Pollinator Attraction Research Articles

Spatial and temporal heterogeneity in pollinator communities maintains within‐species floral odour variation

Flowering plants emit complex bouquets of volatile organic compounds (VOCs) to mediate interactions with their pollinators. These bouquets are undoubtedly influenced by pollinator‐mediated selection, particularly in deceptively‐pollinated species that rely on chemical mimicry. However, many uncertainties remain regarding how spatially and temporally heterogeneous pollinators affect the diversity and distribution of floral odour variation. Here, we characterized and compared the floral odours of ten populations of deceptively‐pollinated Arum maculatum (Araceae), and inter‐annual and decadal variation in pollinator attraction within these populations. Additionally, we transplanted individuals from all sampled populations to two common garden sites dominated by different pollinator species (Psychoda phalaenoides or Psycha grisescens), and compared pollinator attraction rates to investigate whether populations maintained odour blends adapted to a specific pollinator. We identified high within‐ and among‐population variation in a common blend of VOCs found across the range of A. maculatum. We also observed shifts in pollinator community composition within several populations over 1–2 years, as well as over the past decade. Common garden experiments further revealed that transplanted inflorescences generally attracted the dominant local pollinator species in both transplant sites. However, one population (Forêt du Gâvre, France) appears to exclusively attract P. grisescens, even when transplanted to a P. phalaenoides‐dominated site. Together, our results suggest that maintaining diverse floral odour bouquets within populations may be advantageous when pollinator communities vary over short timescales. We propose that temporally‐replicated ecological data are one potential key to understanding variation in complex traits such as floral odour, and in some cases may reveal resiliency to shifting pollinator communities.

Breeding systems of naturalized versus indigenous species provide support for Baker's law on Pohnpei island.

The factors that facilitate successful colonization of islands should be especially evident where the establishment filter is strongest. Colonizers of small, remote oceanic islands should be initially rare, extremely mate-limited and often without pollinators. Hence, plant communities on such islands should reflect an establishment history in which young ‘naturalized’ species are most likely to display self-compatibility and autonomous selfing, whereas ‘indigenous’ species may exhibit more diverse reproductive strategies. To test this prediction, we characterized breeding systems of 28 species on Pohnpei, in the Federated States of Micronesia, a group of remote Pacific islands that are considered a global biodiversity hotspot. Three families with both naturalized and indigenous species were selected—Fabaceae, Malvaceae and Melastomataceae. Measurements included field observations of dichogamy/herkogamy and floral attraction traits, pollen:ovule (P:O) ratios and experimental hand-pollinations for self-compatibility and pollen limitation. Phylogenetic generalized least squares analyses tested for trait correlations between naturalized and indigenous species. Flowers of all 28 species were bisexual, and pollinator attraction features were common. Pollen:ovule ratios ranged from 9 to 557 (median = 87), and all 11 hand-pollinated species were self-compatible. All species had >5 ovules and <3500 pollen grains per flower. Indigenous species did not differ significantly from naturalized species for any trait. There is a dearth of data from remote islands bearing on the question of establishment history. In this study, we inferred all species to have some degree of autogamy and indigenous species were no more likely than naturalized species to display outcrossing mechanisms. On Pohnpei, high ovule numbers, and the inaccessibility of wind pollination and obligate outcrossing strategies, reflect the importance of retaining reproductive assurance mechanisms in the face of pollinator uncertainty.

Metabolomic analysis reveals reliance on secondary plant metabolites to facilitate carnivory in the Cape sundew, Drosera capensis.

