Pollen Transfer Research Articles

Co-flowering neighbor alters pollinator composition and influences reproductive success in a plant pollinated by multiple insects

The effect of the interactions among co-flowering plants for pollination has always been explained by changes in pollinator visiting frequency and/or foraging behavior. For most flowering plants with generalized pollination system, little is known about the influence of co-flowering neighbor on pollinator composition and its ecological consequences. Mazus miquelii, a plant with sensitively bilobed stigma, and its two naturally common co-flowering plants, Glechoma longituba and Ajuga decumbens were used to set the study system. We investigated pollinators’ visiting frequency and behavior, visiting frequency and relative composition of each pollinator type as well as seed production. Our results indicated that plant reproductive success significantly differed with co-flowering plants. However, neither pollinators’ visiting frequency nor foraging behavior could be attributed to the influence of co-flowering neighbor on reproductive success of M. miquelii. Flowers of M. miquelii were pollinated by more individuals of Osmia sp. when co-occurring with A. decumbens, while Halictus sp. was the main pollinator of M. miquelii when it was co-flowering with G. longituba. Comparatively, insects of Osmia sp. had higher effectiveness of pollen transfer than those of Halictus sp. The stigmatic pollen load of M. miquelii was higher co-flowering with A. decumbens than with G. longituba. The co-flowering neighbor significantly altered the pollinator composition and influenced pollen transfer efficiency, which might definitely be responsible for the changes in the focal plant reproductive success. This study improved our understanding on pollinator-mediated plant–plant interactions, particularly the uncertain effects of co-flowering plants on a focal plant from different communities.

Patterns of floral nectar standing crops allow plants to manipulate their pollinators

‘Pollination syndromes’ involving floral nectar have eluded satisfactory evolutionary explanation. For example, floral nectars for vertebrate-pollinated plants average low sugar concentrations, while such animals prefer high concentrations, perplexing pollination biologists and arousing recent controversy. Such relationships should result from evolutionary games, with plants and pollinators adopting Evolutionarily Stable Strategies, and nectar manipulating rather than attracting pollinators. Plant potential to manipulate pollinators depends on relationships between neighbouring flowers within plants, for all nectar attributes, but this has not been investigated. We measured nectar volume, concentration and sugar composition for open flowers on naturally-growing Blandfordia grandiflora plants, presenting classic bird-pollinated plant syndrome. To evaluate potential pollinator manipulation through nectar, we analysed relationships between neighbouring flowers for nectar volume, concentration, proportion sucrose, log(fructose/glucose), and sugar weight. To evaluate potential attraction of repeat-visits to flowers or plants through nectar, we compared attributes between successive days. Nearby flowers were positively correlated for all attributes, except log(fructose/glucose) as fructose≈glucose. Most relationships between nectar attributes for flowers and plants on successive days were non-significant. Nectar-feeding pollinators should therefore decide whether to visit another flower on a plant, based on all attributes of nectar just-obtained, enabling plants to manipulate pollinators through adjusting nectar. Plants are unlikely to attract repeat pollinator-visits through nectar production. Floral nectar evolution is conceptually straightforward but empirically challenging. A mutant plant deviating from the population in attributes of nectar-production per flower would manipulate, rather than attract, nectar-feeding pollinators, altering pollen transfer, hence reproduction. However, links between floral nectar and plant fitness present empirical difficulties.

A fishing expedition to discover the pollinators of several subtropical Apocynaceae.

