A directed network analysis of heterospecific pollen transfer in a biodiverse community

Floral resource partitioning of coexisting bumble bees: Distinguishing species-, colony-, and individual-level effects

Resource partitioning is considered a key factor in alleviating competitive interactions, enabling coexistence among consumer species. However, most studies have focused on resource partitioning between species, ignoring the potentially critical role of intraspecific variation in resource use. We investigated floral resource partitioning across species, colonies, and individuals in a species-rich bumblebee community in the diversification center of bumblebees. We used a total of 10,598 bumblebees belonging to 13 species across 5 years in the Hengduan Mountains of southwest China. First, we evaluated the influence of a comprehensive set of floral traits, including both those related to attractiveness (flower color and shape) and rewards (pollen, sugar ratio, nectar volume, sugar concentration, and amino acid content) on resource partitioning at the species level in bumblebee-plant networks. Then, we explored intraspecific resource partitioning on the colony and individual levels. Our results suggest that bumblebee species differ substantially in their use of the available floral resources, and that this mainly depends on flower attractiveness (floral color and shape). Interestingly, we also detected floral resource partitioning at the colony level within all commonest bumblebee species evaluated. In general, floral resource partitioning between bumblebee individuals decreased with species- and individual-level variation in body size (intertegular span). These results suggest that bumblebee species may coexist via the flexibility in their preferences for specific floral traits, which filters up to support the co-occurrence of high numbers of species and individuals in this global hotspot of species richness.

Flower-bee versus pollen-bee metanetworks in fragmented landscapes

Understanding the organization of mutualistic networks at multiple spatial scales is key to ensure biological conservation and functionality in human-modified ecosystems. Yet, how changing habitat and landscape features affect pollen-bee interaction networks is still poorly understood. Here, we analysed how bee-flower visitation and bee-pollen-transport interactions respond to habitat fragmentation at the local network and regional metanetwork scales, combining data from 29 fragments of calcareous grasslands, an endangered biodiversity hotspot in central Europe. We found that only 37% of the total unique pairwise species interactions occurred in both pollen-transport and flower visitation networks, whereas 28% and 35% were exclusive to pollen-transport and flower visitation networks, respectively. At local level, network specialization was higher in pollen-transport networks, and was negatively related to the diversity of land cover types in both network types. At metanetwork level, pollen transport data revealed that the proportion of single-fragment interactions increased with landscape diversity. Our results show that the specialization of calcareous grasslands' plant-pollinator networks decreases with landscape diversity, but network specialization is underestimated when only based on flower visitation information. Pollen transport data, more than flower visitation, and multi-scale analyses of metanetworks are fundamental for understanding plant-pollinator interactions in human-dominated landscapes.

Heterospecific pollen avoidance strategy prevails in the generalized plant-pollinator network on Yongxing Island

Heterospecific pollen (HP) deposition varies widely among species in communities, which has been explicated by two adaptation strategies: HP avoidance and HP tolerance. Studies of the plant-pollinator network have uncovered that oceanic island communities are highly generalized and strongly connected. It remains unclear, however, which strategy prevails in such communities. We examined stigma pollen deposition on 29 plant species, and assessed patterns of HP load size and diversity in the Yongxing Island community. We assessed the effects of phenotypic specialization and species-level network structural properties of plant species on pollen deposition among species. The hypothesis of three accrual patterns of HP within species was tested by illustrating the relationship between conspecific pollen (CP) and HP receipt. Extensive variation occurred among species in HP receipt, while 75.9% of species received less than 10% HP and one species received more than 40% HP throughout the community. Flower size strongly drives the variation of HP receipt, while network structural properties had no effect on the pollen receipt. Nineteen species showed no relationship between the number of HP and CP loads, and they received smaller HP load sizes and lower HP proportions. Most plant species evolved HP avoidance strategy, and HP receipt was an occasional event for most plant species in the generalized community. HP and CP receipts are independent of each other in plant species with the HP avoidance mechanism. Our results highlight that plants in the generalized pollination system may preferentially select to minimize the HP load on stigmas.

Reproductive interference between alien species in Veronica

Reproductive interference can lead to the exclusive distribution of species. Reports on reproductive interference between alien species are scarce, although alien species are becoming more abundant and evaluations of the interference between aliens and its influence on community structure are essential. We therefore investigated the presence of interference in two alien species of Veronica, V. cymbalaria and V. persica. Hand-pollination experiments revealed that heterospecific pollen adversely affected both species, but asymmetrically: in V. cymbalaria, it reduced the number of seeds both before and after conspecific pollination, but in V. persica it reduced the number of seeds only when it occurred before conspecific pollination. Field observation suggested that the number of seeds produced by V. cymbalaria was more influenced by the number of conspecific flowers than by the frequency of surrounding V. persica flowers. Pot experiments did not show a negative effect of heterospecific individuals on the reproductive success of focal species in either species. Observations of visiting pollinators revealed no pollinator preference between the plant species. Although we demonstrated asymmetrical reproductive interference between V. cymbalaria and V. persica, its effect in the field seemed limited. An essential factor mitigating the adverse effect of reproductive interference might be a shortage of pollinators that could facilitate interspecific pollination. We predict that further invasion of V. cymbalaria into areas where V. persica prevails is unlikely, although swift displacement of V. cymbalaria by V. persica in areas where V. cymbalaria is already established is also unlikely.

The pollen virome: A review of pollen-associated viruses and consequences for plants and their interactions with pollinators

The movement of pollen grains from anthers to stigmas, often by insect pollinator vectors, is essential for plant reproduction. However, pollen is also a unique vehicle for viral spread. Pollen-associated plant viruses reside on the outside or inside of pollen grains, infect susceptible individuals through vertical or horizontal infection pathways, and can decrease plant fitness. These viruses are transferred with pollen between plants by pollinator vectors as they forage for floral resources; thus, pollen-associated viral spread is mediated by floral and pollen grain phenotypes and pollinator traits, much like pollination. Most of what is currently known about pollen-associated viruses was discovered through infection and transmission experiments in controlled settings, usually involving one virus and one plant species of agricultural or horticultural interest. In this review, we first provide an updated, comprehensive list of the recognized pollen-associated viruses. Then, we summarize virus, plant, pollinator vector, and landscape traits that can affect pollen-associated virus transmission, infection, and distribution. Next, we highlight the consequences of plant-pollinator-virus interactions that emerge in complex communities of co-flowering plants and pollinator vectors, such as pollen-associated virus spread between plant species and viral jumps from plant to pollinator hosts. We conclude by emphasizing the need for collaborative research that bridges pollen biology, virology, and pollination biology.

