Beyond buzz-pollination - departures from an adaptive plateau lead to new pollination syndromes

Macroevolution of the plant-hummingbird pollination system

Plant-hummingbird interactions are considered a classic example of coevolution, a process in which mutually dependent species influence each other's evolution. Plants depend on hummingbirds for pollination, whereas hummingbirds rely on nectar for food. As a step towards understanding coevolution, this review focuses on the macroevolutionary consequences of plant-hummingbird interactions, a relatively underexplored area in the current literature. We synthesize prior studies, illustrating the origins and dynamics of hummingbird pollination across different angiosperm clades previously pollinated by insects (mostly bees), bats, and passerine birds. In some cases, the crown age of hummingbirds pre-dates the plants they pollinate. In other cases, plant groups transitioned to hummingbird pollination early in the establishment of this bird group in the Americas, with the build-up of both diversities coinciding temporally, and hence suggesting co-diversification. Determining what triggers shifts to and away from hummingbird pollination remains a major open challenge. The impact of hummingbirds on plant diversification is complex, with many tropical plant lineages experiencing increased diversification after acquiring flowers that attract hummingbirds, and others experiencing no change or even a decrease in diversification rates. This mixed evidence suggests that other extrinsic or intrinsic factors, such as local climate and isolation, are important covariables driving the diversification of plants adapted to hummingbird pollination. To guide future studies, we discuss the mechanisms and contexts under which hummingbirds, as a clade and as individual species (e.g. traits, foraging behaviour, degree of specialization), could influence plant evolution. We conclude by commenting on how macroevolutionary signals of the mutualism could relate to coevolution, highlighting the unbalanced focus on the plant side of the interaction, and advocating for the use of species-level interaction data in macroevolutionary studies.

High floral disparity without pollinator shifts in buzz-bee-pollinated Melastomataceae

Shifts among functional pollinator groups are commonly regarded as sources of floral morphological diversity (disparity) through the formation of distinct pollination syndromes. While pollination syndromes may be used for predicting pollinators, their predictive accuracy remains debated, and they are rarely used to test whether floral disparity is indeed associated with pollinator shifts. We apply classification models trained and validated on 44 functional floral traits across 252 species with empirical pollinator observations and then use the validated models to predict pollinators for 159 species lacking observations. In addition, we employ multivariate statistics and phylogenetic comparative analyses to test whether pollinator shifts are the main source of floral disparity in Melastomataceae. We find strong support for four well-differentiated pollination syndromes ('buzz-bee', 'nectar-foraging vertebrate', 'food-body-foraging vertebrate', 'generalist'). While pollinator shifts add significantly to floral disparity, we find that the most species-rich 'buzz-bee' pollination syndrome is most disparate, indicating that high floral disparity may evolve without pollinator shifts. Also, relatively species-poor clades and geographic areas contributed substantially to total disparity. Finally, our results show that machine-learning approaches are a powerful tool for evaluating the predictive accuracy of the pollination syndrome concept as well as for predicting pollinators where observations are missing.

Harvesting pollen with vibrations: towards an integrative understanding of the proximate and ultimate reasons for buzz pollination

Buzz pollination, a type of interaction in which bees use vibrations to extract pollen from certain kinds of flowers, captures a close relationship between thousands of bee and plant species. In the last 120 years, studies of buzz pollination have contributed to our understanding of the natural history of buzz pollination, and basic properties of the vibrations produced by bees and applied to flowers in model systems. Yet, much remains to be done to establish its adaptive significance and the ecological and evolutionary dynamics of buzz pollination across diverse plant and bee systems. Here, we review for bees and plants the proximate (mechanism and ontogeny) and ultimate (adaptive significance and evolution) explanations for buzz pollination, focusing especially on integrating across these levels to synthesize and identify prominent gaps in our knowledge. Throughout, we highlight new technical and modelling approaches and the importance of considering morphology, biomechanics and behaviour in shaping our understanding of the adaptive significance of buzz pollination. We end by discussing the ecological context of buzz pollination and how a multilevel perspective can contribute to explain the proximate and evolutionary reasons for this ancient bee-plant interaction.

The ontogeny of disparity in Cupressaceae seed cones

Ontogenetic shape change has long been recognized to be important in generating patterns of morphological diversity and may be especially important in plant reproductive structures. We explore how seed cone disparity in Cupressaceae changes over ontogeny by comparing pollination-stage and mature cones. We sampled cones at pollen and seed release and measured cone scales using basic morphometric shape variables. We used multivariate statistical methods, particularly hypervolume overlap calculations, to measure morphospace occupation and disparity. Cone scales at both pollination and maturity exhibit substantial variability, although the disparity is greater at maturity. Mature cone scales are also more clustered in trait space, showing less overlap with other taxa than at pollination. These patterns reflect two growth strategies that generate closed cones over maturation, either through thin laminar scales or relatively thick, peltate scales, resulting in two distinct regions of morphospace occupation. Disparity patterns in Cupressaceae seed cones change over ontogeny, reflecting shifting functional demands that require specific patterns of cone scale growth. The evolution of Cupressaceae reproductive disparity therefore represents selection for trajectories of ontogenetic shape change, a phenomenon that should be widespread across seed plants.

