Pollen Competition Research Articles

Differential selection on pollen and pistil traits in relation to pollen competition in the context of a sexual conflict over timing of stigma receptivity.

Sexual conflict and its evolutionary consequences are understudied in plants, but the theory of sexual conflict may help explain how selection generates and maintains variability. Here, we investigated selection on pollen and pistil traits when pollen arrives sequentially to partially receptive pistils in relation to pollen competition and a sexual conflict over timing of stigma receptivity in the mixed-mating annual Collinsia heterophylla (Plantaginaceae). In this species the conflict is generated by early fertilizing pollen that reduces seed production, which is counteracted by delaying receptivity in the recipient. We performed sequential two-donor pollinations at early floral developmental stages involving two pollen deposition schedules (with or without a time lag of 1 day), using only outcross or self and outcross pollen. We investigated pollen and pistil traits in relation to siring success (male fitness) and seed production (female fitness). In contrast to previous findings in receptive pistils in C. heterophylla and in other species, last arriving pollen donors showed highest siring success in partially receptive pistils. The last male advantage was weaker when self pollen was the first arriving donor. Two measures of germination rate (early and late) and pollen tube growth rate of first arriving donors were important for siring success in crosses with a time lag, while only late germination rate had an effect in contemporary crosses. Curiously, late stigma receptivity was negatively related to seed production in our contemporary crosses, which was opposite to expectation. Our results in combination with previous studies suggest that pollen and pistil traits in C. heterophylla are differentially advantageous depending on stage of floral development and varying pollen deposition schedules. Variation in success of these traits over floral development time may result from sexually antagonistic selection.

BiP3 supports the early stages of female gametogenesis in the absence of BiP1 and BiP2 in Arabidopsis thaliana

Immunoglobulin binding protein (BiP) is an essential heat shock protein 70 (Hsp70) in the endoplasmic reticulum (ER) that functions in various processes including protein translocation, protein folding and quality control. Arabidopsis thaliana harbors ubiquitously expressed genes BIP1 and BIP2, as well as BIP3, which is induced only by ER stress. Recently, we reported that these BIP genes are expressed in male gametophytes and cooperate with each other to support male gametogenesis and pollen competitiveness. Here, we report that the BIP genes cooperate to support female gametogenesis. As reported previously, the bip1 bip2 double mutation causes defects in the fusion of polar nuclei during female gametogenesis. By contrast, the bip triple mutant female gametophytes exhibited defects during the early stages of female gametophyte development, which suggests that BIP3 supports the early stages of female gametophyte development, but not polar nuclear fusion, in the absence of BiP1 and BiP2.

Sexual antagonism in the pistil varies among populations of a hermaphroditic mixed-mating plant.

Sexual conflicts and their evolutionary outcomes may be influenced by population-specific features such as mating system and ecological context; however, very few studies have investigated the link between sexual conflict and mating system. The self-compatible, mixed-mating hermaphrodite Collinsia heterophylla (Plantaginaceae) is thought to exhibit a sexual conflict over timing of stigma receptivity. This conflict involves (i) delayed stigma receptivity, which intensifies pollen competition, and (ii) early fertilization forced by pollen, which reduces seed set. We investigated the potential for the conflict to occur under field conditions and performed glasshouse crosses within eight populations to assess its consistency across populations. Flowers were visited, and produced seeds after pollination, at all developmental stages, suggesting that the conflict can be of significance under natural conditions. In the glasshouse, early pollination imposed costs in all populations. Overall, the timing of first seed set was most strongly affected by the maternal parent, denoting stronger female than male ability to influence the onset of stigma receptivity. Crosses also revealed a negative relationship between donor- and recipient-related onset of receptivity within individuals, a novel result hinting at trade-offs in sex allocation or a history of antagonistic selection. Neither timing of stigma receptivity, timing of first seed set, nor pollen competitive ability covaried with population outcrossing rate. In conclusion, these results indicate that sexually antagonistic selection may be present in varying degrees in different populations of C. heterophylla, but this variation does not appear to be directly related to mating system variation.

Night life on the beach: selfing to avoid pollinator competition between two sympatric Silene species.