Do Silene species with exposed stigmas tolerate interference by heterospecific pollen?

PREMISE

Co-flowering species that have not evolved an avoidance mechanism may have tolerance to heterospecific pollen (HP) deposition as an adaptive strategy to minimize any deleterious effects of HP transfer, but empirical evidence for the tolerance hypothesis remains scarce.

METHODS

To estimate the potential effects of heterospecific pollen deposition (HPD) on female reproductive success, we counted conspecific (CP) and HP pollen grains deposited on stigmas and assessed subsequent seed set of both open- and hand-pollinated flowers in three co-flowering Silene species with exposed stigmas that usually received numerous HP grains on the elongated receptive area.

RESULTS

The percentage of HP grains per flower (HP%) varied from 16.6% to 43.0% among three species. Silene chungtienensis had lower HP%, and the CP-HP relationship was neutral; S. gracilicaulis and S. yunnanensis had a relatively higher HP% with a positive CP-HP relationship. The effects of CP and HP number on natural seed set were positive for all three species, but HP% had stronger negative effects in S. chungtienensis and S. gracilicaulis. In hand-pollinated flowers of the three Silene species, seed set did not decrease with HP whether CP was in excess or insufficient, indicating no negative effects of HPD on seed production.

CONCLUSIONS

Consistent with the tolerance hypothesis, our results indicated that species with higher HP interference are likely to be tolerant to an increase in HP%. These species with generalist-pollinated flowers and exposed large stigmas may benefit from an increase of conspecific pollen deposition, despite the associated increase in heterospecific pollen deposition.

Spatial variation of pollen receipt and effects of heterospecific pollen on seed set in Salvia przewalskii

Generalized pollinators visit multiple co-flowering plant species and may transfer heterospecific pollen grains. Recent studies have indicated that the effect of heterospecific pollen (HP) on reproduction success is variable and depends on the identity of donor and recipient species. However, few studies have documented variation in HP receipt and evaluated the reproductive effects of HP receipt across geographic locations under natural conditions. We investigated the spatial variation of pollen deposition across eight sites and how the pollen receipt related to the seed set of Salvia przewalskii, a subalpine perennial herb in Hengduan Mountain in southwest China. We found that stigmatic pollen loads substantially varied among sites for several metrics, including quantities of conspecific and heterospecific pollen, the proportion of HP, and species composition of HP donors. Five different plant families were the most common HP source at one or two sites, and the proportion of HP ranged from 3.4% to 51.3% across sites. The association of conspecific pollen with seed set was positive and variable among sites, whereas the association of HP receipt and seed set was negative and not significantly different among sites. Our results demonstrate variation in the quantity and fitness effect of pollen receipt across sites, which is a precondition for evolution of local adaptation. Further study of variation in patterns and effects of HP receipt for the same recipient species across natural communities would allow better understanding of the ecological and evolutionary consequences of HP receipt.

Interspecific variation in resistance and tolerance to herbicide drift reveals potential consequences for plant community co-flowering interactions and structure at the agro-eco interface

BACKGROUND AND AIMS

When plant communities are exposed to herbicide 'drift', wherein particles containing the active ingredient travel off-target, interspecific variation in resistance or tolerance may scale up to affect community dynamics. In turn, these alterations could threaten the diversity and stability of agro-ecosystems. We investigated the effects of herbicide drift on the growth and reproduction of 25 wild plant species to make predictions about the consequences of drift exposure on plant-plant interactions and the broader ecological community.

METHODS

We exposed potted plants from species that commonly occur in agricultural areas to a drift-level dose of the widely used herbicide dicamba or a control solution in the glasshouse. We evaluated species-level variation in resistance and tolerance for vegetative and floral traits. We assessed community-level impacts of drift by comparing the species evenness and flowering networks of glasshouse synthetic communities comprised of drift-exposed and control plants.

KEY RESULTS

Species varied significantly in resistance and tolerance to dicamba drift: some were negatively impacted while others showed overcompensatory responses. Species also differed in the way they deployed flowers over time following drift exposure. While drift had negligible effects on community evenness based on vegetative biomass, it caused salient differences in the structure of co-flowering networks within communities. Drift reduced the degree and intensity of flowering overlap among species, altered the composition of groups of species that were more likely to co-flower with each other than with others and shifted species roles (e.g. from dominant to inferior floral producers, and vice versa).

CONCLUSIONS

These results demonstrate that even low levels of herbicide exposure can significantly alter plant growth and reproduction, particularly flowering phenology. If field-grown plants respond similarly, then these changes would probably impact plant-plant competitive dynamics and potentially plant-pollinator interactions occurring within plant communities at the agro-ecological interface.

Conspecific pollen advantage mediated by the extragynoecial compitum and its potential to resist interspecific reproductive interference between two Sagittaria species

The extragynoecial compitum formed by the incomplete fusion of carpel margins, while allowing intercarpellary growth of pollen tubes in apocarpous angiosperms, may also increase the risk of reproductive interference caused by heterospecific pollen (HP) deposition. In Sagittaria, congeneric HP tubes grow via different paths and enter the ovules later than conspecific pollen (CP) tubes. However, it is unclear how the growth advantage of the CP tube helps ensure reproductive success when HP is deposited on the stigmas. We performed molecular characterization of interspecies-pollinated seeds to examine the consequences of interspecific pollen deposition between Sagittaria pygmaea and S. trifolia. We also conducted CP-HP (1:1) mixed pollination and delayed CP pollination treatments to explore the seed-siring abilities of CP and HP. Our results showed that although HP could trigger the development of fruits, the interspecies-pollinated seeds contained partially developed embryos and could not germinate. More than 70% of the embryos in these seeds were molecularly identified as hybrids of both species, suggesting that HP tubes could enter the ovules and fertilize the egg cells. Moreover, CP could sire more offspring (≥70%) after the CP-HP (1:1) mixed pollination treatment, even when HP reached the stigma 0.5-1 h earlier than CP (≥50%). Following adequate CP vs. HP (1:1) pollination on carpels on two sides of the apocarpous gynoecium, both species produced > 70% conspecific seeds, indicating that the CP tubes could occupy ovules that should be occupied by HP via the extragynoecial compitum. Our results reveal that in Sagittaria, pollen deposition from co-existing congeneric heterospecies leads to interspecific seed discounting. However, the CP advantage mediated by the extragynoecial compitum is an effective strategy to mitigate the effects of interspecific pollen deposition. This study improves our understanding of how apocarpous angiosperms with an extragynoecial compitum can maintain species stability and mitigate the negative reproductive interference effect from sympatrically distributed related species.