Plant morpho evo-devo

Evo-devo is often thought of as being the study of which genes underlie which phenotypes. However, evo-devo is much more than this, especially in plant science. In leaf scars along stems, cell changes across wood growth rings, or flowers along inflorescences, plants trace a record of their own development. Plant morpho evo-devo provides data that genes could never furnish on themes such as heterochrony, the evolution of temporal phenotypes, modularity, and phenotype-first evolution. As plant science surges into increasingly -omic realms, it is essential to keep plant morpho evo-devo in full view as an honored member of the evo-devo canon, ensuring that plant scientists can, wherever they are, generate fundamental insights at the appropriate level of biological organization.

Flower morphology as a predictor of pollination mode in a biotic to abiotic pollination continuum

BACKGROUND AND AIMS

Wind pollination has evolved repeatedly in flowering plants, yet the identification of a wind pollination syndrome as a set of integrated floral traits can be elusive. Thalictrum (Ranunculaceae) comprises temperate perennial herbs that have transitioned repeatedly from insect to wind pollination while also exhibiting mixed pollination, providing an ideal system to test for evolutionary correlation between floral morphology and pollination mode in a biotic to abiotic continuum. Moreover, the lack of floral organ fusion across this genus allows testing for specialization to pollination vectors in the absence of this feature.

METHODS

We expanded phylogenetic sampling in the genus from a previous study using six chloroplast loci, which allowed us to test whether species cluster into distinct pollination syndromes based on floral morphology. We then used multivariate analyses on floral traits followed by ancestral state reconstruction of the emerging flower morphotypes and determined whether these traits are evolutionarily correlated under a Bayesian framework with Brownian motion.

KEY RESULTS

Floral traits fell into five distinct clusters, which were reduced to three after considering phylogenetic relatedness and were largely consistent with flower morphotypes and associated pollination vectors. Multivariate evolutionary analyses found a positive correlation between the lengths of floral reproductive structures (styles, stigmas, filaments and anthers). Shorter reproductive structures tracked insect-pollinated species and clades in the phylogeny, whereas longer structures tracked wind-pollinated ones, consistent with selective pressures exerted by biotic vs. abiotic pollination vectors, respectively.

CONCLUSIONS

Although detectable suites of integrated floral traits across Thalictrum were correlated with wind or insect pollination at the extremes of the morphospace distribution, a presumed intermediate, mixed pollination mode morphospace was also detected. Thus, our data broadly support the existence of detectable flower morphotypes from convergent evolution underlying the evolution of pollination mode in Thalictrum, presumably via different paths from an ancestral mixed pollination state.

Evolutionary convergence on hummingbird pollination in Neotropical Costus provides insight into the causes of pollinator shifts

The evolution of hummingbird pollination is common across angiosperms throughout the Americas, presenting an opportunity to examine convergence in both traits and environments to better understand how complex phenotypes arise. Here we examine independent shifts from bee to hummingbird pollination in the Neotropical spiral gingers (Costus) and address common explanations for the prevalence of transitions from bee to hummingbird pollination. We use floral traits of species with observed pollinators to predict pollinators of unobserved species and reconstruct ancestral pollination states on a well-resolved phylogeny. We examine whether independent transitions evolve towards the same phenotypic optimum and whether shifts to hummingbird pollination correlate with elevation or climate. Traits predicting hummingbird pollination include small flower size, brightly colored floral bracts and the absence of nectar guides. We find many shifts to hummingbird pollination and no reversals, a single shared phenotypic optimum across hummingbird flowers, and no association between pollination and elevation or climate. Evolutionary shifts to hummingbird pollination in Costus are highly convergent and directional, involve a surprising set of traits when compared with other plants with analogous transitions and refute the generality of several common explanations for the prevalence of transitions from bee to hummingbird pollination.

Digest: Anther cones increase pollen release in buzz-pollinated Solanum flowers

Do anther arrangements in buzz-pollinated species have a functional significance? In this article, Vallejo-Marin et al. investigated this question by comparing pollen release rates in anther cones and free anther conformations in three species of the genus Solanum. The authors found that vibration transmission among anthers is greater for anther cones than among freely held conformations, resulting in higher rates of pollen release.

Population structure in Neotropical plants: integrating pollination biology, topography and climatic niches

Animal pollinators mediate gene flow among plant populations, but in contrast to well‐studied topographic and (Pleistocene) environmental isolating barriers, their impact on population genetic differentiation remains largely unexplored. Comparing how these multifarious factors drive microevolutionary histories is, however, crucial for better resolving macroevolutionary patterns of plant diversification. Here we combined genomic analyses with landscape genetics and niche modelling across six related Neotropical plant species (424 individuals across 33 localities) differing in pollination strategy to test the hypothesis that highly mobile (vertebrate) pollinators more effectively link isolated localities than less mobile (bee) pollinators. We found consistently higher genetic differentiation (F_ST) among localities of bee‐ than vertebrate‐pollinated species with increasing geographical distance, topographic barriers and historical climatic instability. High admixture among montane populations further suggested relative climatic stability of Neotropical montane forests during the Pleistocene. Overall, our results indicate that pollinators may differentially impact the potential for allopatric speciation, thereby critically influencing diversification histories at macroevolutionary scales.