Background and AimsSecondary metabolites are integral to multiple key plant processes (growth regulation, pollinator attraction and interactions with conspecifics, competitors and symbionts) yet their role in plant adaptation remains an underexplored area of research. Carnivorous plants use secondary metabolites to acquire nutrients from prey, but the extent of the role of secondary metabolites in plant carnivory is not known. We aimed to determine the extent of the role of secondary metabolites in facilitating carnivory of the Cape sundew, Drosera capensis.MethodsWe conducted metabolomic analysis of 72 plants in a time-series experiment before and after simulated prey capture. We used ultra-high-performance liquid chromatography–tandem mass spectrometry (UHPLC–MS/MS) and the retention time index to identify compounds in the leaf trap tissue that changed up to 72 h following simulated prey capture. We identified associated metabolic pathways, and cross-compared these compounds with metabolites previously known to be involved in carnivorous plants across taxa.Key ResultsFor the first time in a carnivorous plant, we have profiled the whole-leaf metabolome response to prey capture. Reliance on secondary plant metabolites was higher than previously thought – 2383 out of 3257 compounds in fed leaves had statistically significant concentration changes in comparison with unfed controls. Of these, ~34 compounds are also associated with carnivory in other species; 11 are unique to Nepenthales. At least 20 compounds had 10-fold changes in concentration, 12 of which had 30-fold changes and are typically associated with defence or attraction in non-carnivorous plants.ConclusionsSecondary plant metabolites are utilized in plant carnivory to an extent greater than previously thought – we found a whole-metabolome response to prey capture. Plant carnivory, at the metabolic level, likely evolved from at least two distinct functions: attraction and defence. Findings of this study support the hypothesis that secondary metabolites play an important role in plant diversification and adaptation to new environments.

A meta-analysis of responses in floral traits and flower-visitor interactions to water deficit.

Alterations in water availability and drought events as predicted by climate change scenarios will increasingly impact natural communities with effects already emerging at present. Water deficit leads to increasing physiological stress in plants, likely affecting floral development and causing changes in floral morphology, nectar and pollen production or scent. Understanding how these floral traits are altered by water deficit is necessary to predict changes in plant-pollinator interactions and how communities are impacted in the future. Here we employ a meta-analysis approach to synthesize the current evidence of experimental water deficit on floral traits and plant-pollinator interactions. Furthermore, we explore experimental factors potentially increasing heterogeneity between studies and provide ideas how to enhance comparability between studies. In the end, we highlight future directions and knowledge gaps for floral traits and plant-pollinator interactions under water deficit. Our analysis showed consistent decreases in floral size, number of flowers and nectar volume to reduced water availability. Other floral traits such as the start of flowering or herkogamy showed no consistent pattern. This indicates that effects of reduced water availability differ between specific traits that are potentially involved in different functions such as pollinator attraction or efficiency. We found no general decreasing visitation rates with water deficit for flower-visitor interactions. Furthermore, the comparison of available studies suggests that increased reporting of plant stress severity and including more hydraulic and physiological measurements will improve the comparability across experiments and aid a more mechanistic understanding of plant-pollinator interactions under altered environmental conditions. Overall, our results show that water deficit has the potential to strongly affect plant-pollinator interactions via changes in specific floral traits. Linking these changes to pollination services and pollinator performance is one crucial step for understanding how changing water availability and drought events under climate change will alter plant and pollinator communities.

Pollination ecology and floral scent chemistry of Philodendron fragrantissimum (Araceae)

ABSTRACT In French Guiana, Philodendron fragrantissimum (Hook.) G.Don was specifically pollinated by a single species of night-active cyclocephaline scarab, Cyclocephala simulatrix Höhne. Its inflorescences exhibited short-lasting anthesis (~30 h) and characteristic floral traits such as floral thermogenesis, edible/nutritious floral tissues, and profuse floral scent emission. Our insect exclusion experiments indicate that entomophilous cross-pollination is obligatory for P. fragrantissimum, with olfactory signalling playing a pivotal role in pollinator attraction. Three volatile organic compounds – methyl benzoate, (Z)-jasmone, and dehydrojasmone – dominate the floral scent of pistillate phase inflorescences (84–99%), but their relative proportions were different according to the headspace sampling method used, dynamic or static. Local pollination services provided by both female and male C. simulatrix are effective, as evidenced by a high visitation rate (>70%) that resulted in 55% of the inflorescences maturing into infructescences. Moreover, pollination was highly efficient, since 2–3 beetles sufficed to pollinate an average 94% of the pistillate flowers.

Color signals of bee-pollinated flowers: the significance of natural leaf background.