PremiseFlowers of the Apocynaceae (milkweed family) have complex structures and pollination mechanisms. Pollen removal and deposition in Angadenia, Pentalinon, and Echites are similar, with anthers releasing pollen onto the sterile style head. The mid‐style head excretes a glue that coats the mouthparts of vistors to aid in the transfer of pollen. Subsequent probes may deposit pollen on the receptive stigmatic surface on the lowest part of the style head, with fertilization resulting after pollination by compatible pollen.MethodsBy employing fishing line of different diameters, which reflected the diameters of the mouthparts of the different insect visitors, we determined the widths best able to remove and deposit pollen, thereby revealing which of the visitors could be effective pollinators, and which may be only nectar robbers.ResultsWe previously found that mouthpart (proboscis) width is correlated with pollen transfer effectiveness in Angadenia berteroi and confirmed this here in two other species, Pentalinon luteum and Echites umbellatus. Our data allowed the prediction of the most effective pollinators of these two other species.DiscussionThe simulation of flower visitor mouthparts using fishing line can provide useful data for evaluating the potential for effective pollen removal and deposition by different visitors.

Sexual and natural selection on pollen morphology in Taraxacum.

Spiny pollen has evolved independently in multiple entomophilous lineages. Sexual selection may act on exine traits that facilitate male mating success by influencing the transfer of pollen from the anther to the body of the pollinator, while natural selection acts to increase pollen survival. We postulated that relative to sexual congeners, apomictic dandelions undergo relaxed selection on traits associated with male mating success. We explored sexual selection on exine traits by measuring the propensity for Taraxacum spp. pollen to attach to hairs of flower-visiting bumblebees (Bombus spp.) or flies (Diptera: Syrphidae and Muscoidea) and assessed natural selection by testing whether pollen traits defend against consumption. Pollen picked up by bumblebees exhibited a narrower subset of spine-spacing phenotypes, consistent with stabilizing selection. Flies picked up larger pollen from flowers than expected at random. Surveys of corbiculae (pollen basket) contents from foraging bumblebees and feces of flies showed that pollen grains consumed by both kinds of visitors are similar in spine characteristics and size to those produced by the donor. When bees visit inflorescences of apomictic T. officinale, they pick up pollen with spine-spacing phenotypes above the mean and shifted toward those of sexual T. ceratophorum. We demonstrate that traits under sexual selection during pollen pickup vary among pollinators, while natural selection for pollen defense is nil in T. ceratophorum. In hybrid zones between apomictic and sexual dandelions, pollen traits place apomictic donors at a dispersal disadvantage, potentially reinforcing reproductive isolation.

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

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. 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. 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.

Floral temperature patterns can function as floral guides

Floral guides are signal patterns that lead pollinators to floral rewards after they have located the flower, and increase foraging efficiency and pollen transfer. Patterns of several floral signalling modalities, particularly colour patterns, have been identified as being able to function as floral guides. Floral temperature frequently shows patterns that can be used by bumblebees for locating and recognising the flower, but whether these temperature patterns can function as a floral guide has not been explored. Furthermore, how combined patterns (using multiple signalling modalities) affect floral guide function has only been investigated in a few modality combinations. We assessed how artificial flowers induce behaviours in bumblebees when rewards are indicated by unimodal temperature patterns, unimodal colour patterns or multimodal combinations of these. Bees visiting flowers with unimodal temperature patterns showed an increased probability of finding rewards and increased learning of reward location, compared to bees visiting flowers without patterns. However, flowers with contrasting unimodal colour patterns showed further guide-related behavioural changes in addition to these, such as reduced reward search times and attraction to the rewarding feeder without learning. This shows that temperature patterns alone can function as a floral guide, but with reduced efficiency. When temperature patterns were added to colour patterns, bees showed similar improvements in learning reward location and reducing their number of failed visits in addition to the responses seen to colour patterns. This demonstrates that temperature pattern guides can have beneficial effects on flower handling both when alone or alongside colour patterns.

Character displacement drives floral variation in Pelargonium (Geraniaceae) communities.