Spatiotemporal variation in the role of floral traits in shaping tropical plant-pollinator interactions

The pollination syndrome hypothesis predicts that plants pollinated by the same pollinator group bear convergent combinations of specific floral functional traits. Nevertheless, some studies have shown that these combinations predict pollinators with relatively low accuracy. This discrepancy may be caused by changes in the importance of specific floral traits for different pollinator groups and under different environmental conditions. To explore this, we studied pollination systems and floral traits along an elevational gradient on Mount Cameroon during wet and dry seasons. Using Random Forest (Machine Learning) models, allowing the ranking of traits by their relative importance, we demonstrated that some floral traits are more important than others for pollinators. However, the distribution and importance of traits vary under different environmental conditions. Our results imply the need to improve our trait-based understanding of plant-pollinator interactions to better inform the debate surrounding the pollination syndrome hypothesis.

Examining the Role of Buzzing Time and Acoustics on Pollen Extraction of Solanum elaeagnifolium

Buzz pollination is a specialized pollination syndrome that requires vibrational energy to extract concealed pollen grains from poricidal anthers. Although a large body of work has examined the ecology of buzz pollination, whether acoustic properties of buzz pollinators affect pollen extraction is less understood, especially in weeds and invasive species. We examined the pollination biology of Silverleaf nightshade (Solanum elaeagnifolium), a worldwide invasive weed, in its native range in the Lower Rio Grande Valley (LRGV) in south Texas. Over two years, we documented the floral visitors on S. elaeagnifolium, their acoustic parameters (buzzing amplitude, frequency, and duration of buzzing) and estimated the effects of the latter two factors on pollen extraction. We found five major bee genera: Exomalopsis, Halictus, Megachile, Bombus, and Xylocopa, as the most common floral visitors on S. elaeagnifolium in the LRGV. Bee genera varied in their duration of total buzzing time, duration of each visit, and mass. While we did not find any significant differences in buzzing frequency among different genera, an artificial pollen collection experiment using an electric toothbrush showed that the amount of pollen extracted is significantly affected by the duration of buzzing. We conclude that regardless of buzzing frequency, buzzing duration is the most critical factor in pollen removal in this species.

The predictive power of pollination syndromes: Passerine pollination in heterantherous Meriania macrophylla (Benth.) Triana (Melastomataceae)

The cloud forest species Meriania macrophylla (Benth.) Triana has pseudocampanulate flowers with bulbous stamen appendages, typical for the passerine pollination syndrome found in the Melastomataceae tribe Merianieae. The species is further characterized by strong stamen dimorphism (heteranthery), a condition otherwise associated with pollen-rewarding bee-pollinated species (both in Melastomataceae and beyond). In passerine-pollinated Merianieae, however, flowers usually only show weak stamen dimorphism. Here, we conducted field and laboratory investigations to determine the pollinators of M. macrophylla and assess the potential role of strong heteranthery in this species. Our field observations in Costa Rica confirmed syndrome predictions and indeed proved pollination by passerine birds in M. macrophylla. The large bulbous set of stamens functions as a food-body reward to the pollinating birds, and as trigger for pollen release (bellows mechanism) as typical for the passerine syndrome in Merianieae. In contrast to other passerine-pollinated Merianieae, the second set of stamens has seemingly lost its rewarding and pollination function, however. Our results demonstrate the utility of the pollination syndrome concept even in light of potentially misleading traits such as strong heteranthery.

Investigation of pollen release by poricidal anthers using mathematical billiards

Buzz pollination is described using a mathematical model considering a billiard approach. Applications to a rough morphology of a typical poricidal anther of a tomato flower (Solanum lycopersicum) experiencing vibrations applied by a bumblebee (Bombus terrestris) are made. The anther is described by a rectangular billiard with a pore on its tip while the borders are perturbed by specific oscillations according to the vibrational properties of the bumblebee. Pollen grains are considered as noninteracting particles that can escape through the pore. Our results not only recover some observed data but also provide a possible answer to an open problem involving buzz pollination.

Judge it by its shape: a pollinator-blind approach reveals convergence in petal shape and infers pollination modes in the genus Erythrina

PREMISE

Pollinators are thought to exert selective pressures on plants, mediating the evolution of convergent floral shape often recognized as pollination syndromes. However, little is known about the accuracy of using petal shape for inferring convergence in pollination mode without a priori pollination information. Here we studied the genus Erythrina L. as a test case to assess whether ornithophyllous pollination modes (hummingbirds, passerines, sunbirds, or mixed pollination) can be inferred based on the evolutionary analysis of petal shape.

METHODS

We characterized the two-dimensional dissected shape of standard, keel, and wing petals from 106 Erythrina species using geometric morphometrics and reconstructed a phylogenetic tree of 83 Erythrina species based on plastid trnL-F and nuclear ribosomal ITS sequences. We then used two phylogenetic comparative methods based on Ornstein-Uhlenbeck models, SURFACE and l1OU, to infer distinct morphological groups using petal shape and identify instances of convergent evolution. The effectiveness of these methods was evaluated by comparing the groups inferred to known pollinators.