Flower color is a primary pollinator attractant and generally adjusted to the cognitive system of the pollinators. The perception of flower color depends on the visual system of pollinators and also on environmental factors such as light conditions and the background against which flowers are displayed. Using bee-pollinated Fabaceae species as a model, we analyzed flower color diversity and compared flower color signals considering both the standard green and the natural leaf background of two tropical seasonally dry vegetations-a mountain rupestrian grassland (campo rupestre) and a woody savanna (cerrado)-compared to a nontropical Mediterranean shrubland. By using natural background, bees discriminated color for 58% of the flowers in the campo rupestre and for only 43% in cerrado. Both vegetations were surpassed by 75% of bee color discrimination in Mediterranean vegetation. Chromatic contrast and purity were similar among the three vegetation types. Green contrast and brightness were similar between the tropical vegetations but differed from the Mediterranean shrubland. Green contrast differences were lost when using a standard green background, and most variables (purity, green contrast, and brightness) differed according to the background (natural or standard green) in all vegetations. The natural background influenced bee perception of flower color regardless of vegetation. The background of the campo rupestre promoted green contrast for flowers, ensuring flower detection by pollinators and, along with bees, may also act as a selective pressure driving the diversity of flower colors in Fabaceae species. We highlight the importance of considering the natural background coloration when analyzing flower color signals.

Optimization of SPME\u2013GC\u2013MS and characterization of floral scents from Aquilegia japonica and A. amurensis flowers

BackgroundThe floral scents of plants play a key role in plant reproduction through the communication between plants and pollinators. Aquilegia as a model species for studying evolution, however, there have been few studies on the floral scents and relationships between floral scents and pollination for Aquilegia taxa.MethodsIn this study, three types of solid-phase micro-extraction (SPME) fiber coatings (DVB/PDMS, CAR/PDMS, DVB/CAR/PDMS) were evaluated for their performance in extracting volatile organic compounds (VOCs) from flowers of Aquilegia amurensis, which can contribute to the future studies of elucidating the role of floral scents in the pollination process.ResultsIn total, 55 VOCs were identified, and among them, 50, 47 and 45 VOCs were extracted by the DVB/CAR/PDMS fiber, CAR/PDMS fiber and DVB/PDMS fibers, respectively. Only 30 VOCs were detected in A. japonica taxa. Furthermore, the relative contents of 8 VOCs were significant different (VIP > 1 and p < 0.05) between the A. amurensis and A. japonica.ConclusionsThe results can be applied in new studies of the relationships between the chemical composition of floral scents and the processes of attraction of pollinator. It may provide new ideas for rapid evolution and frequent interspecific hybridization of Aquilegia.

Inoculation of pear flowers with Metschnikowia reukaufii and Acinetobacter nectaris enhances attraction of honeybees and hoverflies, but does not increase fruit and seed set.

Currently, one of the most important challenges is to provide sufficient and affordable food and energy for a fast-growing world population, alongside preserving natural habitats and maintaining biodiversity. About 35% of the global food production depends on animals for pollination. In recent years, an alarming worldwide decline in pollinators has been reported, putting our food production under additional pressure. Therefore, there is an urgent need to find sustainable ways to ensure this crucial ecosystem service. Recent studies have shown that floral nectar is generally colonized by microorganisms, specifically yeasts and bacteria, which may alter nectar chemistry and enhance attraction of pollinators. In this study, we investigated changes in pollinator foraging behavior and pollination success in European pear (Pyrus communis L.) cultivars ‘Regal Red’ and ‘Sweet Sensation’ (red sports of ‘Doyenné de Comice’) after flower inoculation with the typical nectar-inhabiting microorganisms Metschnikowia reukaufii and Acinetobacter nectaris, and a combination of both. Pollination success was monitored by measuring the number of flower visits, fruit set and seed set in two consecutive years, 2019 and 2020. Results revealed that application of a mixture of M. reukaufii and A. nectaris resulted in significantly higher visitation rates of honeybees and hoverflies. By contrast, no effects on flower visits were found when yeasts and bacteria were applied separately. Fruit set and seed set were not significantly affected by any of the inoculation treatments. The only factors affecting fruit set were initial number of flower clusters on the trees and the year. The absence of treatment effects can most likely be attributed to the fact that pollination was not a limiting factor for fruit set in our experiments. Altogether, our results show that inoculation of flowers with nectar microbes can modify pollinator foraging patterns, but did not lead to increased pollination success under the conditions tested.