Pollinator individual-based networks reveal the specialized plant-pollinator mutualism in two biodiverse communities

Generalization of pollination systems is widely accepted by ecologists in the studies of plant-pollinator interaction networks at the community level, but the degree of generalization of pollination networks remains largely unknown at the individual pollinator level. Using potential legitimate pollinators that were constantly visiting flowers in two alpine meadow communities, we analyzed the differences in the pollination network structure between the pollinator individual level and species level. The results showed that compared to the pollinator species-based networks, the linkage density, interaction diversity, interaction evenness, the average plant linkage level, and interaction diversity increased, but connectance, degree of nestedness, the average of pollinator linkage level, and interaction diversity decreased in the pollinator individual-based networks, indicating that pollinator individuals had a narrower food niche than their counterpart species. Pollination networks at the pollinator individual level were more specialized at the network level (H'2) and the plant species node level (d') than at the pollinator species-level networks, reducing the chance of underestimating levels of specialization in pollination systems. The results emphasize that research into pollinator individual-based pollination networks will improve our understanding of the pollination networks at the pollinator species level and the coevolution of flowering plants and pollinators.

Pollination success increases with plant diversity in high-Andean communities

Pollinator-mediated plant–plant interactions have traditionally been viewed within the competition paradigm. However, facilitation via pollinator sharing might be the rule rather than the exception in harsh environments. Moreover, plant diversity could be playing a key role in fostering pollinator-mediated facilitation. Yet, the facilitative effect of plant diversity on pollination remains poorly understood, especially under natural conditions. By examining a total of 9371 stigmas of 88 species from nine high-Andean communities in NW Patagonia, we explored the prevalent sign of the relation between conspecific pollen receipt and heterospecific pollen diversity, and assessed whether the incidence of different outcomes varies with altitude and whether pollen receipt relates to plant diversity. Conspecific pollen receipt increased with heterospecific pollen diversity on stigmas. In all communities, species showed either positive or neutral but never negative relations between the number of heterospecific pollen donor species and conspecific pollen receipt. The incidence of species showing positive relations increased with altitude. Finally, stigmas collected from communities with more co-flowering species had richer heterospecific pollen loads and higher abundance of conspecific pollen grains. Our findings suggest that plant diversity enhances pollination success in high-Andean plant communities. This study emphasizes the importance of plant diversity in fostering indirect plant–plant facilitative interactions in alpine environments, which could promote species coexistence and biodiversity maintenance.

Pollination generalization and reproductive assurance by selfing in a tropical montane ecosystem

Pollination and reproduction are important processes for understanding plant community dynamics. Information regarding pollination and reproduction is urgent for threatened ecosystems, such as tropical montane ecosystems. In tropical mountains, pollination patterns are expected to conform to the reproductive assurance theory (due to low pollinator activity) and old, climatically buffered and infertile landscapes (OCBIL) theory (due to restricted plant range size). For 82 plant species of the Itatiaia National Park (including endemic and endangered species), we evaluated at least one of the following features: pollinator identity, flower color and size, flowering phenology, and pollinator dependence. Most plant species (ca. 60%) were pollinated by two or more functional groups of pollinators (generalized pollination), with high importance of flies as pollinators. There was low pollinator activity overall (less than one visit per flower per hour). Notably, the invasive honeybee Apis mellifera L. performed half of the visits to this entire plant community, suggesting an impact on the native pollinator fauna and consequently on the native flora. Most endemic plants were generalized with white and small flowers, while endangered species were pollination-specialized with colorful and large flowers. Thus, endangered species are susceptible to changes in pollinator fauna. Flowering seasonality reflected the importance of climatic constraints in this environment. One-third of the plant species were autogamous. Our data suggest that pollinator scarcity may have promoted reproductive assurance strategies such as generalization and pollinator independence. Our community-level study highlighted consistent pollination patterns for tropical mountains and emphasized threats for specialized endangered species.

Pollinators contribute to the maintenance of flowering plant diversity

Mechanisms that favour rare species are key to the maintenance of diverse communities^1-3. One of the most critical tasks for conservation of flowering plant biodiversity is to understand how plant-pollinator interactions contribute to the maintenance of rare species^4-7. Here we show that niche partitioning in pollinator use and asymmetric facilitation confer fitness advantage of rarer species in a biodiversity hotspot using phylogenetic structural equation modelling that integrates plant-pollinator and interspecific pollen transfer networks with floral functional traits. Co-flowering species filtered pollinators via floral traits, and rarer species showed greater pollinator specialization leading to higher pollination-mediated male and female fitness than more abundant species. When plants shared pollinator resources, asymmetric facilitation via pollen transport dynamics benefitted the rarer species at the cost of more abundant species, serving as an alternative diversity-promoting mechanism. Our results emphasize the importance of community-wide plant-pollinator interactions that affect reproduction for biodiversity maintenance.

Spatial variation in the intensity of interactions via heterospecific pollen transfer may contribute to local and global patterns of plant diversity

BACKGROUND

Studies that aim to understand the processes that generate and organize plant diversity in nature have a long history in ecology. Among these, the study of plant-plant interactions that take place indirectly via pollinator choice and floral visitation has been paramount. Current evidence, however, indicates that plants can interact more directly via heterospecific pollen (HP) transfer and that these interactions are ubiquitous and can have strong fitness effects. The intensity of HP interactions can also vary spatially, with important implications for floral evolution and community assembly.

SCOPE

Interest in understanding the role of heterospecific pollen transfer in the diversification and organization of plant communities is rapidly rising. The existence of spatial variation in the intensity of species interactions and their role in shaping patterns of diversity is also well recognized. However, after 40 years of research, the importance of spatial variation in HP transfer intensity and effects remains poorly known, and thus we have ignored its potential in shaping patterns of diversity at local and global scales. Here, I develop a conceptual framework and summarize existing evidence for the ecological and evolutionary consequences of spatial variation in HP transfer interactions and outline future directions in this field.

CONCLUSIONS

The drivers of variation in HP transfer discussed here illustrate the high potential for geographic variation in HP intensity and its effects, as well as in the evolutionary responses to HP receipt. So far, the study of pollinator-mediated plant-plant interactions has been almost entirely dominated by studies of pre-pollination interactions even though their outcomes can be influenced by plant-plant interactions that take place on the stigma. It is hence critical that we fully evaluate the consequences and context-dependency of HP transfer interactions in order to gain a more complete understanding of the role that plant-pollinator interactions play in generating and organizing plant biodiversity.