RESULTS

We found significant petal shape differences between hummingbird- and passerine-pollinated Erythrina species. Our analyses also revealed that petal combinations generally provided better inferences of pollinator types than individual petals and that the method and optimization criterion can affect the results.

CONCLUSIONS

We show that model-based approaches using petal shape can detect convergent evolution of floral shape and relatively accurately infer pollination modes in Erythrina. The inference power of the keel petals argues for a deeper investigation of their role in the pollination biology of Erythrina and other bird-pollinated legumes.

Buzz pollination

Pritchard and Vallejo-Marín introduce the process and evolution of buzz pollination.

Connective appendages in Huberia bradeana (Melastomataceae) affect pollen release during buzz pollination

Floral structures, such as stamen appendages, play crucial roles in pollinator attraction, pollen release dynamics and, ultimately, the reproductive success of plants. The pollen-rewarding, bee buzz-pollinated flowers of Melastomataceae often bear conspicuous staminal appendages. Surprisingly, their functional role in the pollination process remains largely unclear. We use Huberia bradeana Bochorny & R. Goldenb. (Melastomataceae) with conspicuously elongated, twisted stamen appendages to investigate their functional role in the pollination process. We studied the effect of stamen appendages on pollinator behaviour and reproductive success by comparing manipulated flowers (appendages removed) with unmanipulated flowers. To assess bee pollinator behaviour, we measured three properties of buzzes (vibrations) produced by bees on Huberia flowers: frequency, duration and number of buzzes per flower visit. We measured male and female reproductive success by monitoring pollen release and deposition after single bee visits. Finally, we used artificial vibrations and laser vibrometry to assess how flower vibrational properties change with the removal of stamen appendages. Our results show that the absence of staminal appendages does not modify bee buzzing behaviour. Pollen release was higher in unmanipulated flowers, but stigmatic pollen loads differ only marginally between the two treatments. We also detected lower vibration amplitudes in intact flowers as compared to manipulated flowers in artificial vibration experiments. The presence of connective appendages are crucial in transmitting vibrations and assuring optimal pollen release. Therefore, we propose that the high diversity of colours, shapes and sizes of connective appendages in buzz-pollinated flowers may have evolved by selection through male fitness.

Low bee visitation rates explain pollinator shifts to vertebrates in tropical mountains

Evolutionary shifts from bee to vertebrate pollination are common in tropical mountains. Reduction in bee pollination efficiency under adverse montane weather conditions was proposed to drive these shifts. Although pollinator shifts are central to the evolution and diversification of angiosperms, we lack experimental evidence of the ecological processes underlying such shifts. Here, we combine phylogenetic and distributional data for 138 species of the Neotropical plant tribe Merianieae (Melastomataceae) with pollinator observations of 11 and field pollination experiments of six species to test whether the mountain environment may indeed drive such shifts. We demonstrate that shifts from bee to vertebrate pollination coincided with occurrence at high elevations. We show that vertebrates were highly efficient pollinators even under the harsh environmental conditions of tropical mountains, whereas bee pollination efficiency was lowered significantly through reductions in flower visitation rates. Furthermore, we show that pollinator shifts in Merianieae coincided with the final phases of the Andean uplift and were contingent on adaptive floral trait changes to alternative rewards and mechanisms facilitating pollen dispersal. Our results provide evidence that abiotic environmental conditions (i.e. mountain climate) may indeed reduce the efficiency of a plant clade's ancestral pollinator group and correlate with shifts to more efficient new pollinators.

Trade-off mitigation: a conceptual framework for understanding floral adaptation in multispecies interactions

Explanations of floral adaptation to diverse pollinator faunas have often invoked visitor‐mediated trade‐offs in which no intermediate, generalized floral phenotype is optimal for pollination success, i.e. fitness valleys are created. In such cases, plant species are expected to specialize on particular groups of flower visitors. Contrary to this expectation, it is commonly observed that flowers interact with various groups of visitors, while at the same time maintaining distinct phenotypes among ecotypes, subspecies, or congeners. This apparent paradox may be due to a gap in our understanding of how visitor‐mediated trade‐offs could affect floral adaptation. Here we provide a conceptual framework for analysing visitor‐mediated trade‐offs with the hope of stimulating empirical and theoretical studies to fill this gap. We propose two types of visitor‐mediated trade‐offs to address negative correlations among fitness contributions of different visitors: visitor‐mediated phenotypic trade‐offs (phenotypic trade‐offs) and visitor‐mediated opportunity trade‐offs (opportunity trade‐offs). Phenotypic trade‐offs occur when different groups of visitors impose conflicting selection pressures on a floral trait. By contrast, opportunity trade‐offs emerge only when some visitors’ actions (e.g. pollen collection) remove opportunities for fitness contribution by more beneficial visitors. Previous studies have observed disruptive selection due to phenotypic trade‐offs less often than expected. In addition to existing explanations, we propose that some flowers have achieved ‘adaptive generalization’ by evolving features to avoid or eliminate the fitness valleys that phenotypic trade‐offs tend to produce. The literature suggests a variety of pathways to such ‘trade‐off mitigation’. Trade‐off mitigation may also evolve as an adaptation to opportunity trade‐offs. We argue that active exclusion, or floral specialization, can be viewed as a trade‐off mitigation, occurring only when flowers cannot otherwise avoid strong opportunity trade‐offs. These considerations suggest that an evolutionary strategy for trade‐off mitigation is achieved often by acquiring novel combinations of traits. Thus, phenotypic diversification of flowers through convergent evolution of certain trait combinations may have been enhanced not only through adaptive specialization for particular visitors, but also through adaptive generalization for particular visitor communities. Explorations of how visitor‐mediated trade‐offs explain the recurrent patterns of floral phenotypes may help reconcile the long‐lasting controversy on the validity of pollination syndromes.