Ozone Pollution Alters Olfaction and Behavior of Pollinators

Concentration of air pollutants, particularly ozone (O3), has dramatically increased since pre-industrial times in the troposphere. Due to the strong oxidative potential of O3, negative effects on both emission and lifetime in the atmosphere of plant volatile organic compounds (VOCs) have already been highlighted. VOCs alteration by O3 may potentially affect the attraction of pollinators that rely on these chemical signals. Surprisingly, direct effects of O3 on the olfaction and the behavioral response of pollinators have not been investigated so far. We developed a comprehensive experiment under controlled conditions to assess O3 physiological and behavioral effects on two pollinator species, differing in their ecological traits. Using several realistic concentrations of O3 and various exposure times, we investigated the odor antennal detection and the attraction to VOCs present in the floral scents of their associated plants. Our results showed, in both species, a clear effect of exposure to high O3 concentrations on the ability to detect and react to the floral VOCs. These effects depend on the VOC tested and its concentration, and the O3 exposure (concentration and duration) on the pollinator species. Pollination systems may, therefore, be impaired in different ways by increased levels of O3, the effects of which will likely depend on whether the exposure is chronic or, as in this study, punctual, likely causing some pollination systems to be more vulnerable than others. While several studies have already shown the negative impact of O3 on VOCs emission and lifetime in the atmosphere, this study reveals, for the first time, that this impact alters the pollinator detection and behavior. These findings highlight the urgent need to consider air pollution when evaluating threats to pollinators.

Floral scent is different between sexual phases within individuals in a synchronously dichogamous shrub (Canella winterana) but there is no distinct female or male scent profile across individuals

Floral scent is a highly complex and variable floral characteristic that is involved in pollinator attraction. One possible cause for variation in floral scent can be sexual identity of the flower. Here, we examine the floral scent bouquet of a synchronously dichogamous shrub, Canella winterana (L.) Gaertn.We used dynamic headspace extraction and gas chromatography – mass spectroscopy, followed by statistical analysis using non-metric multidimensional scaling, SIMPER, MANOVA, and PERMANOVA to identify and compare the scent profiles of Canella winterana in its female and male phase for multiple individuals as well as multiple inflorescences of a single tree over one cycle of its entire sexual phase transition. The scent profile of C. winterana is composed of 49 volatile organic compounds and dominated by five compounds. We found no evidence for distinct male or female scent profiles; however, there were significant differences in scent emission between different sexes within some individuals. Two compounds explained over half of the variation between sexual stages within individuals. Our exploration of a single tree's sexual phase transition, including neuter phase, found that five compounds dominated the female phase scent bouquet and that female phase was distinct from male and neuter phase. This study offers new insight into the role that variability in floral scent between sexual phases might play in variable pollinator behavioral responses. These results suggest partial support for two distinct hypotheses regarding the differences between the sex phases (1) honest signaling and (2) sexual mimicry.

Floral scent and pollinator visitation in relation to floral colour morph in the mixed‐mating annual herb Collinsia heterophylla

Even though floral scent is of major importance for pollinator attraction, it is less investigated than other floral traits. Previous studies suggest the importance of joint exploration of olfactory and visual floral cues to understand plant–pollinator interactions. We investigated flower scents in Collinsia heterophylla, a bee‐pollinated, annual herb a with mixed‐mating system combining self‐ and outcross‐pollination. In Collinsia, floral size and development variation is related to mating system, ranging from large‐flowered mixed‐mating species to small‐flowered self‐pollinated species. However, to our knowledge, flower scent has not been described in any species in the genus. We also studied whether flower‐emitted volatiles were coupled to presence versus absence of a coloured band on the upper lip within a population in C. heterophylla, and if these colour morphs affected pollinator visitation. We performed headspace collections of volatiles in the greenhouse from potted flowering plants, and compared these to controls in the bud stage. Flower‐specific volatiles were highly dominated by terpenoid compounds typical of bee‐pollinated plants, such as β‐myrcene, (Z)‐ and (E)‐ocimene and sesquiterpenes (E)‐α‐bergamotene and β‐sesquiphellandrene. The aliphatic ester methyl hexanoate was also prominent, together with additional esters, whereas methyl cinnamate constituted the only aromatic compound. Floral colour morphs showed no qualitative difference in volatiles, but the coloured morph produced significantly higher quantities for seven of the 26 individual flower compounds. A field experiment performed within a natural population, using behavioural observations and florescent dyes dusted on the flowers, could not detect any differences in pollinator visitation between colour morphs. We conclude that C. heterophylla flowers emit volatile compounds commonly associated with attraction of their most important pollinators. It would be highly interesting to explore the function of floral scent for pollinator attraction and relate floral scent to mating system variation across Collinsia for a better understanding of pollinator influence on floral evolution.