Evolution of autonomous selfing may be advantageous because it allows for reproductive assurance. In co-flowering plants competing for pollinators, the least common and/or attractive could suffer pollen limitations. Silene niceensis and S. ramosissima are taxonomically related species sharing the same habitat, although S. ramosissima is less abundant and has a more restricted distribution. They also have the same a priori nocturnal pollinator syndrome, and show an overlapping flowering phenology. The aim of this study was to investigate whether a selfing strategy in S. ramosissima allows it to avoid pollinator competition and/or interspecific pollen transfer with S. niceensis, which would thus enable both species to reach high levels of fruit and seed set. The breeding system, petal colour, flower life span and degree of overlap between male and female phases, floral visitor abundance and visitation rates were analysed in two sympatric populations of S. niceensis and S. ramosissima in southern Spain. Autonomous selfing in S. ramosissima produced very high fruit and seed set, which was also similar to open-pollinated plants. Silene niceensis showed minimum levels of autonomous selfing, and pollen/ovule ratios were within the range expected for the breeding system. In contrast to S. niceensis, flower life span was much shorter in S. ramosissima, and male and female organs completely overlapped in space and time. Upper surface petals of both species showed differing brightness, chroma and hue. Flowers of S. niceensis were actively visited by moths, hawkmoths and syrphids, whereas those of S. ramosissima were almost never visited. The findings show that different breeding strategies exist between the sympatric co-flowering S. niceensis and S. ramosissima, the former specializing in crepuscular-nocturnal pollination and the latter mainly based on autonomous selfing. These two strategies allow both species to share the restricted dune habitat in which they exist, with a high female reproductive success due to the absence of pollinator competition and/or interspecific pollen flow.

Competition for Pollination and the Evolution of Flowering Time.

Competition for Pollination and the Evolution of Flowering Time.

Costs and benefits of reproducing under unfavorable conditions: an integrated view of ecological and physiological constraints in a cerrado shrub

Disentangling the relative importance of biotic and abiotic constraints in plant reproduction is a major challenge in reproductive ecology. Here, we tested the ‘resource limitation hypothesis’ that predicts a high-level ovule abortion under resource scarcity; the ‘flowering displacement hypothesis’ that predicts low levels of pollen limitation driven by relaxed competition for pollinators; and the ‘herbivory escape hypothesis’ that predicts low impact by natural enemies during unfavorable conditions. We followed reproductive phenology, measured the seasonal variation in resource abundance, and calculated initial ovule budgets to estimate the relative importance of each limiting factor on final reproductive output. Ovule fate was determined of ovules in different positions within the pods, and a germination experiment was conducted to identify bottlenecks at the germination stage. Despite marked decreases in resource availability during the dry season, reproduction consistently occurred during mid-to-late dry season. Destruction by natural enemies and abortion were the most likely ovule fates, with only 2.2 % of flower buds converted into ripe fruits. Ovule fates were not random along fruit positions, with higher likelihood of well-formed seeds in fruit tips and higher likelihood of ovule abortion and non-fertilized ovules near the fruit bases. The benefits derived from flowering displacement to the dry season include reduced competition for pollinators and synchronization of seedling establishment with the onset of the rainy season. However, we found no support for the herbivory escape hypothesis. We argue that a cost–benefit approach is a useful framework to understand the evolutionary ecology of phenological strategies in seasonal environments.

The effect of polyploidy and hybridization on the evolution of floral colour in Nicotiana (Solanaceae).

Background and Aims Speciation in angiosperms can be accompanied by changes in floral colour that may influence pollinator preference and reproductive isolation. This study investigates whether changes in floral colour can accompany polyploid and homoploid hybridization, important processes in angiosperm evolution.Methods Spectral reflectance of corolla tissue was examined for 60 Nicotiana (Solanaceae) accessions (41 taxa) based on spectral shape (corresponding to pigmentation) as well as bee and hummingbird colour perception in order to assess patterns of floral colour evolution. Polyploid and homoploid hybrid spectra were compared with those of their progenitors to evaluate whether hybridization has resulted in floral colour shifts.Key Results Floral colour categories in Nicotiana seem to have arisen multiple times independently during the evolution of the genus. Most younger polyploids displayed an unexpected floral colour, considering those of their progenitors, in the colour perception of at least one pollinator type, whereas older polyploids tended to resemble one or both of their progenitors.Conclusions Floral colour evolution in Nicotiana is weakly constrained by phylogeny, and colour shifts do occur in association with both polyploid and homoploid hybrid divergence. Transgressive floral colour in N. tabacum has arisen by inheritance of anthocyanin pigmentation from its paternal progenitor while having a plastid phenotype like its maternal progenitor. Potentially, floral colour evolution has been driven by, or resulted in, pollinator shifts. However, those polyploids that are not sympatric (on a regional scale) with their progenitor lineages are typically not divergent in floral colour from them, perhaps because of a lack of competition for pollinators.

