Fire effects on pollination and plant reproduction: a quantitative review.

BACKGROUND AND AIMS: Fire may favour plant flowering by opening the vegetation and increasing abiotic resource availability. Increased floral display size can attract more pollinators and increase the absolute fruit and seed production immediately after the fire. However, anthropogenic increases in fire frequency may alter these responses. We aim to assess the effects of fire on pollination and reproductive success of plants at the global scale. METHODS: We performed a systematic literature review and meta-analyses to examine overall fire effects as well as different fire parameters on pollination and on plant reproduction. We also explored to what extent the responses vary among pollinators, pollination vectors, plant regeneration strategies, compatibility systems, vegetation types and biomes. KEY RESULTS: Most studies were conducted in fire-prone ecosystems. Overall, single fires increased pollination and plant reproduction but this effect was overridden by recurrent fires. Floral visitation rates of pollinators were enhanced immediately following a wildfire, and especially in bee-pollinated plants. Fire increased the absolute production of fruits or seeds but not the fruit or seed set. The reproductive benefits were mostly observed in wind-pollinated (graminoids), herbaceous and resprouter species. Finally, fire effects on pollination were positively correlated with fire effects on plant reproductive success. CONCLUSIONS: Fire has a central role in pollination and plant sexual reproduction in fire-prone ecosystems. The increase in the absolute production of fruits and seeds suggests that fire benefits on plant reproduction are likely driven by increased abiotic resources and the consequent floral display size. However, reproduction efficiency, as measured by fruit or seed set, does not increase with fire. In contrast, when assessed on the same plant simultaneously, fire effects on pollination are translated into reproduction. Increased fire frequency due to anthropogenic changes can alter the nature of the response to fire.

Alpine Rhododendron population contractions lead to spatial distribution mismatch with their pollinators under climate change.

The effect of global climate change on plant-pollinator interaction is not limited to changes in phenology and richness within communities but also includes the spatial mismatch caused by the inconsistency of geographical distribution changes. Subsequently, the pollinator interaction network may be remodeled or even disrupted. In this study, we simulated the suitable habitat niche of 15 Rhododendron species and their eight pollinator species as well as their overlapping versus geographical mismatch under the current and three future climate change scenarios in 2090s, using MaxEnt. Results showed that the suitable habitat of all Rhododendron species would decrease in 2090s. In particular, 10, 8, and 13 Rhododendron-pollinator assemblages would have a reduced spatial match region under the climate change scenarios, mainly due to the contraction of the suitable habitat of Rhododendron species. The results provide novel insights into the response of plant-pollinator interactions to global warming, useful to prioritize conservation actions of alpine plant ecosystems.

Ozone alters the chemical signal required for plant - insect pollination: The case of the Mediterranean fig tree and its specific pollinator.

Tropospheric ozone (O3) is likely to affect the chemical signal emitted by flowers to attract their pollinators through its effects on the emission of volatile organic compounds (VOCs) and its high reactivity with these compounds in the atmosphere. We investigated these possible effects using a plant-pollinator interaction where the VOCs responsible for pollinator attraction are known and which is commonly exposed to high O3 concentration episodes: the Mediterranean fig tree (Ficus carica) and its unique pollinator, the fig wasp (Blastophaga psenes). In controlled conditions, we exposed fig trees bearing receptive figs to a high-O3 episode (5 h) of 200 ppb and analyzed VOC emission. In addition, we investigated the chemical reactions occurring in the atmosphere between O3 and pollinator-attractive VOCs using real-time monitoring. Finally, we tested the response of fig wasps to the chemical signal when exposed to increasing O3 mixing ratios (0, 40, 80, 120 and 200 ppb). The exposure of the fig tree to high O3 levels induced a significant decrease in leaf stomatal conductance, a limited change in the emission by receptive figs of VOCs not involved in pollinator attraction, but a major change in the relative abundances of the compounds among pollinator-attractive VOCs in O3-enriched atmosphere. Fig VOCs reacted with O3 in the atmosphere even at the lowest level tested (40 ppb) and the resulting changes in VOC composition significantly disrupted the attraction of the specific pollinator. These results strongly suggest that current O3 episodes are probably already affecting the interaction between the fig tree and its specific pollinator.