Causes and consequences of variation in heterospecific pollen receipt in Oenothera fruticosa

PREMISE

Heterospecific pollen transfer, the transfer of pollen between species, is common among co-flowering plants, yet the amount of pollen received is extremely variable among species. Intraspecific variation in heterospecific pollen receipt can be even greater, but we lack an understanding of its causes and fitness consequences in wild populations.

METHODS

We examined potential drivers of variation in heterospecific pollen receipt in Oenothera fruticosa. We evaluated the relationship between heterospecific and conspecific pollen receipt and considered how visitation by different pollinator groups, local floral neighborhood composition, and flowering phenology affect the total amount and proportion of heterospecific pollen received. Finally, we tested whether variation in heterospecific pollen receipt translated into lower seed production.

RESULTS

Heterospecific pollen was ubiquitous on O. fruticosa stigmas, but the amount received was highly variable and unrelated to conspecific pollen receipt. Heterospecific pollen receipt depended on pollinator type, the proportion of nearby conspecific flowers, and flowering date. Significant interactions revealed that the effects of pollinator type and neighborhood were not independent, further contributing to variation in heterospecific pollen. Naturally occurring levels of heterospecific pollen were sufficient to negatively impact seed set, but large amounts of conspecific pollen counteracted this detrimental effect.

CONCLUSIONS

Although selection could act on floral traits that attract quality pollinators and promote synchronous flowering in O. fruticosa, the risk of heterospecific pollen is equally dependent on local floral context. This work highlights how extrinsic and intrinsic factors contribute to intraspecific variation in heterospecific pollen receipt in wild plants, with significant fitness consequences.

Pollinator effectiveness is affected by intraindividual behavioral variation

Variation in pollinator quality is fundamental to the evolution of plant-pollinator mutualisms and such variation frequently results from differences in foraging behavior. Surprisingly, despite substantial intraindividual variation in pollinator foraging behavior, the consequences for pollen removal and deposition on flowers are largely unknown. We asked how two pollen foraging behaviors of a generalist pollinator (Bombus impatiens) affect removal and deposition of heterospecific and conspecific pollen, key aspects of pollinator quality, for multiple plant species. In addition, we examined how bee body size and pollen placement among body parts shaped pollen movement. We manipulated foraging behavior types using artificial flowers, which donated pollen that captive bees then deposited on three recipient plant species. While body size primarily affected donor pollen removal, foraging behavior primarily affected donor pollen deposition. How behavior affected donor pollen deposition depended on the plant species and the quantity of donor pollen on the bee's abdomen. Plant species with smaller stigmas received significantly less pollen and fewer bees successfully transferred pollen to them. For a single plant species, heterospecific pollen interfered with conspecific pollen deposition, such that more heterospecific pollen on the bee's abdomen resulted in less conspecific pollen deposition on the flower. Thus, intraindividual variation in foraging behavior and its interaction with the amount and placement of acquired pollen and with floral morphology can affect pollinator quality and may shape plant fitness via both conspecific and heterospecific pollen transfer.

Impacts of plant invasions in native plant-pollinator networks

The disruption of mutualisms by invasive species has consequences for biodiversity loss and ecosystem function. Although invasive plant effects on the pollination of individual native species has been the subject of much study, their impacts on entire plant-pollinator communities are less understood. Community-level studies on plant invasion have mainly focused on two fronts: understanding the mechanisms that mediate their integration; and their effects on plant-pollinator network structure. Here we briefly review current knowledge and propose a more unified framework for evaluating invasive species integration and their effects on plant-pollinator communities. We further outline gaps in our understanding and propose ways to advance knowledge in this field. Specifically, modeling approaches have so far yielded important predictions regarding the outcome and drivers of invasive species effects on plant communities. However, experimental studies that test these predictions in the field are lacking. We further emphasize the need to understand the link between invasive plant effects on pollination network structure and their consequences for native plant population dynamics (population growth). Integrating demographic studies with those on pollination networks is thus key in order to achieve a more predictive understanding of pollinator-mediated effects of invasive species on the persistence of native plant biodiversity.

Diversity and composition of pollen loads carried by pollinators are primarily driven by insect traits, not floral community characteristics

Flowering plants require conspecific pollen to reproduce but they often also receive heterospecific pollen, suggesting that pollinators carry mixed pollen loads. However, little is known about drivers of abundance, diversity or composition of pollen carried by pollinators. Are insect-carried pollen loads shaped by pollinator traits, or do they reflect available floral resources? We quantified pollen on 251 individual bees and 95 flies in a florally diverse community. We scored taxonomic order, sex, body size, hairiness and ecological specialization of pollinators, and recorded composition of available flowers. We used phylogenetically controlled model selection to compare relative influences of pollinator traits and floral resources on abundance, diversity and composition of insect-carried pollen. We tested congruence between composition of pollen loads and available flowers. Pollinator size, specialization and type (female bee, male bee, or fly) described pollen abundance, diversity and composition better than floral diversity. Pollen loads varied widely among insects (10-80,000,000 grains, 1-16 species). Pollen loads of male bees were smaller, but vastly more diverse than those of female bees, and equivalent in size but modestly more diverse than those of flies. Pollen load size and diversity were positively correlated with body size but negatively correlated with insect ecological specialization. These traits also drove variation in taxonomic and phylogenetic composition of insect-carried pollen loads, but composition was only weakly congruent with available floral resources. Qualities of pollinators best predict abundance and diversity of carried pollen indicating that functional composition of pollinator communities may be important to structuring heterospecific pollen transfer among plants.

