatial aggregation of morphs on sexual reproduction in a distylous, aquatic plant

In distylous, self-incompatible plants, clonal propagation, unbalanced floral morph frequencies, and reduced population size can interfere with the functioning of distyly by compromising legitimate intermorph pollinations, resulting in reduced reproductive output. Here, we examined the mating system and the impact of mate availability, population size, and spatial aggregation of morphs on reproductive output in the distylous, clonal, aquatic plant Hottonia palustris. Controlled pollinations under greenhouse conditions detected no spontaneous selfing without the action of a pollen vector (autonomous autogamy) and demonstrated very low fruit and seed development after self-pollination. Intermorph (legitimate) crossings resulted in high reproductive output in both floral morphs (long- and short-styled individuals), whereas intramorph (illegitimate) crossings decreased fruit and seed development by more than 50%, indicating that the species has partial intramorph-incompatibility. In natural populations, small population size and increasing deviation of floral morph frequencies negatively affected reproductive outcome. Individuals of the majority morph type developed significantly fewer fruit and seeds than individuals of the minority morph type. This rapid decline in fecundity was symmetrical, indicating that regardless of which morph was in the majority, the same patterns of negative frequency-dependent mating occurred. Increasing spatial isolation between compatible morphs significantly reduced fruit and seed set in both morphs similarly. This study provides clear indications of frequency- and context-dependent mating in natural populations of a distylous plant species.

Is there wind pollination in the southern heaths (Styphelioideae, Ericaceae)?

Wind pollination is associated with particular plant groups including conifers, grasses and some tree species. However sporadically (but quite frequently) it has evolved in lineages that are predominantly biotically pollinated. Factors which have been proposed as forcing this change are often associated with extreme environmental conditions that could limit pollinator attention to the flowers of the species. The Styphelioideae has been considered exclusively biotically pollinated, with insects the predominant visitors and vertebrates somewhat less important. However at least three species of Richea seem to have a number of characteristics which point towards wind as the main pollen vector. This study enumerates some of the characteristics of selected Richea species and evaluates the likelihood that wind pollination has evolved in R. sprengelioides, R. procera and R. victoriana. The genus seems to have exaptations, for example the operculum type of corolla, which would facilitate development of wind pollination from an insect-pollinated ancestor. Comparison with two Erica species illustrates that different modifications may faciliate change to achieve a similar outcome.

Reproductive biology and mating system estimates of two Andean melocacti, Melocactus schatzlii and M. andinus (Cactaceae).

The genus Melocactus comprises 36 species of globose cacti with the most derived traits in the Cereeae tribe. It is the proper study system to examine what are the most derived reproductive strategies within that tribe. This study aims to characterize the reproductive biology and to estimate the mating system parameters of two Andean melocacti, Melocactus schatzlii and M. andinus. The reproductive attributes of the two species were described, including floral morphology, anthesis patterns, floral rewards, floral visitors and visitation patterns. Levels of self-compatibility and autonomous self-pollination were estimated by hand-pollination experiments. Mating system estimates were obtained by conducting progeny array analyses using isozymes. The flowers of the two species present the typical hummingbird-pollination syndrome. Despite their morphological resemblance, the two species differ in flower size, pollen and ovule production and anthesis pattern. Their main pollinator agents are hummingbirds, four species in M. schatzlii and one species in M. andinus. Both cacti are self-compatible and capable of self-pollination without the aid of pollen vectors. Population-level outcrossing rate was higher for M. schatzlii (t(m)=0.9) than for M. andinus (t(m)=0.4). At the family level, outcrossing rates for most mothers of M. schatzlii were higher (t(m)>0.8) than for M. andinus (t(m)<0.5). Although the two cacti are capable of selfing, M. schatzlii is a predominantly outcrossing species, while M. andinus behaves as a mixed-mating cactus. Hummingbirds are the only pollinators responsible for outcrossing and gene flow events in these species. In their absence, both melocacti set seeds by selfing. Based on its low population size, restricted distribution in Venezuela, low rates of floral visits, and high levels of inbreeding, M. andinus is considered to be an endangered species deserving further study to define its conservation status.

The influence of pollinator abundance on the dynamics and efficiency of pollination in agricultural Brassica napus: implications for landscape‐scale gene dispersal

Summary It is important to understand the pollination processes that generate landscape‐scale gene dispersal in plants, particularly in crop plants with genetically modified (GM) varieties. In one such crop, Brassica napus, the situation is complicated by uncertainty over the relative importance of two pollen vectors, wind and insects. We investigated pollination in two fields of B. napus that bloomed at different times of year (April vs. July) and attracted different abundances of foraging social bees. Rates of pollen transfer were quantified by counting the pollen grains deposited on stigmas and remaining in the anthers at intervals after flower opening. Flowers open in April were adequately pollinated only after 5 days and only 10% received even a single bee visit. Flowers open in July received three bee visits per hour and were fully pollinated within 3 h. Based on published measurements of airborne pollen dispersal, we estimate that wind‐pollination from a hypothetical field 1 km distant could have fertilized up to 0·3% of the field's seed when bees were scarce in April but only up to 0·007% when bees were abundant in July. The efficiency of pollination (the proportion of pollen released from anthers that landed on receptive stigmas) was seven times greater in July (1·5%) than in April (0·2%). The relatively high efficiency of insect pollination may help to explain the evolutionary maintenance of entomophily. Synthesis and applications. Our results begin to resolve a long‐standing inconsistency among previous studies by suggesting that the susceptibility of fields of B. napus to long‐distance cross‐pollination by wind depends on the level of bee activity. Models for predicting GM gene flow at the landscape‐scale in this crop should take this into account.