Cross-Species Metabolic Profiling of Floral Specialized Metabolism Facilitates Understanding of Evolutional Aspects of Metabolism Among Brassicaceae Species.

Plants produce a variety of floral specialized (secondary) metabolites with roles in several physiological functions, including light-protection, attraction of pollinators, and protection against herbivores. Pigments and volatiles synthesized in the petal have been focused on and characterized as major chemical factors influencing pollination. Recent advances in plant metabolomics have revealed that the major floral specialized metabolites found in land plant species are hydroxycinnamates, phenolamides, and flavonoids albeit these are present in various quantities and encompass diverse chemical structures in different species. Here, we analyzed numerous floral specialized metabolites in 20 different Brassicaceae genotypes encompassing both different species and in the case of crop species different cultivars including self-compatible (SC) and self-incompatible (SI) species by liquid chromatography-mass spectrometry (LC-MS). Of the 228 metabolites detected in flowers among 20 Brassicaceae species, 15 metabolite peaks including one phenylacyl-flavonoids and five phenolamides were detected and annotated as key metabolites to distinguish SC and SI plant species, respectively. Our results provide a family-wide metabolic framework and delineate signatures for compatible and incompatible genotypes thereby providing insight into evolutionary aspects of floral metabolism in Brassicaceae species.

Abiotic Environment Predicts Micro- but Not Macroevolutionary Patterns of Flower Color in Monkeyflowers (Phrymaceae).

Anthocyanin pigments are responsible for many of the vivid pink, purple, red, and blue flower colors across angiosperms and frequently vary within and between closely related species. While anthocyanins are well known to influence pollinator attraction, they are also associated with tolerance to abiotic stressors such as extreme temperatures, reduced precipitation, and ultraviolet radiation. Using a comparative approach, we tested whether abiotic variables predict floral anthocyanin in monkeyflowers (Phrymaceae) across western North America. Within two polymorphic species, we found that abiotic variables predicted flower color across their geographic ranges. In Erythranthe discolor, the frequency of pink flowered (anthocyanin producing) individuals was greater in populations with reduced precipitation. In Diplacus mephiticus, the frequency of pink flowered individuals was greater at higher elevations that had reduced precipitation and lower temperatures but less ultraviolet radiation. At the macroevolutionary scale, across two parallel radiations of North American monkeyflowers, species with floral anthocyanins (pink, purple, or red corollas) occupied areas with reduced precipitation in Erythranthe but not Diplacus. However, after accounting for phylogenetic relatedness, we found no evidence for the joint evolution of flower color and environmental affinity in either clade. We conclude that although abiotic stressors may play a role in the evolution of flower color within polymorphic species, we found no evidence that these processes lead to macroevolutionary patterns across monkeyflowers.

An unusual tricosatriene is crucial for male fungus gnat attraction and exploitation by sexually deceptive Pterostylis orchids