A half-day flowering pattern helps plants sharing pollinators in an oceanic island community

The temporal pattern of flower opening and closure is a feature of the biology of many plant species, particularly those inhabiting oceanic islands where flowering generally lasts for only a few hours per day. Additionally, flower visitors often seek different floral sources on a timely basis, thus the relative timing of interactions is central to their status in pollination competition, or in the facilitation of pollination among co-flowering plants sharing pollinators. However, few studies have examined the impacts of daily temporal variation in flowering patterns on the pollinator network and competition on a community scale. In order to examine whether the daily pattern of flower opening and closure can impose temporal dynamics on interspecific interactions within a single day, plant–pollinator interaction networks (AM subweb and PM subweb) were quantified, and the relevant interactions between the two subwebs were compared using the Bray–Curtis dissimilarity of visitation frequencies in an oceanic island community (Paracel Islands, South China Sea). The role of species within networks and its variation between two subwebs were assessed by calculating the species-level specialization and species strength of each plant and pollinator species. The quantitative plant–pollinator interaction dissimilarity between morning and afternoon subsets was 0.69, and this value dropped to 0.58 when considering plant species flowering throughout the day. In our study, this dissimilarity between the two subwebs might be explained by the morning peak activity rather than a preference for morning flowers. No significant differences were detected in the species-level specialization and species strength of plants flowering all day from morning to afternoon at the community level. The flower visitation rates of native honeybee Apis cerana were not significantly different between morning and afternoon for most of the whole-day flowering plants. However, plant species only flowering either in the morning or the afternoon differed in the rate of visitation by A. cerana. The analyses of variation in the visitation rates of pollinators shared by plants within a single day in the studied community suggest that daily structuring at a community level and half-day staggered flowering during the morning or afternoon might reduce competitive interactions in oceanic insular habitats.

Breeding systems of floral colour forms in the Drosera cistiflora species complex

Variation in plant breeding systems has implications for pollinator-mediated selection on floral traits and the ecology of populations. Here we evaluate pollinator contribution to seed production, self-compatibility and pollen limitation in different floral colour forms of Drosera cistiflora sensu lato (Droseraceae). These insectivorous perennial plants are endemic to fynbos and renosterveld vegetation in the Cape Floristic Region of South Africa, and the species complex includes five floral colour forms (pink, purple, red, white and yellow), some of which are known to be pollinated by beetles. Controlled hand-pollination experiments were conducted in 15 populations of D. cistiflora s.l. (two to four populations per floral colour form) to test whether the colour forms vary in their degree of self-compatibility and their ability to produce seeds through autonomous self-fertilization. Yellow-flowered forms were highly self-incompatible, while other floral colour forms exhibited partial self-compatibility. Seed set resulting from autonomous selfing was very low, and pollinator dependence indices were high in all populations. Since hand cross-pollination resulted in greater seed set than open pollination in 13 of the 15 populations, we inferred that seed production is generally pollen-limited. Drosera cistiflora s.l. typically exhibits high levels of pollinator dependence and pollen limitation. This is unusual among Drosera species worldwide and suggests that pollinators are likely to mediate strong selection on attractive traits such as floral colour and size in D. cistiflora s.l. These results also suggest that the floral colour forms of D. cistiflora s.l. which are rare and threatened are likely to be vulnerable to local extinction if mutualisms were to collapse indefinitely.

Post-pollination barriers enable coexistence of pollinator-sharing ornithophilous Erica species

Some evolutionary radiations produce a number of closely-related species that continue to coexist. In such plant systems, when pre-pollination barriers are weak, relatively strong post-pollination reproductive barriers are required to maintain species boundaries. Even when post-pollination barriers are in place, however, reproductive interference and pollinator dependence may strengthen selection for pre-pollination barriers. We assessed whether coexistence of species from the unusually speciose Erica genus in the fynbos biome, South Africa, is enabled through pre-pollination or post-pollination barriers. We also tested for reproductive interference and pollinator dependence. We investigated this in natural populations of three bird-pollinated Erica species (Erica plukenetii, E. curviflora and E. coccinea), which form part of a large guild of congeneric species that co-flower and share a single pollinator species (Orange-breasted Sunbird Anthobaphes violacea). At least two of the three pre-pollination barriers assessed (distribution ranges, flowering phenology and flower morphology) were weak in each species pair. Hand-pollination experiments revealed that seed set from heterospecific pollination (average 8%) was significantly lower than seed set from outcross pollination (average 50%), supporting the hypothesis that species boundaries are maintained through post-pollination barriers. Reproductive interference, assessed in one population by applying outcross pollen three hours after applying heterospecific pollen, significantly reduced seed set compared to outcross pollen alone. This may drive selection for traits that enhance pre-pollination barriers, particularly given that two of the three species were self-sterile, and therefore pollinator dependent. This study suggests that post-pollination reproductive barriers could facilitate the coexistence of congeneric species, in a recent radiation with weak pre-pollination reproductive barriers.

Floral traits are associated with the quality but not quantity of heterospecific stigmatic pollen loads

Background

In flowering communities, plant species commonly share pollinators and therefore plant individuals receive heterospecific pollen (HP). However, the patterns of HP transfers can deviate from patterns of plant-pollinator visitations. Although flower-visitor interactions are known to be mediated by floral traits, e.g. floral size or nectar tube depth, the explanatory power of these traits for HP transfer patterns remains elusive. Here, we have explored pollen transfer patterns at three sites in Southern Germany on three dates (early, mid and late summer). At the plant level, we tested whether flower abundance and floral traits are correlated with HP reception and donation. At the community level, we determined whether flower and bee diversity are correlated with network modularity and whether floral traits explain the module affiliation of plant species. We collected the stigmas of flowering plant species, analysed HP and conspecific pollen (CP) loads and measured floral traits, flower and bee diversity.

Results

Our results show that the degree and intensity of HP reception or donation at the plant level do not correlate with floral traits, whereas at the community level, the module affiliation of who is sharing pollen with whom is well-explained by floral traits. Additionally, variation in network modularity between communities is better explained by plant diversity and abundance than by bee diversity and abundance.

Conclusions

Overall, our results indicate that floral traits that are known to mediate flower-visitor interactions can improve our understanding of qualitative HP transfer but only provide limited information about the quantity of HP transfer, which more probably depends on other floral traits, flower-visitor identity or community properties.

Foraging behavior and pollination efficiency of generalist insects in an understory dioecious shrub Helwingia japonica

PREMISE

It has been hypothesized that pollination success in animal-pollinated dioecious plants relies on opportunistic pollinators with no discrimination against female flowers. However, empirical studies of pollinator foraging behavior and pollination effectiveness in dioecious species are few.

METHODS

To investigate potential pollinators in Helwingia japonica, a dioecious shrub with small, inconspicuous flowers, we compared floral visitors and visit frequency to female and male plants for three flowering seasons in two field populations in subtropical forests in southwest China. Pollen placement on the insect bodies of four groups (solitary bees and other bees, fungus gnats, and other flies) was compared, and insect foraging behavior was observed. Pollen removal and conspecific and heterospecific pollen deposition per visit were measured to compare pollination effectiveness among the four groups.