Pollen dispensing schedules in buzz-pollinated plants: experimental comparison of species with contrasting floral morphologies

PREMISE

Plants can mitigate the fitness costs associated with pollen consumption by floral visitors by optimizing pollen release rates. In buzz-pollinated plants, bees apply vibrations to remove pollen from anthers with small pores. These poricidal anthers potentially function as mechanism staggering pollen release, but this has rarely been tested across plant species differing in anther morphology.

METHODS

In Solanum Section Androceras, three pairs of buzz-pollinated species have undergone independent evolutionary shifts between large- and small-flowers, which are accompanied by replicate changes in anther morphology. We used these shifts in anther morphology to characterize the association between anther morphology and pollen dispensing schedules. We applied simulated bee-like vibrations to anthers to elicit pollen release, and compared pollen dispensing schedules across anther morphologies. We also investigated how vibration velocity affects pollen release.

RESULTS

Replicate transitions in Solanum anther morphology are associated with consistent changes in pollen dispensing schedules. We found that small-flowered taxa release their pollen at higher rates than their large-flowered counterparts. Higher vibration velocities resulted in quicker pollen dispensing and more total pollen released. Finally, both the pollen dispensing rate and the amount of pollen released in the first vibration were negatively related to anther wall area, but we did not observe any association between pore size and pollen dispensing.

CONCLUSIONS

Our results provide the first empirical demonstration that the pollen dispensing properties of poricidal anthers depend on both floral characteristics and bee vibration properties. Morphological modification of anthers could thus provide a mechanism to exploit different pollination environments.

Bene"fit" Assessment in Pollination Coevolution: Mechanistic Perspectives on Hummingbird Bill-Flower Matching

One of the reasons why flowering plants became the most diverse group of land plants is their association with animals to reproduce. The earliest examples of this mutualism involved insects foraging for food from plants and, in the process, pollinating them. Vertebrates are latecomers to these mutualisms, but birds, in particular, present a wide variety of nectar-feeding clades that have adapted to solve similar challenges. Such challenges include surviving on small caloric rewards widely scattered across the landscape, matching their foraging strategy to nectar replenishment rate, and efficiently collecting this liquid food from well-protected chambers deep inside flowers. One particular set of convergent traits among plants and their bird pollinators has been especially well studied: the match between the shape and size of bird bills and ornithophilous flowers. Focusing on a highly specialized group, hummingbirds, we examine the expected benefits from bill-flower matching, with a strong focus on the benefits to the hummingbird and how to quantify them. Explanations for the coevolution of bill-flower matching include (1) that the evolution of traits by bird-pollinated plants, such as long and thin corollas, prevents less efficient pollinators (e.g., insects) from accessing the nectar and (2) that increased matching, as a result of reciprocal adaptation, benefits both the bird (nectar extraction efficiency) and the plant (pollen transfer). In addition to nectar-feeding, we discuss how interference and exploitative competition also play a significant role in the evolution and maintenance of trait matching. We present hummingbird-plant interactions as a model system to understand how trait matching evolves and how pollinator behavior can modify expectations based solely on morphological matching, and discuss the implications of this behavioral modulation for the maintenance of specialization. While this perspective piece directly concerns hummingbird-plant interactions, the implications are much broader. Functional trait matching is likely common in coevolutionary interactions (e.g., in predator-prey interactions), yet the physical mechanisms underlying trait matching are understudied and rarely quantified. We summarize existing methods and present novel approaches that can be used to quantify key benefits to interacting partners in a variety of ecological systems.

Stamen dimorphism in bird-pollinated flowers: Investigating alternative hypotheses on the evolution of heteranthery

Heteranthery, the presence of distinct stamen types within a flower, is commonly explained as functional adaptation to alleviate the "pollen dilemma," defined as the dual and conflicting function of pollen as pollinator food resource and male reproductive agent. A single primary hypothesis, "division of labor," has been central in studies on heteranthery. This hypothesis postulates that one stamen type functions in rewarding pollen-collecting pollinators and the other in reproduction, thereby minimizing pollen loss. Only recently, alternative functions (i.e., staggered pollen release), were proposed, but comparative and experimental investigations are lagging behind. Here, we used 63 species of the tribe Merianieae (Melastomataceae) to demonstrate that, against theory, heteranthery occurs in flowers offering rewards other than pollen, such as staminal food bodies or nectar. Although shifts in reward type released species from the "pollen dilemma," heteranthery has evolved repeatedly de novo in food-body-rewarding, passerine-pollinated flowers. We used field investigations to show that foraging passerines discriminated between stamen types and removed large stamens more quickly than small stamens. Passerines removed small stamens on separate visits towards the end of flower anthesis. We propose that the staggered increase in nutritive content of small stamens functions to increase chances for outcross-pollen transfer.