Cross-kingdom mimicry of female insect sex pheromones by sexually deceptive orchids has evolved multiple times.1 Fungus gnats (Diptera) are predicted to be pollinators of hundreds of sexually deceptive orchids,2-4 yet unlike orchids that sexually attract bees and wasps (Hymenoptera),5-11 the chemistry of fungus gnat-pollinated orchids remains unknown. Furthermore, despite the importance of fungus gnats as pollinators, pests, and decomposers of organic material, and evidence for sex pheromones since 1971,12-17 no structure of any fungus gnat sex pheromone has to date been confirmed. In this study, we found a mixture of five hydrocarbons shared between Pterostylis orbiculata orchids and female Mycomya sp. (Mycetophilidae) fungus gnats, which included three alkanes, a C23 diene, and a C23 triene. The triene was an undescribed natural product, which we synthesized and confirmed to be (6Z,9Z)-1,6,9-tricosatriene. Field bioassays with a synthetic blend of the five hydrocarbons elicited attraction and sexual behavior from male gnats. The triene alone elicited attraction and low levels of sexual behavior, but the blend without it was unattractive, suggesting that this compound is a key component of orchid pollinator attraction and the female fungus gnat sex pheromone. In two closely related Pterostylis species, we found related C23 trienes, but not (6Z,9Z)-1,6,9-tricosatriene. These results suggest that unusual long-chain unsaturated hydrocarbons hold the key to sexual deception in Pterostylis orchids, and are an important step toward deciphering female fungus gnat sex pheromones.

Divergence in floral scent and morphology, but not thermogenic traits, associated with pollinator shift in two brood-site-mimicking Typhonium (Araceae) species

Flowers which imitate insect oviposition sites probably represent the most widespread form of floral mimicry, exhibit the most diverse floral signals and are visited by two of the most speciose and advanced taxa of insect - beetles and flies. Detailed comparative studies on brood-site mimics pollinated exclusively by each of these insect orders are lacking, limiting our understanding of floral trait adaptation to different pollinator groups in these deceptive systems. Two closely related and apparent brood-site mimics, Typhonium angustilobum and T. wilbertii (Araceae) observed to trap these distinct beetle and fly pollinator groups were used to investigate potential divergence in floral signals and traits most likely to occur under pollinator-mediated selection. Trapped pollinators were identified and their relative abundances enumerated, and thermogenic, visual and chemical signals and morphological traits were examined using thermocouples and quantitative reverse transcription-PCR, reflectance, gas chromatography-mass spectrometry, floral measurements and microscopy. Typhonium angustilobum and T. wilbertii were functionally specialized to trap saprophagous Coleoptera and Diptera, respectively. Both species shared similar colour and thermogenic traits, and contained two highly homologous AOX genes (AOX1a and AOX1b) most expressed in the thermogenic tissue and stage (unlike pUCP). Scent during the pistillate stage differed markedly - T. angustilobum emitted a complex blend of sesquiterpenes, and T. wilbertii, a dung mimic, emitted high relative amounts of skatole, p-cresol and irregular terpenes. The species differed significantly in floral morphology related to trapping mechanisms. Functional specialization and pollinator divergence were not associated with differences in anthesis rhythm and floral thermogenic or visual signals between species, but with significant differences in floral scent and morphological features, suggesting that these floral traits are critical for the attraction and filtering of beetle or fly pollinators in these two brood-site mimics.

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.

Evolution of floral scent in relation to self-incompatibility and capacity for autonomous self-pollination in the perennial herb Arabis alpina.

Background and AimsThe transition from outcrossing to selfing is a frequent evolutionary shift in flowering plants and is predicted to result in reduced allocation to pollinator attraction if plants can self-pollinate autonomously. The evolution of selfing is associated with reduced visual floral signalling in many systems, but effects on floral scent have received less attention. We compared multiple populations of the arctic–alpine herb Arabis alpina (Brassicaceae), and asked whether the transition from self-incompatibility to self-compatibility has been associated with reduced visual and chemical floral signalling. We further examined whether floral signalling differ between self-compatible populations with low and high capacity for autonomous self-pollination, as would be expected if benefits of signalling decrease with reduced dependence on pollinators for pollen transfer.MethodsIn a common garden we documented flower size and floral scent emission rate and composition in eight self-compatible and nine self-incompatible A. alpina populations. These included self-compatible Scandinavian populations with high capacity for autonomous self-pollination, self-compatible populations with low capacity for autonomous self-pollination from France and Spain, and self-incompatible populations from Italy and Greece.Key ResultsThe self-compatible populations produced smaller and less scented flowers than the self-incompatible populations. However, flower size and scent emission rate did not differ between self-compatible populations with high and low capacity for autonomous self-pollination. Floral scent composition differed between self-compatible and self-incompatible populations, but also varied substantially among populations within the two categories.ConclusionsOur study demonstrates extensive variation in floral scent among populations of a geographically widespread species. Contrary to expectation, floral signalling did not differ between self-compatible populations with high and low capacity for autonomous self-pollination, indicating that dependence on pollinator attraction can only partly explain variation in floral signalling. Additional variation may reflect adaptation to other aspects of local environments, genetic drift, or a combination of these processes.