RESULTS

Floral visitors usually did not discriminate between male and female flowers and did not gather pollen into collections. Our measurements of pollen transfer efficiency showed that solitary bees were the most effective pollinators with the highest conspecific pollen deposition. These insects seemed to be opportunistic visitors because pollen grains of H. japonica were distributed evenly over different regions of the visitor's body, and heterospecific pollen accounted for over 50% of total pollen loads on stigmas in the two populations.

CONCLUSIONS

Our investigations indicated that potential pollinators were generalists and did not discriminate against female flowers, as predicted for dioecious species pollinated by insects. A perspective of pollen removal by floral visitors offers insights into the evolution of plant sexual systems.

Pollen grain size associated with pollinator feeding strategy

Angiosperm pollen grain diameter varies greatly from a few microns to over 100, but the selective forces driving the interspecific variation in pollen size remain unclear. Although both pre- and post-pollination hypotheses have been proposed, empirical evidence remains scarce. Here we propose that visits by pollen-foraging pollinators have selected against large pollen grains. An association between pollinator behaviour and pollen grain size was confirmed by field studies of 80 flowering species in natural communities, showing that pollinators positively collected pollen in those species with relatively smaller pollen grains but rarely did so in species with larger ones. Allowing for the confounding effects of pollinator type, flower size or style length and pollen grain number, we found a significant effect of pollen-foraging behaviour on variation in pollen grain size, particularly in bee-pollinated plants. While these results suggest that many plant species whose pollen is collected or consumed by pollinators produce small pollen grains, it remains unclear whether pollen grain size is directly affected by pollinator foraging habit or indirectly mediated by pollen number trade-offs.

Effect of heterospecific pollen deposition on pollen tube growth depends on the phylogenetic relatedness between donor and recipient

Co-flowering plant species may interact via pollinators leading to heterospecific pollen transfer with consequences for plant reproduction. What determines the severity of heterospecific pollen effect on conspecific pollen performance is unclear, but it may depend on the phylogenetic relatedness of the interactors (pollen donors and recipient). The heterospecific pollen effect might also depend on the extent to which plants are exposed to heterospecific pollen over ecological or evolutionary timescales. For instance, generalist-pollinated plant species might tolerate heterospecific pollen more than specialists. Here, we tested whether heterospecific pollen effects are stronger between closely related species than phylogenetically distant ones in a tropical highland community. Then, based on these results, we determined whether responses to heterospecific pollen were stronger in generalized vs. specialized plant species. We applied heterospecific pollen from close (congeneric) or distant (different families) donors alone or with conspecific pollen on stigmas of three recipient species (one generalist, Sisyrinchium wettsteinii; and two specialists, Fuchsia campos-portoi and Fuchsia regia) and scored pollen tube performance in styles. In all species, pollen from closely related donors grew pollen tubes to the base of the style indicating a high potential to interfere with seed set. Conversely, distantly related heterospecific pollen had no effect on either specialist Fuchsia species, whereas enhanced performance of conspecific pollen was observed in generalist S. wettsteinii. The strong effect of phylogenetic relatedness of donor and recipient might have obscured the role of pollination specialization, at least for the three species examined here. Therefore, phylogenetic relatedness mediated the effect of heterospecific pollen on post-pollination success, with possible consequences for reproductive trait evolution and community assembly for further studies to explore.

Pollen on stigmas as proxies of pollinator competition and facilitation: complexities, caveats and future directions

BACKGROUND

Pollen transfer via animals is necessary for reproduction by ~80 % of flowering plants, and most of these plants live in multispecies communities where they can share pollinators. While diffuse plant-pollinator interactions are increasingly recognized as the rule rather than the exception, their fitness consequences cannot be deduced from flower visitation alone, so other proxies, functionally closer to seed production and amenable for use in a broad variety of diverse communities, are necessary.

SCOPE

We conceptually summarize how the study of pollen on stigmas of spent flowers can reflect key drivers and functional aspects of the plant-pollinator interaction (e.g. competition, facilitation or commensalism). We critically evaluate how variable visitation rates and other factors (pollinator pool and floral avoidance) can give rise to different relationships between heterospecific pollen and (1) conspecific pollen on the stigma and (2) conspecific tubes/grain in the style, revealing the complexity of potential interpretations. We advise on best practices for using these proxies, noting the assumptions and caveats involved in their use, and explicate what additional data are required to verify interpretation of given patterns.

CONCLUSIONS

We conclude that characterizing pollen on stigmas of spent flowers provides an attainable indirect measure of pollination interactions, but given the complex processes of pollen transfer that generate patterns of conspecific-heterospecific pollen on stigmas these cannot alone determine whether competition or facilitation are the underlying drivers. Thus, functional tests are also needed to validate these hypotheses.

Importance of Pollinator-Mediated Interspecific Pollen Transfer for Angiosperm Evolution

Understanding how pollen moves between species is critical to understanding speciation, diversification, and evolution of flowering plants. For co-flowering species that share pollinators, competition through interspecific pollen transfer (IPT) can profoundly impact floral evolution, decreasing female fitness via heterospecific pollen deposition on stigmas and male fitness via pollen misplacement during visits to heterospecific flowers. The pollination literature demonstrates that such reproductive interference frequently selects for reproductive character displacement in floral traits linked to pollinator attraction, pollen placement, and mating systems and has also revealed that IPT between given pairs of species is typically asymmetric. More recent work is starting to elucidate its importance to the speciation process, clarifying the link between IPT and current and historical patterns of hybridization, the evolution of phenotypic novelty through adaptive introgression, and the rise of reproductive isolation. Our review aims to stimulate further research on IPT as a ubiquitous mechanism that plays a central role in angiosperm diversification.

Color-matching between pollen and corolla: hiding pollen via visual crypsis?

Visual signals attractive to friends may also attract enemies.  The bright colors of anthers and pollen have generally been thought to attract pollinators. We hypothesize that visual crypsis of anthers, and particularly pollen, should be favored in flowering plants because protection from pollen collectors reduces the loss of male gametes. To understand adaptive strategies relating to the color of pollen, we measured the color of pollen, undehisced anther sacs, and their background, the corolla, with a spectrometer for 104 insect-pollinated flowering species from a natural community in Hengduan Mountains, southwest China. The colors of anthers, pollen and corollas were diverse in these species. The color diversity of exposed pollen was significantly higher than that of concealed pollen (i.e. where anthers are enclosed or shielded by corollas). The color contrast between pollen and corolla was significantly smaller in species with exposed pollen than in those with concealed pollen. Unlike anther color, exposed pollen color tended to match its background corolla color. Our phylogenetic comparative analysis showed contrasting effects of pollen color patterns between flowers with exposed pollen and those with concealed pollen, revealing a strategy of hiding pollen from pollen thieves via visual crypsis.