Interactions between plant community members are an underexplored driver of angiosperm floral variation. We investigate character displacement as a potential contributor to floral variation in Pelargonium communities. Pelargoniums all place pollen on the ventral sides of their pollinators, potentially leading to interspecific pollen transfer (IPT) in sympatry. We show that the positions of pollen placement and receipt are determined by anther and style exsertion lengths. Using field experiments, we demonstrate that heterospecific species experience higher IPT if they have similar style lengths than when they have greater style length differences. Using crosses, we show that IPT has negative consequences on seed set. In combination, these results suggest that character displacement in style length is likely to reduce IPT and increase female fitness in sympatry. Patterns of style length variation across 29 different Pelargonium communities suggest that character displacement has occurred in multiple communities. Furthermore, analyses using a wide-ranging species pair show that style lengths are more different between sympatric populations than they are between allopatric populations. In addition to pollinators as agents of floral divergence, this study suggests that variation in Pelargonium community structure has driven style length variation through character displacement.

Habitat partitioning in native Geranium species through reproductive interference.

Heterospecific pollen transfer may reduce the fitness of recipient species, a phenomenon known as reproductive interference. A theoretical study has predicted that distributions of species pairs affected by reproductive interference may be syntopic under negligible reproductive interference, sympatric but with partitioning at small spatial scale (i.e. allotopic) under weak interference, or exclusive when reproductive interference is strong. Verifying these predictions is essential for evaluation of the applicability of reproductive interference as a general assembly rule of biological communities. The aim of this study was to test these predictions in two sympatrically distributed wild Geranium species, G. thunbergii and G. wilfordii. To measure the effect of reproductive interference, the associations between the relative abundance of the counterpart species and seed set in the focal species, and seed set reduction following mixed pollination, were analysed. The possibility of hybridization with viable offspring was examined by genotyping plants in the field and after mixed pollination. Fertility of putative hybrids was based on their seed set and the proportion of pollen grains with apertural protrusions. A transect study was conducted to examine spatial partitioning, and possible influences of environmental conditions (canopy openness and soil moisture content) on partitioning between the species were analysed. Neither abundance of the counterpart species nor heterospecific pollen deposition significantly affected seed set in the focal species, and hybridization between species was almost symmetrical. Putative hybrids had low fertility. The two species were exclusively distributed at small scale, although environmental conditions were not significantly different between them. The allotopy of the two species may be maintained by relatively weak reproductive interference through bidirectional hybridization. Re-evaluation of hybridization may allow ongoing or past reproductive interference to be recognized and provide insight into the distributional relationships between the interacting plants.

Mating strategies dictate the importance of insect visits to native plants in urban fragments

Plant species conservation relies on their reproductive success and likelihood of population persistence. Knowledge of plant mating systems, particularly the relationship between plants and their pollinators, is fundamental to inform conservation efforts. This knowledge could be critical for prioritising efforts in human-dominated fragmented landscapes such as the world’s biodiversity hotspots, where reproductive success can be compromised due to habitat loss, limited access to pollinators or other factors. Yet, fundamental data on plant mating systems are lacking for many Australian plants. Here we determined the mating systems of native plant species growing in native woodland fragments within Perth’s urban landscape in south-western Australia. We manipulated insect access to flowers and pollen transfer on five locally common native species, then observed floral visitors and examined reproductive success. Hemiandra pungens and Patersonia occidentalis had mixed mating systems with some ability to self-pollinate, whereas Dianella revoluta and Jacksonia sericea were reliant on insects for outcross pollination. The fruits and seeds produced by Tricoryne elatior were too low to draw conclusions about its mating system. The introduced honey bee (Apis mellifera) was the sole visitor to the mixed mating species, whereas native bees visited D. revoluta and J. sericea (one bee species each). Overall, our data suggest that D. revoluta and J. sericea are more vulnerable to fragmentation than H. pungens and P. occidentalis. Although insects contributed significantly to the reproductive output of the two former plant species, our observations suggested low frequency and richness of insect visitors to these urban fragments. More research is required to determine the generality of our findings. A comparative study in larger native woodland fragments would help estimate the effect of fragmentation on insect pollinators and consequences for the insect-reliant plant species.

Population Genomics of Transitions to Selfing in Brassicaceae Model Systems.