Reproductive character displacement shapes a spatially structured petal color polymorphism in Leavenworthia stylosa.

Character displacement is a potentially important process driving trait evolution and species diversification. Floral traits may experience character displacement in response to pollinator-mediated competition (ecological character displacement) or the risk of forming hybrids with reduced fitness (reproductive character displacement). We test these and alternative hypotheses to explain a yellow-white petal color polymorphism in Leavenworthia stylosa, where yellow morphs are spatially associated with a white-petaled congener (Leavenworthia exigua) that produces hybrids with complete pollen sterility. A reciprocal transplant experiment found limited evidence of local adaptation of yellow color morphs via increased survival and seed set. Pollinator observations revealed that Leavenworthia attract various pollinators that generally favor white petals and exhibit color constancy. Pollen limitation experiments showed that yellow petals do not alleviate competition for pollination. Interspecific pollinator movements were infrequent and low hybridization rates (∼0.40-0.85%) were found in each morph, with natural rates likely being lower. Regardless, hybridization rates were significantly higher in white morphs of L. stylosa, yielding a small selection coefficient of s = 0.0042 against this phenotype in sympatry with L. exigua. These results provide support for RCD as a mechanism contributing to the pattern of petal color polymorphism in L. stylosa.

Competition for pollinators and intra-communal spectral dissimilarity of flowers.

Competition for pollinators occurs when, in a community of flowering plants, several simultaneously flowering plant species depend on the same pollinator. Competition for pollinators increases interspecific pollen transfer rates, thereby reducing the number of viable offspring. In order to decrease interspecific pollen transfer, plant species can distinguish themselves from competitors by having a divergent phenotype. Floral colour is an important signalling cue to attract potential pollinators and thus a major aspect of the flower phenotype. In this study, we analysed the amount of spectral dissimilarity of flowers among pollinator-competing plants in a Dutch nature reserve. We expected pollinator-competing plants to exhibit more spectral dissimilarity than non-competing plants. Using flower visitation data of 2 years, we determined the amount of competition for pollinators by different plant species. Plant species that were visited by the same pollinator were considered specialist and competing for that pollinator, whereas plant species visited by a broad array of pollinators were considered non-competing generalists. We used principal components analysis to quantify floral reflectance, and found evidence for enhanced spectral dissimilarity among plant species within specialist pollinator guilds (i.e. groups of plant species competing for the same pollinator). This is the first study that examined intra-communal dissimilarity in floral reflectance with a focus on the pollination system.

Selection by pollinators on floral traits in generalized Trollius ranunculoides (Ranunculaceae) along altitudinal gradients.

Abundance and visitation of pollinator assemblages tend to decrease with altitude, leading to an increase in pollen limitation. Thus increased competition for pollinators may generate stronger selection on attractive traits of flowers at high elevations and cause floral adaptive evolution. Few studies have related geographically variable selection from pollinators and intraspecific floral differentiation. We investigated the variation of Trollius ranunculoides flowers and its pollinators along an altitudinal gradient on the eastern Qinghai-Tibet Plateau, and measured phenotypic selection by pollinators on floral traits across populations. The results showed significant decline of visitation rate of bees along altitudinal gradients, while flies was unchanged. When fitness is estimated by the visitation rate rather than the seed number per plant, phenotypic selection on the sepal length and width shows a significant correlation between the selection strength and the altitude, with stronger selection at higher altitudes. However, significant decreases in the sepal length and width of T. ranunculoides along the altitudinal gradient did not correspond to stronger selection of pollinators. In contrast to the pollinator visitation, mean annual precipitation negatively affected the sepal length and width, and contributed more to geographical variation in measured floral traits than the visitation rate of pollinators. Therefore, the sepal size may have been influenced by conflicting selection pressures from biotic and abiotic selective agents. This study supports the hypothesis that lower pollinator availability at high altitude can intensify selection on flower attractive traits, but abiotic selection is preventing a response to selection from pollinators.