Patterns and effects of heterospecific pollen transfer between an invasive and two native plant species: the importance of pollen arrival time to the stigma

PREMISE

Invasive plant species can integrate into native plant-pollinator communities, but the underlying mechanisms are poorly understood. Competitive interactions between invasive and native plants via heterospecific pollen (HP) and differential invasive HP effects depending on HP arrival time to the stigma may mediate invasion success, but these have been little studied.

METHODS

We evaluated patterns and effects of HP receipt on pollen tube growth in two native and one invasive species in the field. We also used hand-pollination experiments to evaluate the effect of invasive HP pollen and its arrival time on native reproductive success.

RESULTS

Native species receive smaller and less-diverse HP loads (5-7 species) compared to invasive species (10 species). The load size of HP had a negative effect on the proportion of pollen tubes in both native species but not in the invasive, suggesting higher HP tolerance in the latter. Invasive HP arrival time differentially affected pollen tube success in native species.

CONCLUSIONS

Our results highlight the need to study reciprocal HP effects between invasive and native species and the factors that determine differential responses to HP receipt to fully understand the mechanisms facilitating invasive species integration into native plant-pollinator communities.

Plant species dominance increases pollination complementarity and plant reproductive function

Worldwide, anthropogenic change is causing biodiversity loss, disrupting many critical ecosystem functions. Most studies investigating the relationship between biodiversity and ecosystem functioning focus on species richness, predominantly within the context of productivity-related functions. Consequently, there is limited understanding of how other biodiversity measures, such as species evenness (the distribution of abundance among species), affect complex multitrophic functions such as pollination. We explore the effect of species evenness on the ecosystem function of pollination using a controlled experiment with selected plants and insects in flight cages. We manipulated the relative abundances of plant and pollinator species, while holding species richness, composition, dominance order, and total abundance constant. Then, we tested how numerical species evenness affected network structure and consequently, seed production, in our artificial communities. Contrary to our expectation, numerical dominance in plant communities increased complementarity in pollinator use (reduced pollinator sharing) among plant species. As predicted by theory, this increased complementarity resulted in higher seed production for the most dominant and rare plant species in our cages. Our results show that in a controlled experimental setting, numerical species evenness can alter important aspects of plant-pollinator networks and plant reproduction, irrespective of species richness, composition, and total abundance. Extending this understanding of how species evenness affects ecosystem functioning to natural systems is crucial as anthropogenic disturbances continue to alter species' abundances, likely disrupting ecosystem functions long before extinctions occur.

Global geographic patterns of heterospecific pollen receipt help uncover potential ecological and evolutionary impacts across plant communities worldwide

Species interactions are known to be key in driving patterns of biodiversity across the globe. Plant-plant interactions through heterospecific pollen (HP) transfer by their shared pollinators is common and has consequences for plant reproductive success and floral evolution, and thus has the potential to influence global patterns of biodiversity and plant community assembly. The literature on HP transfer is growing and it is therefore timely to review patterns and causes of among-species variation in HP receipt at a global scale, thus uncovering its potential contribution to global patterns of biodiversity. Here we analyzed published data on 245 species distributed across five continents to evaluate latitudinal and altitudinal patterns of HP receipt. We further analyzed the role of floral symmetry and evolutionary history in mediating patterns of HP receipt. Latitude and elevation affected the likelihood and intensity of HP receipt indicating that HP transfer increases in species-rich communities and in areas with high abundance of vertebrate pollinators. Floral symmetry and evolutionary history determined HP load size across plant communities worldwide. Overall, our results suggest that HP receipt may have the potential to contribute to global geographic patterns of plant diversity by imposing strong selective pressures in species-rich areas across the globe.

Pollen grain morphology is not exclusively responsible for pollen collectability in bumble bees

Bee-pollinated plants face a dilemma in that bees both passively transport pollen grains among conspecific flowers and actively collect pollen to feed their larvae. Therefore, mechanisms that reduce pollen collection by bees have evolved in melittophilous plants. Malvaceae pollen is uncollectable for corbiculate bees which has previously been ascribed to pollen size, spines, and pollenkitt. We analysed the influence of pollen grain properties (diameter, spine length, spine density) on the collectability of echinate (spiny) pollen by bumble bees (Bombus terrestris). Workers individually foraging on one of eight plant species from six families performed significantly less pollen foraging on plants which have large, echinate pollen grains. Nevertheless, neither pollen grain size, spine length, nor spine density prove to be an absolute disqualifier for collectability. While pollen foragers did not shift to nectar collection but seized visiting flowers with uncollectable pollen, nectar foragers performed regular foraging bouts on these plants. Pollen that is uncollectable for corbiculate bees limits pollen depletion by generalist bumble bees and probably also honey bees while maintaining them as pollinators, which is an effective solution to the pollen dilemma. As previous assumptions about the impact of pollen morphology on its collectability are disproved, potentially determining factors are discussed.

Effects of spatial patterning of co-flowering plant species on pollination quantity and purity

BACKGROUND AND AIMS

If two plant species share pollinators, it has been proposed that the interaction between them may range from competitive to facilitative, depending on the way in which they intermingle. In particular, the presence of a rewarding plant species may increase the rate of pollinator visitation to a less rewarding species in its vicinity, but the beneficial increase in visitation may be counteracted by a detrimental increase in heterospecific pollen transfer. We assessed this trade-off using bumble-bees foraging over a gradual spatial transition between two plant species in an indoor cage experiment.

METHODS

We used two 'species' of artificial flowers - one more rewarding than the other - in arrays that varied in the degree of species intermingling. The flowers dispensed and received powdered food dyes serving as pollen analogues. Captive bumble-bees visited to collect sucrose solution. We quantified dye delivery to the adhesive-tape 'stigmas' in flowers by spectrophotometry.

KEY RESULTS

Across the spatial transition between species, the less attractive species received more dye (more bee visits) when in proximity to the more attractive species than it did when alone, but the larger dye loads were less pure (more heterospecific pollen transfer). The decline in purity cancelled out the gain in acquisition, so conspecific pollen receipt by the less attractive species was neutrally affected. The more attractive species received fewer visits when surrounded by the less attractive species, so the interaction between the two species was amensalism when considering conspecific pollen reception.