Complex interactions underlie the correlated evolution of floral traits and their association with pollinators in a clade with diverse pollination systems

Natural selection by pollinators is an important factor in the morphological diversity and adaptive radiation of flowering plants. Selection by similar pollinators in unrelated plants leads to convergence in floral morphology, or "floral syndromes." Previous investigations into floral syndromes have mostly studied relatively small and/or simple systems, emphasizing vertebrate pollination. Despite the importance of multiple floral traits in plant-pollinator interactions, these studies have examined few quantitative traits, so their co-variation and phenotypic integration have been underexplored. To gain better insights into pollinator-trait dynamics, we investigate the model system of the phlox family (Polemoniaceae), a clade of ∼400 species pollinated by a diversity of vectors. Using a comprehensive phylogeny and large dataset of traits and observations of pollinators, we reconstruct ancestral pollination system, accounting for the temporal history of pollinators. We conduct phylogenetically controlled analyses of trait co-variation and association with pollinators, integrating many analyses over phylogenetic uncertainty. Pollinator shifts are more heterogeneous than previously hypothesized. The evolution of floral traits is partially constrained by phylogenetic history and trait co-variation, but traits are convergent and differences are associated with different pollinators. Trait shifts are usually gradual, rather than rapid, suggesting complex genetic and ecological interactions of flowers at macroevolutionary scales.

Flower-visiting insects of genus Melastoma (Myrtales: Melastomataceae) at the Fushan Botanical Garden, Taiwan

Background

We investigated the diversity and behaviour of insects that visit flowers of four native Melastoma (Family Melastomataceae) species of Taiwan and a horticultural hybrid Melastoma species at the Fushan Botanical Garden, Taiwan biweekly from May to August 2020. Visits of flower-visiting insects were classified into seven behavioural categories, based on the insects' behaviour and positions on the flower. The data are further assigned into four insect-flower interactions, namely pollination, herbivory, commensalism and neutralism. Our goal is to provide baseline data of insect-plant interactions of Melastoma, which is a common, but understudied plant genus in the country.

New information

A total of 1,289 visits to flowers were recorded by at least 63 insect morphospecies belonging to seven orders. The number of insect species recorded per Melastoma species ranged from 9 to 39. Visiting, sonication and passing were the three most frequently recorded types of behaviour, collectively accounting for 90.2% (n = 1,240) of the total observations. Pollination was the most dominant insect-flower interaction, accounting for 70.2% of the total observations, followed by neutralism (20.0%), herbivory (6.3%) and commensalism (3.5%). Sweat bees of the genera Lasioglossum and Maculonomia (Hymenoptera: Halictidae) are considered key pollinators to Melastoma species in Fushan Botanical Garden, based on their high number of visits and sonication behaviour. Our study provides the first list of insects that visit the flowers of all Taiwan's known Melastoma species and description of their interactions with the plants.

Evolution of pollination syndromes and corolla symmetry in Balsaminaceae reconstructed using phylogenetic comparative analyses

BACKGROUND AND AIMS

Floral diversity as a result of plant-pollinator interactions can evolve by two distinct processes: shifts between pollination systems or divergent use of the same pollinator. Although both are pollinator driven, the mode, relative importance and interdependence of these different processes are rarely studied simultaneously. Here we apply a phylogenetic approach using the Balsaminaceae (including the species-rich genus Impatiens) to simultaneously quantify shifts in pollination syndromes (as inferred from the shape and colour of the perianth), as well as divergent use of the same pollinator (inferred from corolla symmetry).

METHODS

For 282 species we coded pollination syndromes based on associations between floral traits and known pollination systems, and assessed corolla symmetry. The evolution of these traits was reconstructed using parsimony- and model-based approaches, using phylogenetic trees derived from phylogenetic analyses of nuclear ribosomal and plastid DNA sequence data.

KEY RESULTS

A total of 71 % of studied species have a bee pollination syndrome, 22 % a bimodal syndrome (Lepidoptera and bees), 3 % a bird pollination syndrome and 5 % a syndrome of autogamy, while 19 % of species have an asymmetrical corolla. Although floral symmetry and pollination syndromes are both evolutionarily labile, the latter shifts more frequently. Shifts in floral symmetry occurred mainly in the direction towards asymmetry, but there was considerable uncertainty in the pattern of shift direction for pollination syndrome. Shifts towards asymmetrical flowers were associated with a bee pollination syndrome.

CONCLUSION

Floral evolution in Impatiens has occurred through both pollination syndrome shifts and divergent use of the same pollinator. Although the former appears more frequent, the latter is likely to be underestimated. Shifts in floral symmetry and pollination syndromes depend on each other but also partly on the region in which these shifts take place, suggesting that the occurrence of pollinator-driven evolution may be determined by the availability of pollinator species at large geographical scales.