Pollen (microgametophyte) competition: an assessment of its significance in the evolution of flowering plant diversity, with particular reference to seed germination

A hypothesis by David L. Mulcahy published in Science in 1979 on pollen competition as an important force in the evolution of flowering plant diversity, via selection of favourable genes in the microgametophyte that are passed on to, and inherited in, the sporophyte, stimulated quite a bit of research on the topic. The primary aim of this opinion paper is to assess the evidence for the effect of microgametophyte competition on the sporophyte, with particular reference to seed germination. Pollen competition was associated with an increase in seed germination in 14 of 30 case studies (30 published papers). Seed mass was related to better germination in only two of the 14 cases in which there was a positive effect of pollen competition on germination. An evaluation of the four criteria that must be met to validate Mulcahy's hypothesis cast some doubt on the significance of pollen competition as an important selective force in the diversification of angiosperms. Insect pollination, via which many pollen grains are deposited on a stigma, may not be the answer to Darwin's ‘abominable mystery’ concerning the reason for the abrupt origin and rapid diversification of flowering plants during the Cretaceous.

Relative density and dispersion pattern of two southern African Asteraceae affect fecundity through heterospecific interference and mate availability, not pollinator visitation rate

Summary Since co‐flowering plants often share pollinators, their fecundity is likely affected by pollen transfer within and among plant species. Changes in pollinator visitation rates (e.g. through facilitation or competition for pollinators) are usually thought to exert the predominant influence on plant fecundity. However, the transfer of conspecific relative to heterospecific pollen between individuals may also play an important role. Indeed, the relative importance of these determinants of fecundity is expected to depend on the density, relative abundance and spatial arrangement of con‐ and heterospecifics. We investigated the effects of con‐ and heterospecific density and spatial distribution pattern on pollinator visitation and plant fecundity within and between two annual, self‐incompatible co‐flowering species [Dimorphotheca pinnata and Ursinia cakilefolia (Asteraceae)] by manipulating their relative abundance, overall patch density and dispersion patterns in experimental arrays in Namaqualand, South Africa. We quantified pollinator visitation rates and fruit set in arrays of varying density and aggregation. This enabled us to determine which mechanism(s) were driving variation of fecundity, particularly through their influence on visitation rates, mate availability and heterospecific pollen interference. To test whether autogamy offers reproductive assurance when individuals are scattered among a dense population of heterospecifics, we included an autogamous species (U. anthemoides) in a separate experiment. We found that increased fecundity with increasing conspecific density was not the result of higher visitation rates, but rather increased mate availability. Furthermore, increased spatial aggregation of conspecifics at low density significantly increased fecundity through reduced heterospecific interference. In contrast to results for self‐incompatible species, fruit set in U. anthemoides was consistently high and unaffected by scattered distribution patterns. This suggests that autogamy may offer reproductive assurance when mates are limited and the potential for interspecific pollen transfer (IPT) is high. Synthesis. In this study of annual daisies, variation in fruit set is primarily driven by factors that affect the transfer of conspecific relative to heterospecific pollen, independent of pollinator visitation rate. Our findings demonstrate that mate limitation and IPT negatively affect fruit set and that these effects can be mitigated by intraspecific aggregation and the ability to autonomously self‐pollinate.

Inbreeding effects in a mixed-mating vine: effects of mating history, pollen competition and stress on the cost of inbreeding.

Inbreeding depression is assumed to be a central factor contributing to the stability of plant mating systems. Predicting the fitness consequence of inbreeding in natural populations is complicated, however, because it may be affected by the mating histories of populations generating variation in the amount of purging of deleterious alleles. Furthermore, the level of inbreeding depression may depend on environmental conditions and the intensity of pollen competition. In a greenhouse experiment comparing four populations of the neotropical vine Dalechampia scandens (Euphorbiaceae), we tested whether inbreeding depression for early-life fitness depended on the inferred mating history of each population, as indicated by genetically determined differences in herkogamy and autofertility rates. We also tested whether the intensity of pollen competition and the level of stress encountered by the seeds and seedlings affected the amount of inbreeding depression observed. Herkogamy was a good predictor of autofertility in each population. However, we found only limited evidence for inbreeding depression in any population, and inbreeding depression varied independently of the intensity of pollen competition and amount of stress encountered by the seeds and seedlings. Thus, the population's rate of autofertility did not predict the amount of inbreeding depression. Overall, we found no evidence supporting the expectations that more inbred populations experience less inbreeding depression, and that pollen competition reduces the cost of inbreeding. These results suggest that additional factors may be responsible for the maintenance of the mixed mating systems of D. scandens populations.