Many plants harbor complex mechanisms that promote outcrossing and efficient pollen transfer. These include floral adaptations as well as genetic mechanisms, such as molecular self-incompatibility (SI) systems. The maintenance of such systems over long evolutionary timescales suggests that outcrossing is favorable over a broad range of conditions. Conversely, SI has repeatedly been lost, often in association with transitions to self-fertilization (selfing). This transition is favored when the short-term advantages of selfing outweigh the costs, primarily inbreeding depression. The transition to selfing is expected to have major effects on population genetic variation and adaptive potential, as well as on genome evolution. In the Brassicaceae, many studies on the population genetic, gene regulatory, and genomic effects of selfing have centered on the model plant Arabidopsis thaliana and the crucifer genus Capsella. The accumulation of population genomics datasets have allowed detailed investigation of where, when and how the transition to selfing occurred. Future studies will take advantage of the development of population genetics theory on the impact of selfing, especially regarding positive selection. Furthermore, investigation of systems including recent transitions to selfing, mixed mating populations and/or multiple independent replicates of the same transition will facilitate dissecting the effects of mating system variation from processes driven by demography.

Active pollination drives selection for reduced pollen-ovule ratios.

Variation in pollen-ovule ratios is thought to reflect the degree of pollen transfer efficiency-the more efficient the process, the fewer pollen grains needed. Few studies have directly examined the relationship between pollen-ovule ratio and pollen transfer efficiency. For active pollination in the pollination brood mutualisms of yuccas and yucca moths, figs and fig wasps, senita and senita moths, and leafflowers and leafflower moths, pollinators purposefully collect pollen and place it directly on the stigmatic surface of conspecific flowers. The tight coupling of insect reproductive interests with pollination of the flowers in which larvae develop ensures that pollination is highly efficient. We used the multiple evolutionary transitions between passive pollination and more efficient active pollination to test if increased pollen transfer efficiency leads to reduced pollen-ovule ratios. We collected pollen and ovule data from a suite of plant species from each of the pollination brood mutualisms and used phylogenetically controlled tests and sister-group comparisons to examine whether the shift to active pollination resulted in reduced pollen-ovule ratios. Across all transitions between passive and active pollination in plants, actively pollinated plants had significantly lower pollen-ovule ratios than closely related passively pollinated taxa. Phylogenetically corrected comparisons demonstrated that actively pollinated plant species had an average 76% reduction in the pollen-ovule ratio. The results for active pollination systems support the general utility of pollen-ovule ratios as indicators of pollination efficiency and the central importance of pollen transfer efficiency in the evolution of pollen-ovule ratio.

Pollination biology in Roepera (Zygophyllaceae): How flower structure and shape influence foraging activity

AbstractThe foraging behavior of bees is a complex phenomenon that depends on numerous physical features of flowers. Of particular importance are accessibility of floral rewards, floral proportions, symmetry and orientation. The flowers of Roepera are characterized by the presence of staminal scales (SS), which play an important role in nectar protection. We studied two species of Roepera with different symmetry and flower orientation, which are mainly visited by honeybees (Apis mellifera). We aimed to show how the foraging behavior of honey bees is affected by the function of SS, floral symmetry and orientation. The foraging behavior was documented by video photography. Handling time, access to nectar, percentage of pollen/nectar foraging, percentage of pollen contact and pollen deposition site on the honey bee's body were assessed. The morphometric features of the honey bees and flowers were analyzed. We found that the SS restricted pollinator access to nectar. Our results indicated consistency of visitation patterns in zygomorphic, laterally oriented flowers of R. fuscata versus random patterns in actinomorphic, diversely oriented flowers of R. leptopetala. The relative proportions of SS and proboscis length appear to be crucial for the success of pollinators. The directionality of the honey bees' movement, together with the different positioning of reproductive organs, plays an important role in the accuracy of pollen transfer and pollination efficiency.