Bee-Mediated Selection Favors Floral Sex Specialization in a Heterantherous Species: Strategies to Solve the Pollen Dilemma

Animal-pollinated plants show a broad variation in floral morphology traits and gametophyte production within populations. Thus, floral traits related to plant reproduction and sexuality are usually exposed to pollinator-mediated selection. Such selective pressures may be even stronger in heterantherous and pollen flowers, in which pollen contributes to both bee feeding and pollination, overcoming the “pollen dilemma” or the inability to perform both functions simultaneously. We describe the phenotypic gender and sexual organ morphology of flowers in two populations of Macairea radula (Melastomataceae), a heterantherous and buzz-pollinated species with pollen flowers. We estimated selection gradients on these traits through female and male fitness components. Both populations showed sizeable phenotypic gender variation, from strict hermaphrodites to increased femaleness or maleness. We found a continuous variation in style and stamen size, and this variation was correlated with corresponding shape values of both sexual organs. We detected bee-mediated selection towards short and long styles through seed number and towards intermediate degrees of heteranthery through pollen removal in one population, and selection towards increased maleness through pollen dispersal in both populations. Our results suggest that bee-mediated selection favors floral sex specialization and stylar dimorphism in M. radula, optimizing reproductive success and solving the pollen dilemma.

Pollination syndromes in the 21st century: where do we stand and where may we go?

Pollination syndromes, recurring suites of floral traits appearing in connection with specific functional pollinator groups, have served for decades to organise floral diversity under a functional-ecological perspective. Some potential caveats, such as over-simplification of complex plant-animal interactions or lack of empirical observations, have been identified and discussed in recent years. Which of these caveats do indeed cause problems, which have been solved and where do future possibilities lie? I address these questions in a review of the pollination-syndrome literature of 2010 to 2019. I show that the majority of studies was based on detailed empirical pollinator observations and could reliably predict pollinators based on a few floral traits such as colour, shape or reward. Some traits (i.e. colour) were less reliable in predicting pollinators than others (i.e. reward, corolla width), however. I stress that future studies should consider floral traits beyond those traditionally recorded to expand our understanding of mechanisms of floral evolution. I discuss statistical methods suitable for objectively analysing the interplay of system-specific evolutionary constraints, pollinator-mediated selection and adaptive trade-offs at microecological and macroecological scales. I exemplify my arguments on an empirical dataset of floral traits of a neotropical plant radiation in the family Melastomataceae.

Plants are visited by more pollinator species than pollination syndromes predicted in an oceanic island community

The pollination syndrome concept has provided powerful utility in understanding the evolution and adaptation of floral traits. However, the utility of this conception has been questioned on the grounds that flowers usually attract a broader spectrum of visitors than one might expect. Furthermore, the relationship between plant specialization and floral traits is poorly understood. Here, we examined the applicability of using the pollination syndrome to predict the pollinators of plants on Yongxing Island. We used the species-level specialization of pollination networks to compare the difference of plant ecological specialization among floral traits. The result of full model was not significant, indicating that floral traits did not affect the pollinator functional groups. The five floral traits explained only 22.5% of the pollinator's visitation preference. Our results showed that plants were visited by more pollinator species than pollination syndromes predicted. Plants with restrictive flowers showed higher specialization than those with unrestrictive flowers, while other floral traits exhibited no significant effect on plant specialization. Generalized pollination system on oceanic island might influence the predictive accuracy of pollination syndromes and the relationship between floral traits and plant ecological specialization. Our findings highlighted the utility and limitations of pollination syndromes concept in oceanic island communities.

Unraveling the Spiraling Radiation: A Phylogenomic Analysis of Neotropical Costus L

The family of pantropical spiral gingers (Costaceae Nakai; c. 125 spp.) can be used as a model to enhance our understanding of the mechanisms underlying Neotropical diversity. Costaceae has higher taxonomic diversity in South and Central America (c. 72 Neotropical species, c. 30 African, c. 23 Southeast Asian), particularly due to a radiation of Neotropical species of the genus Costus L. (c. 57 spp.). However, a well-supported phylogeny of the Neotropical spiral gingers including thorough sampling of proposed species encompassing their full morphologic and geographic variation is lacking, partly due to poor resolution recovered in previous analyses using a small sampling of loci. Here we use a phylogenomic approach to estimate the phylogeny of a sample of Neotropical Costus species using a targeted enrichment approach. Baits were designed to capture conserved elements' variable at the species level using available genomic sequences of Costus species and relatives. We obtained 832 loci (generating 791,954 aligned base pairs and 31,142 parsimony informative sites) for samples that encompassed the geographical and/or morphological diversity of some recognized species. Higher support values that improve the results of previous studies were obtained when including all the available loci, even those producing unresolved gene trees and having a low proportion of variable sites. Concatenation and coalescent-based species trees methods converge in almost the same topology suggesting a robust estimation of the relationships, even under the high levels of gene tree conflict presented here. The bait set design here presented made inferring a robust phylogeny to test taxonomic hypotheses possible and will improve our understanding of the origins of the charismatic diversity of the Neotropical spiral gingers.