CONCLUSIONS

Pollinator-mediated interactions between plant species depend on pollination quantity and purity, both of which can depend on spatial intermingling.

Consequences of invasion for pollen transfer and pollination revealed in a tropical island ecosystem

Pollination is known to be sensitive to environmental change but we lack direct estimates of how quantity and quality of pollen transferred between plant species shifts along disturbance gradients. This limits our understanding of how species compositional change impacts pollen receipt per species and structure of pollen transfer networks. We constructed pollen transfer networks along a plant invasion gradient in the Hawaiian dry tropical forest ecosystem. Flowers and stigmas were collected from both native and introduced plants, pollen was identified and enumerated and floral traits were measured. We also characterized pollen loads carried by individuals of the dominant invasive pollinator, Apis mellifera. Species flowering in native-dominated sites were more tightly connected by pollen transfer than those in heavily invaded sites. Compositional turnover in the pollen loads of A. mellifera was correlated (70%) with turnover in the composition of pollen transfer networks. Floral traits predicted species roles within pollen transfer networks, but many of these differed qualitatively depending on whether plants were native or introduced. Our work indicates that pollen transfer networks change with invasion. Floral morphology and foraging behaviour of the introduced super-generalist pollinator are implicated as key in determining the roles introduced species play within native pollen transfer networks.

A paradoxical mismatch between interspecific pollinator moves and heterospecific pollen receipt in a natural community

Pollinators visiting multiple plant species may cause heterospecific pollen transfer (HPT). To test a null model that more pollinator interspecific moves result in higher HPT among interacting species, we quantified the comparative magnitudes of the two networks involving 14 co-flowering species in an alpine meadow in the eastern Himalaya, southwest China. Interspecific moves accounted for 4% of the total visits,whereas heterospecific pollen constituted 22% of the total stigmatic pollen loads. On average, plant species received interspecific moves and HPT from 6.9 and 9.7 other species, respectively. Although the two networks were largely concordant, 21.6% of interspecific move links were not correspondingly linked by HPT, and 44.1% of heterospecific pollen transfer links were not linked by moves. Plant species with more outgoing pollinator moves tended to disperse more of their own pollen to others, as expected. Surprisingly, our data reveal that plant species which received more pollinator moves from other species tended to receive less HP, implying that only species with low acceptance of HP were likely to permit frequent pollinator moves. These new findings unveil a paradoxical relation between pollinator interspecific moves and HP receipt, suggesting an adaptive strategy of co-flowering species that reduces deleterious effects of HPT.

Variation in sampling effort affects the observed richness of plant-plant interactions via heterospecific pollen transfer: implications for interpretation of pollen transfer networks

PREMISE OF THE STUDY

There is growing interest in understanding plant-plant interactions via pollen transfer at the community level. Studies on the structure and spatial variability of pollen transfer networks have been valuable to this understanding. However, there is high variability in the intensity of sampling used to characterize pollen transfer interactions, which could influence network structure. To date, there is no knowledge of how sampling effort influences the richness of pollen on stigmas and thereby transfer interactions observed, nor how this may vary across species and study sites.

METHODS

We use rarefaction curves on 16 species to characterize the relationship between sampling effort (number of stigmas analyzed) and the richness of pollen transfer interactions recorded. We further assess variability in this relationship among species, plant community types, and sites within a single plant community.

KEY RESULTS

We show high among-species variation in the amount of sampling required to sufficiently characterize interspecific pollen transfer. We further reveal variability in the sampling effort-interaction richness relationship among different plant communities and even for the same species growing in different sites.

CONCLUSIONS

The wide heterogeneity in the sampling effort required to accurately characterize pollen transfer interactions observed has the potential to influence the characterization of pollen transfer dynamics. Thus, sampling completeness should be considered in future studies to avoid overestimation of modularity and specialization in pollen transfer networks that may bias the predicted causes and expected consequences of such processes for plant-plant interactions.

Pollination ecology in China from 1977 to 2017

China is one of most biodiverse countries in the world, containing at least 10% of all angiosperm species. Therefore, we should anticipate a diverse, pollinator fauna. China also has a long history of applied ethnobiology, including a sustainable agriculture based on apiculture and plant-pollinator interactions. However, the science of pollination ecology is a far younger sub-discipline in China, compared to in the West. Chinese studies in pollination ecology began in the 1970s. For this review, we compiled a complete reference database (>600 publications) of pollination studies in China. Using this database, we identified and analyzed gaps and limitations in research on the pollination systems of native and naturalized species. Specifically, we asked the following questions: 1) What do we know about the pollination systems of native, Chinese species? 2) How does Chinese pollination ecology compare with the development of pollination research abroad and which aspects of research should be pursued by Chinese anthecologists in the near future? 3) What research on pollination in China will advance our understanding and contribute to our ongoing analyses of endemism and conservation? Subsequently, we segregated and identified prospective lines of future research that are unique to China and can only be done in China. This requires discussing priorities within a systematic approach.

Comparative intra- and interspecific sexual organ reciprocity in four distylous Primula species in the Himalaya-Hengduan Mountains

Distyly is a mechanism promoting cross-pollination within a balanced polymorphism. Numerous studies show that the degree of inter-morph sexual organ reciprocity (SOR) within species relates to its pollen-mediated gene flow. Similarly, a lower interspecific SOR should promote interspecific isolation when congeners are sympatric, co-blooming and share pollinators. In this comparative study, we address the significance of SOR at both intra- and interspecific levels. Seventeen allopatric and eight sympatric populations representing four Primula species (P. anisodora, P. beesiana, P. bulleyana and P. poissonii) native to the Himalaya-Hengduan Mountains were measured for eight floral traits in both long- and short-styled morphs. GLMM and spatial overlap methods were used to compare intra- and interspecific SOR. While floral morphology differed among four Primula species, SOR within species was generally higher than between species, but in species pairs P. poissonii/P. anisodora and P. beesiana/P. bulleyana, the SOR was high at both intra- and interspecific levels. We did not detect a significant variation in intraspecific SOR or interspecific SOR when comparing allopatric versus sympatric populations for all species studied. As intraspecific SOR increased, disassortative mating may be promoted. As interspecific SOR decreased, interspecific isolation between co-flowering species pairs also may increase. Hybridisation between congeners occurred when interspecific SOR increased in sympatric populations, as confirmed in two species pairs, P. poissonii/P. anisodora and P. beesiana/P. bulleyana.