Evaluating restoration outcomes through assessment of pollen dispersal, mating system, and genetic diversity

Ecological genetics can provide a novel contribution to assessing the achievement of restoration objectives. We used paternity assignment to infer realized pollen dispersal within, and pollen immigration into, a restoration population of Hakea nitida, a common near‐coastal shrub or small tree in southwest Australia. We compared mating system parameters and genetic diversity with a nearby remnant reference population and assessed genetic divergence among the restoration and reference populations. We found realized pollen dispersal events closely tracked the frequency distributions of the distances between all plants within the restoration focus area. Mean realized pollen dispersal distance (359 m) approached the mean of the distances between all plants (407 m), far exceeding mean nearest neighbor distance (12 m). Maximum realized pollen dispersal distance (869 m) approached the maximum distance between all plants in the study area (1,033 m). Pollen immigration into the restoration study area was limited (4%). The mating system revealed moderate outcrossing rates (tm = 0.861 restoration and tm = 0.745 reference population), with significant and similar biparental inbreeding (tm − ts = 0.180, tm− ts = 0.186) but greater correlated paternity (rpm) in the restoration (0.519) than in the reference (0.188) population. Genetic divergence among the restoration and reference remnant population was moderate (FST = 0.094, DST = 0.239). Patterns of pollen dispersal and mating system parameters imply the attraction of pollinators within the restoration population as a key factor in progressing towards the establishment of self‐sustaining populations.

Multiple Attack to Inflorescences of an Annual Plant Does Not Interfere with the Attraction of Parasitoids and Pollinators

Plants in the flowering stage need to ensure reproduction by protecting themselves from attack and by preserving interactions with mutualist pollinators. When different plant mutualists are using the same type of cues, such as volatile compounds, attraction of parasitoids and pollinators may trade off. To explore this, we compared volatile emission of Brassica nigra plants in response to single or dual attack on their inflorescences. Additionally, we recorded flower visitation by pollinators and the attraction of parasitoids in the greenhouse and/or field. Brassica nigra were exposed in the flowering stage to one or two of the following three attackers: Brevicoryne brassicae aphids, Pieris brassicae caterpillars, and Xanthomonas campestris pv. raphani bacteria. We found that single attack by caterpillars, and dual attack by caterpillars plus aphids, induced the strongest changes in plant volatile emission. The caterpillars’ parasitoid C. glomerata did not exhibit preference for plants exposed to caterpillars only vs. plants exposed to caterpillars plus aphids or plus bacteria. However, the composition of the pollinator community associated with flowers of B. nigra was affected by plant exposure to the attackers, but the total number of pollinators visiting the plants did not change upon attack. We conclude that, when B. nigra were exposed to single or dual attack on their inflorescences, the plants maintained interactions with natural enemies of the insect attackers and with pollinators. We discuss how chemical diversity may contribute to plant resilience upon attack.

Evolution of honest reward signal in flowers

Some flowering plants signal the abundance of their rewards by changing their flower colour, scent or other floral traits as rewards are depleted. These floral trait changes can be regarded as honest signals of reward states for pollinators. Previous studies have hypothesized that these signals are used to maintain plant-level attractiveness to pollinators, but the evolutionary conditions leading to the development of honest signals have not been well investigated from a theoretical basis. We examined conditions leading to the evolution of honest reward signals in flowers by applying a theoretical model that included pollinator response and signal accuracy. We assumed that pollinators learn floral traits and plant locations in association with reward states and use this information to decide which flowers to visit. While manipulating the level of associative learning, we investigated optimal flower longevity, the proportion of reward and rewardless flowers, and honest- and dishonest-signalling strategies. We found that honest signals are evolutionarily stable only when flowers are visited by pollinators with both high and low learning abilities. These findings imply that behavioural variation in learning within a pollinator community can lead to the evolution of an honest signal even when there is no contribution of rewardless flowers to pollinator attractiveness.