ASSESSMENT OF POLLEN-MEDIATED TRANSGENE FLOW IN CITRUS UNDER EXPERIMENTAL FIELD CONDITIONS

Despite potential benefits granted by genetically modified (GM) citrus trees, their release and commercialization raises concerns about their potential environmental impact. The transfer via pollen of transgenes to crosscompatible cultivars is deemed to be the greatest source for environmental exposure. In this work, three different citrus genotypes carrying the uidA (GUS) tracer marker gene (pollen donors) and a non-GM selfincompatible contiguous citrus genotype (recipient) were used in conditions allowing natural entomophilous pollination to occur. The examination of 603 to 2990 seeds per year showed unexpectedly low frequencies (0.17-2.86%) of transgene flow. Paternity analyses of the progeny of subsets of recipient plants using 10 microsatellite (SSR) loci demonstrated a higher mating competence of trees from another non-GM pollen source population that greatly limited the mating chance of the contiguous cross-compatible and floweringsynchronized transgenic pollen source. This mating superiority could be explained by a much higher pollen competition capacity of the non-GM genotypes, as was confirmed through mixed-hand pollinations, indicating that pollen competition strongly contributed to transgene confinement. This is the first study on transgene flow in citrus. It provide crucial information on the safety and field performance of GM citrus that can serve as a basis for further field trials and as a guide for (case-by-case) regulatory policies. (Resume d'auteur)

Despite Extensive Pollinator Sharing, Invasive Blackberry has Negligible Impacts on Reproductive Success of a Rare Native Wildflower

Abstract Plant invasions have the potential to disrupt community dynamics and impair essential ecosystem services, including the pollination of native flowers. Invaders most likely to invoke pollinator competition are characterized by conspicuous, resource-rich blooms and long flowering periods, while the natives most likely to be impacted are obligate out-crossers that are sensitive to disturbance or locally rare. We investigated the effects of a widespread showy invader of the Pacific Northwest, Rubus armeniacus (Himalayan blackberry), on an imperiled endemic wildflower, Sidalcea hendersonii. We observed pollinators, quantified pollen deposition and conducted pollen-supplementation experiments on S. hendersonii plants growing at three distances (1 m, 15 m and 50 m) from well-established blackberry patches in five adjacent field plots. Individual R. armeniacus flowers received more than three times as many total visits as S. hendersonii inflorescences; however, there was minimal overlap between the polli...

In vitro Competition of Betula alnoides Pollens from Two Types of Habitats in a Heterogeneous Landscape in South China

Abstract Pollens of Betula alnoides were collected in its natural forests at two types of sites with soils originated from limestone (Site L) and granite (Site G) in a heterogeneous landscape at Jingxi County, Guangxi in 2010 to 2012 to investigate whether nutritional differences between the sites influenced its pollen competition. In vitro pollen germinations were conducted separately and in pairs for these samples as well as nutrient contents of pollens and soils were measured. There was no significant difference in total nitrogen, phosphorus, potassium and calcium contents of dried pollen and in efficient ones of aqueous extracts of pollens between two types of sites although the soil nutrient contents were quite different between them, and less remarkable correlation was found between these nutrient contents of soil and pollen. The results of pollen competition of B. alnoides were much diverse among three years, it seemed that pollen-pollen interaction did not depend on site type, and might vary according to genotype and/or combination of genotypes.

Intrinsic and extrinsic factors acting on the reproductive process in alpine‐snowbed plants: roles of phenology, biological interaction, and breeding system

AbstractPollinator activity and competition for pollinators lead to quantitative and qualitative pollen limitations on seed production and affect the reproductive success of plant species, depending on their breeding system (e.g., self‐compatibility and heterospecific compatibility) and genetic load (e.g., inbreeding depression and hybrid inviability). In alpine ecosystems, snowmelt regimes determine the distribution and phenology of plant communities. Plant species growing widely along a snowmelt gradient often grow with different species among local populations. Their pollinators also vary in their abundance, activity, and behavior during the season. These variations may modify plant–pollinator and plant–plant interactions. We integrated a series of our studies on the alpine dwarf shrub, Phyllodoce aleutica (Ericaceae), to elucidate the full set of intrinsic (species‐specific breeding system) and extrinsic factors (snow condition, pollinator activity, and interspecific competition) acting on their reproductive process. Seasonality of pollinator activity led to quantitative pollen limitation in the early‐blooming populations, whereas in the late‐blooming populations, high pollinator activity ensured pollination service, but interspecific competition for pollinators led to qualitative and quantitative pollen limitation in less competitive species. However, negative effects of illegitimate pollen receipt on seed‐set success might be reduced when cryptic incompatibility systems (i.e., outcross pollen grains took priority over self‐ and heterospecific pollen grains) could effectively prevent ovule and seed discounting. Our studies highlight the importance of species‐specific responses of plant reproduction to changing pollinator availability along environmental gradients to understand the general features of pollination networks in alpine ecosystems.