Pollinator effectiveness of a specialist bee exploiting a generalist plant\u2014tracking pollen transfer by Heriades truncorum with quantum dots

Heriades truncorum (Megachilidae) is a specialist bee that forages on Asteraceae and collects pollen by tapping its abdomen on pollen-presenting florets which places the grains directly in the ventral scopa. We tracked pollen transfer by female H. truncorum between conspecific inflorescences of Inula ensifolia and Pulicaria dysenterica by labelling pollen with quantum dots. On average, bees transferred 31.14 (I. ensifolia) and 9.96 (P. dysenterica) pollen grains from the last visited inflorescence, 39% and 45% of which were placed on receptive styles. Pollen germination ratio is significantly lower for inflorescences of P. dysenterica visited by one H. truncorum (0.13%) compared with open control inflorescences (0.51%), which suggests that the bees mainly transfer self-pollen of these self-incompatible plants. Thus, a single visit by H. truncorum does not grant the plant high reproductive success, but the bees’ abundance and flower constancy might reduce this disadvantage.

The Landscape Genetic Signature of Pollination by Trapliners: Evidence From the Tropical Herb, Heliconia tortuosa

Animal-mediated pollination is essential for the maintenance of plant reproduction, especially in tropical ecosystems, where pollination networks have been thought to have highly generalized structures. However, accumulating evidence suggests that not all floral visitors provide equally effective pollination services, potentially reducing the number of realized pollinators and increasing the cryptic specialization of pollination networks. Thus, there is a need to understand how different functional groups of pollinators influence pollination success. Here, we examined whether patterns of contemporary pollen-mediated gene flow in Heliconia tortuosa are consistent with the foraging strategy of its territorial or traplining hummingbird pollinators. Territorial hummingbirds defend clumps of flowers and are expected to transfer pollen locally. In contrast, traplining hummingbirds forage across longer distances, thereby increasing pollen flow among forest fragments, and are thought to repeatedly visit particular plants. If trapliners indeed visit the same plants repeatedly along their regular routes, this could lead to a situation where neighboring plants sample genetically distinct pollen pools. To test this hypothesis, we genotyped 720 seeds and 71 mother plants from 18 forest fragments at 11 microsatellite loci. We performed TwoGener analysis to test pollen pool differentiation within sites (among neighboring plants within the same forest fragment: Φ SC) and between sites (among forest fragments: Φ CT). We found strong, statistically significant pollen pool differentiation among neighboring mother plants (Φ SC = 0.0506), and weaker, statistically significant differentiation among sites (Φ CT = 0.0285). We interpret this pattern of hierarchical pollen pool differentiation as the landscape genetic signature of the foraging strategy of traplining hummingbirds, where repeatable, long-distance, and high-fidelity routes transfer pollen among particular plants. Although H. tortuosa is also visited by territorial hummingbirds, our results suggest that these pollinators do not contribute substantially to successful pollination, highlighting differences in realized pollination efficiency. This cryptic reduction in the number of realized pollinators potentially increases the vulnerability of pollination success to the decline of populations of traplining hummingbirds, which have been shown to be sensitive to forest fragmentation. We conclude that maintaining habitat connectivity to sustain the foraging routes of trapliners may be essential for the maintenance of pollen-mediated gene flow in human-modified landscapes.

Effective pollination of Aeschynanthus acuminatus (Gesneriaceae) by generalist passerines, in sunbird-absent East Asia

Aeschynanthus (Gesneriaceae), a genus comprising approximately 160 species in subtropical Southeast Asia, has red, tubular flowers, typical of a sunbird pollination syndrome. A. acuminatus, the species that is distributed extending to the northern edge of the genus, where the specialized nectarivorous sunbirds are absent, possesses reddish-green flowers and a wide-open corolla tube, flowering time shifts from summer to winter and the species achieves high fruiting success. This atypical flower led us to investigate the pollination biology of this species. Three species of generalist passerines, Grey-cheeked Fulvetta (Alcippe morrisonia, Sylviidae), White-eared Sibia (Heterophasia auricularis, Leiothrichidae) and Taiwan Yuhina (Yuhina brunneiceps, Zosteropidae), were recorded visiting A. acuminatus flowers. Pollination effectiveness was quantified via conspecific pollen presence on stigmas and natural fruit set. The significantly high natural fruit set (60%) and conspecific pollen transfer rate (94%) indicate high reproductive success facilitated by the accurate pollen placement on the birds. The existence of copious (61 µL) and highly diluted (7%) hexose-dominant nectar, together with a major reflectance peak of corolla lobe in the long-wavelength red color spectrum, is consistent with the pollination syndrome of generalist passerines. The high pollination effectiveness of A. acuminatus due to the recruitment of generalist passerines as pollinators, and the specializations of floral traits to match generalist bird pollination, appear crucial in the successful colonization on islands such as Taiwan that lack specialized bird pollinators.