Floral Aroma and Pollinator Relationships in Two Sympatric Late-Summer-Flowering Mediterranean Asparagus Species.

This research delves into plant-pollinator relationships within the Mediterranean region, focusing on two synchronous and sympatric asparagus species: A. acutifolius and A. albus. For the first time, the floral scents of the genus Asparagus are reported. We investigate the volatile organic compounds (VOCs) present in their floral scents and their impact on pollinator attraction. Captured flower-emitted VOCs underwent solid-phase microextraction of headspace (SPME-HS) and gas chromatography and mass spectrometry (GC-MS) analysis. The investigation confirms distinctive aroma profiles for each species. A. albus predominantly emits benzene derivatives and sesquiterpenes, while A. acutifolius is characterized by carotenoid derivatives, monoterpenes, and sesquiterpenes. The only shared compounds between the two species are the sesquiterpenes (Z,E)-α-farnesene and (E,E)-α-farnesene. A positive correlation links peak floral aroma intensity (benzenoids in A. albus and ionones in A. acutifolius) with a higher pollinator visit frequency, emphasizing the critical role of intense floral scents in pollinator attraction. The study of reproductive aspects reveals almost complete gynodioecy in A. acutifolius, influencing unique dynamics for the two species. These adaptations hold significant importance within the Mediterranean ecosystem, particularly during the late dry summer period, when a limited number of plant species vie for a shared primary pollinator.

An elevational gradient in floral traits and pollinator assemblages in the Neotropical species Costus guanaiensis var. tarmicus in Peru.

Different populations of plant species can adapt to their local pollinators and diverge in floral traits accordingly. Floral traits are subject to pollinator-driven natural selection to enhance plant reproductive success. Studies on temperate plant systems have shown pollinator-driven selection results in floral trait variation along elevational gradients, but studies in tropical systems are lacking. We analyzed floral traits and pollinator assemblages in the Neotropical bee-pollinated taxon Costus guanaiensis var. tarmicus across four sites along a steep elevational gradient in Peru. We found variations in floral traits of size, color, and reward, and in the pollinator assemblage along the elevational gradient. We examined our results considering two hypotheses, (1) local adaptation to different bee assemblages, and (2) the early stages of an evolutionary shift to a new pollinator functional group (hummingbirds). We found some evidence consistent with the adaptation of C. guanaiensis var. tarmicus to the local bee fauna along the studied elevational gradient. Corolla width across sites was associated with bee thorax width of the local most frequent pollinator. However, we could not rule out the possibility of the beginning of a bee-to-hummingbird pollination shift in the highest-studied site. Our study is one of the few geographic-scale analyses of floral trait and pollinator assemblage variation in tropical plant species. Our results broaden our understanding of plant-pollinator interactions beyond temperate systems by showing substantial intraspecific divergence in both floral traits and pollinator assemblages across geographic space in a tropical plant species.