Habitat selection in a deceptive predator: maximizing resource availability and signal efficacy

Many animals deceive prey by presenting a deceptive signal that lures prey into close proximity. These predators are predicted to select habitats that maximize the efficacy of their deceptive signal and offer high levels of prey traffic. The orchid mantis Hymenopus coronatus preys upon pollinators that are deceived by the predator’s resemblance to a flower. Habitat patches that contain flowers can offer orchid mantises high levels of insect activity, yet may be detrimental to the success of their deceptive signaling strategy. Currently 2 exclusive hypotheses—previously applied to deceptive flowers—predict the interaction between signaling efficacy under different flower densities. Increased competition for pollinators from nearby flowers suggests that orchid mantis signals should be more effective when in isolation from flowers (remote habitats hypothesis). Alternatively, orchid mantises may benefit from increased insect activity when near flowers (magnet species hypothesis). We found that orchid mantises had no preference for inhabiting flowers over leafy vegetation. They were also no more effective in attracting prey when sitting upon flowers compared to when in isolation from flowers. However, field experiments revealed that the density of flowers in their immediate vicinity did have an effect on orchid mantis attractiveness. Mantises were visited by more flying insects when in patches of high flower density, suggesting that they benefit from the magnet species effect. Co-occurring flowers do not negatively impact the efficacy of the orchid mantis’ deceptive signal and areas with high densities of flowers may allow them access to high levels of prey availability.

Density dependence in flower visitation rates of cockroach-pollinatedClusia blattophilaon the Nouragues inselberg, French Guiana

Abstract:The effective floral neighbourhood is the radius around a plant where the density of flowering plants and other factors affect visitation rates and pollination success of plants. This study aims to determine this radius and focuses on the effects of conspecific plant density, plant sex and the amount of shrub vegetation on visitation rates ofClusia blattophila, a dioecious bush pollinated byAmazonina platystylatacockroaches. The number of visits did not differ between flower sexes but cockroaches spent less time on the rewardless female flowers. The density effect was scale dependent. The distribution of flowering individuals within the 15-m radius had a significant positive effect on flower visitation rates. At a larger scale (35–45-m radius), an increase in density of male plants led to a decrease in visitation rates, indicating competition for pollinators. Within the smaller radius, the facilitative effect was probably induced by elevated floral advertisement and high mobility of cockroaches at this scale. Within the larger radii, the results indicated that cockroach mobility was restricted and population density was stable at this scale. Density of male plants affected visitation rates because only male plants reward pollinators. Hence, male plants were the true competitors.

Invasive conifers reduce seed set of a native Andean cedar through heterospecific pollination competition

The invasion of natural forest communities by introduced woody species may threat processes that are critical for ecosystem integrity, including plant pollination and reproduction. In particular, invasive conifers (e.g., Pseudotsuga menziesii, Pinus spp.) escaped from forest plantations may impair seed production of anemophilous native conifers by altering the dynamics of wind-borne pollen flow among trees. In this study, we analyzed the effects of invasive conifers on the pollination and sexual reproduction of Andean cedar (Austrocedrus chilensis), a dioecious conifer native to the temperate forests of southern South America. Wind-dispersed pollen captured in pollen traps revealed that the timing of pollen release between the invasive and native conifers overlapped extensively, and the pollen cloud surrounding female cedar trees in invaded forest was heavily contaminated with pollen from the invasive conifers. Increasing abundance of invasive conifers in the proximity of female cedars decreased the local density of air-borne cedar pollen and increased contamination of the pollen cloud with pollen from the invasive conifers. This increased the chance of finding heterospecific pollen from these invaders in the micropyles of the native cedar. In turn, the presence of pollen grains from invasive conifers, particularly of P. menziesii, in the cedar’s micropyles was associated with a strong increase in the percentage of seed abortion. Our results clearly support the hypothesis that invasive conifers can interfere with the pollination and sexual reproduction of native wind-pollinated conifers. Thus, plantations of introduced and highly invasive conifers should not occur in the proximity of stands of native conifers in order to guarantee their reproduction and long-term population viability.