Effects of invasive Cirsium arvense on pollination in a southern Appalachian floral community vary with spatial scale and floral symmetry

Invasive plants can alter pollination dynamics by disrupting pollinator visitation and pollen transfer dynamics. However, a consensus regarding the direction of their overall effects (competitive vs. facilitative) remains elusive. Here, we evaluate the role of floral traits and spatial scale (community vs. floral neighborhood) in mediating invasive Cirsium arvense effects on resident plant species at multiple stages of the pollination process. C. arvense decreased pollinator visitation rate at the community level only for species with radial floral symmetry. At the floral neighborhood scale, pollinator visitation rate to all resident species was lower in the presence of C. arvense regardless of symmetry. C. arvense altered patterns of conspecific pollen receipt at the floral neighborhood scale, but the direction of the effect varied by plant species. We argue that these scale-dependent effects may be mediated by differences in foraging range and behavior of the main pollinators in the community. C. arvense, however, did not affect the overall reproductive success of resident species at either scale, suggesting that plants at our study sites may not be pollen limited. We further show evidence suggesting that C. arvense may alter the structure of community-level plant–plant interactions via heterospecific pollen transfer by subverting the roles of resident pollen donors within the pollen transfer network. Overall, our results suggest that generalized species (with radial flowers) may be more susceptible to invasive species’ effects than specialized ones (bilateral flowers) and highlight the need to consider scale-dependent effects in order to develop a more predictive understanding of invasive species effects in nature.

Breeding Systems in Diploid and Polyploid Hawthorns (Crataegus): Evidence from Experimental Pollinations of C. monogyna, C. subsphaerica, and Natural Hybrids

Background and Objectives: Polyploidisation and frequent hybridisation play an important role in speciation processes and evolutionary history and have a large impact on reproductive systems in the genus Crataegus. Reproductive modes in selected diploid and polyploid taxa in eastern Slovakia were investigated and analysed for the first time. Materials and Methods: Diploid, triploid, and tetraploid hawthorns were tested for self-pollination, self-compatibility, and self-fertilisation. Pollination experiments were performed within and between diploid and triploid species to determine the possibilities and directions of pollen transfer under natural conditions. Seeds from crossing experiments and open pollinations were analysed using the flow cytometric seed screen method. Results: These experiments demonstrated that sexual reproduction, cross-pollination, and self-incompatibility are typical of the diploid species Crataegus monogyna and C. kyrtostyla. Seeds produced by self-fertile tetraploid C. subsphaerica were derived from both meiotically reduced and unreduced megagametophytes. Conclusions: Experimental results concerning triploid C. subsphaerica and C. laevigata × C. subsphaerica are ambiguous but suggest that seeds are almost exclusively created through apomixis, although a few sexually generated seeds were observed. In the genus Crataegus, pseudogamy is a common feature of polyploid taxa, as in all cases pollination is essential for regular seed development. Research Highlights: We suggest that all studied Crataegus taxa produce reduced pollen irrespective of ploidy level. Moreover, we emphasise that triploids produce apparently aneuploid pollen grains as a result of irregular meiosis. They are also capable of utilising pollen from 2x, 3x, or 4x donors for pseudogamous formation of endosperm.