Morphological specialization of heterantherous Rhynchanthera grandiflora (Melastomataceae) accommodates pollinator diversity

The tropical Melastomataceae are characterized by poricidal anthers which constitute a floral filter selecting for buzz-pollinating bees. Stamens are often dimorphic, sometimes with discernible feeding and pollinating functions. Rhynchanthera grandiflora produces nectarless flowers with four short stamens and one long stamen; all anthers feature a narrow elongation with an upwards facing pore. We tested pollen transfer by diverse foraging bees and viability of pollen from both stamen types. The impact of anther morphology on pollen release direction and scattering angle was studied to determine the plant's reproductive strategy. Medium-sized to large bees sonicated flowers in a specific position, and the probability of pollen transfer correlated with bee size even among these legitimate visitors. Small bees acted as pollen thieves or robbers. Anther rostrum and pore morphology serve to direct and focus the pollen jet released by floral sonication towards the pollinator's body. Resulting from the ventral and dorsal positioning of the short and long stamens, respectively, the pollinator's body was widely covered with pollen. This improves the plant's chances of outcrossing, irrespective of which bee body part contacts the stigma. Consequently, R. grandiflora is also able to employ bee species of various sizes as pollen vectors. The strategy of spreading pollen all over the pollinator's body is rather cost-intensive but counterbalanced by ensuring that most of the released pollen is in fact transferred to the bee. Thus, flowers of R. grandiflora illustrate how specialized morphology may serve to improve pollination by a functional group of pollinators.

Modularity increases rate of floral evolution and adaptive success for functionally specialized pollination systems

Angiosperm flowers have diversified in adaptation to pollinators, but are also shaped by developmental and genetic histories. The relative importance of these factors in structuring floral diversity remains unknown. We assess the effects of development, function and evolutionary history by testing competing hypotheses on floral modularity and shape evolution in Merianieae (Melastomataceae). Merianieae are characterized by different pollinator selection regimes and a developmental constraint: tubular anthers adapted to specialized buzz-pollination. Our analyses of tomography-based 3-dimensional flower models show that pollinators selected for functional modules across developmental units and that patterns of floral modularity changed during pollinator shifts. Further, we show that modularity was crucial for Merianieae to overcome the constraint of their tubular anthers through increased rates of evolution in other flower parts. We conclude that modularity may be key to the adaptive success of functionally specialized pollination systems by making flowers flexible (evolvable) for adaptation to changing selection regimes.

Dellinger et al. report the characterization of different pollinator selection regimes on 3D flower models. They show that pollinators selected for functional floral modules, and that this modularity allowed certain species to overcome morphological constraints by increased evolutionary rates in other flower parts.

Buzz pollination: studying bee vibrations on flowers

Approximately 6% of flowering plant species possess flowers with anthers that open through small pores or slits. Extracting pollen from this type of specialised flower is achieved most efficiently by vibrating the anthers, a behaviour that has evolved repeatedly among bees. Here I provide a brief overview of the study of vibrations produced by bees and their effects on pollen release. I discuss how bee morphology and behaviour affect the mechanical properties of vibrations, and how floral traits may influence the transmission of those vibrations from the bee to the anther, thus mediating pollen release, and ultimately bee and plant fitness. I suggest that understanding the evolution of buzz pollination requires a study of the biomechanics of bee vibrations and their transmission on flowers.

Does body size predict the buzz-pollination frequencies used by bees?

Body size is an important trait linking pollinators and plants. Morphological matching between pollinators and plants is thought to reinforce pollinator fidelity, as the correct fit ensures that both parties benefit from the interaction. We investigated the influence of body size in a specialized pollination system (buzz-pollination) where bees vibrate flowers to release pollen concealed within poricidal stamens. Specifically, we explored how body size influences the frequency of buzz-pollination vibrations. Body size is expected to affect frequency as a result of the physical constraints it places on the indirect flight muscles that control the production of floral vibrations. Larger insects beat their wings less rapidly than smaller-bodied insects when flying, but whether similar scaling relationships exist with floral vibrations has not been widely explored. This is important because the amount of pollen ejected is determined by the frequency of the vibration and the displacement of a bee's thorax. We conducted a field study in three ecogeographic regions (alpine, desert, grassland) and recorded flight and floral vibrations from freely foraging bees from 27 species across four families. We found that floral vibration frequencies were significantly higher than flight frequencies, but never exceeded 400 Hz. Also, only flight frequencies were negatively correlated with body size. As a bee's size increased, its buzz ratio (floral frequency/flight frequency) increased such that only the largest bees were capable of generating floral vibration frequencies that exceeded double that of their flight vibrations. These results indicate size affects the capacity of bees to raise floral vibration frequencies substantially above flight frequencies. This may put smaller bees at a competitive disadvantage because even at the maximum floral vibration frequency of 400 Hz, their inability to achieve comparable thoracic displacements as larger bees would result in generating vibrations with lower amplitudes, and thus less total pollen ejected for the same foraging effort.