Diferentes poblaciones de una especie de planta pueden adaptarse a sus polinizadores locales y, en consecuencia, divergir en los rasgos florales. Los rasgos florales están sujetos a la selección natural impulsada por los polinizadores para mejorar el éxito reproductivo de la planta. Los estudios en sistemas de plantas de zonas templadas han demostrado que la selección impulsada por los polinizadores da como resultado una variación de los rasgos florales a lo largo de gradientes de altitud, pero faltan estudios en sistemas de plantas de zonas tropicales. Analizamos las características florales y los ensamblajes de polinizadores en el taxón Neotropical polinizado por abejas Costus guanaiensis var. tarmicus en cuatro sitios a lo largo de un empinado gradiente altitudinal en Perú. Encontramos variación en los rasgos florales de tamaño, color y recompensa, y en los ensamblajes de polinizadores a lo largo del gradiente altitudinal. Examinamos nuestros resultados considerando dos hipótesis, (1) la adaptación local a diferentes ensamblajes de abejas polinizadoras y (2) las primeras etapas de un desplazamiento evolutivo a un nuevo grupo funcional polinizador (colibríes). Encontramos evidencia que respalda la adaptación de C. guanaiensis var. tarmicus a la fauna de abejas local a lo largo del gradiente altitudinal estudiado. El ancho de la corola de la flor a lo largo de los sitios de estudio se asoció con el ancho del tórax de la especie de abeja local más frecuentemente registrada en las flores. Sin embargo, no pudimos descartar la posibilidad del comienzo de un desplazamiento de polinización de abeja a colibrí en el sitio más alto estudiado. Nuestro estudio es uno de los pocos análisis a escala geográfica de variación de rasgos florales y ensamblaje de polinizadores en una especie tropical. Nuestros resultados amplían la comprensión de las interacciones planta­polinizador más allá de los sistemas templados al mostrar una sustancial divergencia intraespecífica tanto en los rasgos florales como en los ensamblajes de polinizadores en una especie tropical.

Proximity to oilseed rape fields affects plant pollination and pollinator-mediated selection on a co-flowering plant on the Tibetan Plateau.

The ecological effects of mass-flowering crops on pollinator abundance and species richness of neighbouring habitats are well established, yet the potential evolutionary consequences remain unclear. We studied effects of proximity to a mass-flowering crop on the pollination of local co-flowering plants and on patterns of natural selection on a pollination-generalised plant on the Tibetan Plateau. We recorded pollinator visitation rates and community composition at different distances (near vs. far) to oilseed rape (Brassica napus) fields in two habitat types and quantified pollinator-mediated selection on attractive traits of Trollius ranunculoides. The proximity to oilseed rape increased pollinator visitation in neighbouring alpine meadows and changed pollinator composition in neighbouring shrub meadows. Trollius ranunculoides in the alpine meadow near oilseed rape received three times more pollinator visits (mainly bees) and consequently had a 16.5% increase in seed set but also received slightly more heterospecific pollen per stigma. In contrast, pollinator visitation to T. ranunculoides in the shrub meadow near oilseed rape was three times lower (mainly flies), leading to a 10.7% lower seed despite no effect on pollen deposition. The proximity to the oilseed rape field intensified pollinator-mediated selection on flower size and weakened selection on flower height of T. ranunculoides in the alpine meadow but did not affect phenotypic selection on either trait in the shrub meadow. Our study highlights context-dependent variation in plant-pollinator interactions close to mass-flowering oilseed rape, suggesting potential effects on the evolution of flower traits of native plants through altered pollinator-mediated selection. However, context dependence may make these effects difficult to predict.

Phylogenetic Analyses of Some Key Genes Provide Information on Pollinator Attraction in Solanaceae.

Floral syndromes are known by the conserved morphological traits in flowers associated with pollinator attraction, such as corolla shape and color, aroma emission and composition, and rewards, especially the nectar volume and sugar concentration. Here, we employed a phylogenetic approach to investigate sequences of genes enrolled in the biosynthetic pathways responsible for some phenotypes that are attractive to pollinators in Solanaceae genomes. We included genes involved in visible color, UV-light response, scent emission, and nectar production to test the hypothesis that these essential genes have evolved by convergence under pollinator selection. Our results refuted this hypothesis as all four studied genes recovered the species' phylogenetic relationships, even though some sites were positively selected. We found differences in protein motifs among genera in Solanaceae that were not necessarily associated with the same floral syndrome. Although it has had a crucial role in plant diversification, the plant-pollinator interaction is complex and still needs further investigation, with genes evolving not only under the influence of pollinators, but by the sum of several evolutionary forces along the speciation process in Solanaceae.

The Ability of Bumblebees Bombus terrestris (Hymenoptera: Apidae) to Detect Floral Humidity is Dependent Upon Environmental Humidity.