Male flowers of Aconitum compensate for toxic pollen with increased floral signals and rewards for pollinators

Many plants require animal pollinators for successful reproduction; these plants provide pollinator resources in pollen and nectar (rewards) and attract pollinators by specific cues (signals). In a seeming contradiction, some plants produce toxins such as alkaloids in their pollen and nectar, protecting their resources from ineffective pollinators. We investigated signals and rewards in the toxic, protandrous bee-pollinated plant Aconitum napellus, hypothesizing that male-phase flower reproductive success is pollinator-limited, which should favour higher levels of signals (odours) and rewards (nectar and pollen) compared with female-phase flowers. Furthermore, we expected insect visitors to forage only for nectar, due to the toxicity of pollen. We demonstrated that male-phase flowers emitted more volatile molecules and produced higher volumes of nectar than female-phase flowers. Alkaloids in pollen functioned as chemical defences, and were more diverse and more concentrated compared to the alkaloids in nectar. Visitors actively collected little pollen for larval food but consumed more of the less-toxic nectar. Toxic pollen remaining on the bee bodies promoted pollen transfer efficiency, facilitating pollination.

The role of hummingbirds in the evolution and diversification of Bromeliaceae: unsupported claims and untested hypotheses

AbstractAt least half of the 3600 species of Bromeliaceae are pollinated by hummingbirds. There is little doubt that the four to 12 evolutionary shifts towards and c. 32 shifts away from hummingbird pollination opened new evolutionary spaces for bromeliad diversification, and that hummingbird pollination has led to increased bromeliad diversification rates. However, the mechanisms leading to these increased rates remain unclear. We here propose that there are four main types of mechanisms that may increase diversification rates of hummingbird-pollinated bromeliad clades: (1) bromeliad speciation through adaptation to different hummingbird species; (2) increased allopatric speciation in hummingbird-pollinated clades due to lower pollen transfer efficiency compared with other pollinators; (3) differential speciation rates in hummingbird-pollinated clades dependent on of flowering phenology and hummingbird behaviour; and (4) higher speciation rates of bromeliads in montane environments (where hummingbird pollination predominates) due to topographic population fragmentation. To date, none of these hypotheses has been appropriately tested, partly due to a lack of data, but also because research so far has focused on documenting the pattern of increased diversification in hummingbird-pollinated clades, implicitly assuming that this pattern supports an underlying mechanism while ignoring the fact that several competing mechanisms may be considered. The aim of the present review is to increase awareness of these mechanisms and to trigger research aimed at specifically testing them. We conclude that much additional research on the roles of hummingbird behaviour and gene flow between bromeliad species is needed to elucidate their contribution to the evolution of diversity in bromeliads and other plant families.

Hawkmoth pollination of the orchid Habenaria clavata: mechanical wing guides, floral scent and electroantennography

Abstract Floral morphology can play a key role in mechanically guiding pollinators towards reproductive structures, particularly when visibility is limited at night, but the functional significance of morphological traits has seldom been considered in this context. Here we describe a remarkably intricate pollination mechanism in the hawkmoth-pollinated African grassland orchid Habenaria clavata, and also document aspects of the reproductive success and chemical ecology of this pollination system. The flowers are pollinated by several short-tongued hawkmoths, particularly Basiothia schenki, which was the most frequent visitor and occurred at all sites. Moths are probably attracted by the strong scent, which was dominated by several oxygenated aromatics that also elicited strong electrophysiological responses from antennae of B. schenki. Apart from the white rostellum lobes and stigma, which serve as a visual guide to the spur entrance, the flower parts are entirely green and indistinguishable from leaves in terms of spectral information. Using motion-activated video cameras we established that the leading edges of the forewings of foraging hawkmoths contact the two upwardly curving petal lobes, and that hawkmoths are then apparently mechanically guided down onto the reproductive structures. Pollinaria are attached in an unusual place – among hairs on the ventral surface of the thorax, between the middle legs – and are brushed over the protruding stigma lobes when the proboscis is fully inserted in the 41-mm-long spur. These results highlight how multiple traits (morphology, spectral reflectance and scent) can act synergistically to ensure transfer of pollen among flowers.