Flowers produce local humidity that is often greater than that of the surrounding environment, and studies have shown that insect pollinators may be able to use this humidity difference to locate and identify suitable flowers. However, environmental humidity is highly heterogeneous, and is likely to affect the detectability of floral humidity, potentially constraining the contexts in which it can be used as a salient communication pathway between plants and their pollinators. In this study, we use differential conditioning techniques on bumblebees Bombus terrestris audax (Harris) to explore the detectability of an elevated floral humidity signal when presented against different levels of environmental noise. Artificial flowers were constructed that could be either dry or humid, and individual bumblebees were presented with consistent rewards in either the humid or dry flowers presented in an environment with four levels of constant humidity, ranging from low (~20% RH) to highly saturated (~95% RH). Ability to learn was dependent upon both the rewarding flower type and the environment: the bumblebees were able to learn rewarding dry flowers in all environments, but their ability to learn humid rewarding flowers was dependent on the environmental humidity, and they were unable to learn humid rewarding flowers when the environment was highly saturated. This suggests that floral humidity might be masked from bumblebees in humid environments, suggesting that it may be a more useful signal to insect pollinators in arid environments.

The Effects of Flower Patch Density on Pollinator Visitation.

There is conflicting research regarding how conspecific plant density can affect pollinator visitation, with some studies indicating dense flower patches will receive more visitors and other studies demonstrating the opposite. This study investigated the effects of conspecific density on pollinator visitation in a restored prairie. Three plant species, Penstemon digitalis (Nutt. ex Sims) (Lamiales: Plantaginaceae), Monarda fistulosa (L.) (Lamiales: Lamiaceae), and Eryngium yuccifolium (Michx.) (Apiales: Apiaceae), were observed, with visiting pollinators recorded. Conspecific density did not have an effect on total pollinator visitation rates for any of the focal plant species. However, different groups of pollinators varied in their responses to flower density, notably with larger Bombus spp. tending to visit dense flowering patches more than did other groups of bees. This suggests that plant density may impact certain pollinators differently than others. These results also indicate a possible mechanism through which multiple pollinator species can coexist while only one flowering resource is available, with the foraging behavior of smaller bees potentially allowing them to avoid competition with larger, social bees. Furthermore, a comparison of seed weight demonstrated that E. yuccifolium plants tended to have larger seed sets in isolated individuals, suggesting that flowers in large patches may be pollinated less effectively and are competing for, rather than facilitating, pollinator visits.

A new brood-pollination mutualism between Stellera chamaejasme and flower thrips Frankliniella intonsa.

BACKGROUND: Brood pollination mutualism is a special type of plant-pollinator interaction in which adult insects pollinate plants, and the plants provide breeding sites for the insects as a reward. To manifest such a mutualism between Stellera chamaejasme and flower thrips of Frankliniella intonsa, the study tested the mutualistic association of the thrips life cycle with the plant flowering phenology and determined the pollination effectiveness of adult thrips and their relative contribution to the host's fitness by experimental pollinator manipulation. RESULTS: The adult thrips of F. intonsa, along with some long-tongue Lepidoptera, could serve as efficient pollinators of the host S. chamaejasme. The thrips preferentially foraged half-flowering inflorescences of the plants and oviposited in floral tubes. The floral longevity was 11.8 ± 0.55 (mean ± se) days, which might precisely accommodate the thrips life cycle from spawning to prepupation. The exclusion of adult thrips from foraging flowers led to a significant decrease in the fitness (i.e., seed set) of host plants, with a corresponding reduction in thrips fecundity (i.e., larva no.) in the flowers. CONCLUSIONS: The thrips of F. intonsa and the host S. chamaejasme mutualistically interact to contribute to each other's fitness such that the thrips pollinate host plants and, as a reward, the plants provide the insects with brooding sites and food, indicating the coevolution of the thrips life cycle and the reproductive traits (e.g., floral longevity and morphology) of S. chamaejasme.

Pollinator Community Predicts Trait Matching between Oil-Producing Flowers and a Guild of Oil-Collecting Bees.

AbstractThe impact of pollinator community diversity on trait matching in plant-pollinator interactions is poorly studied, even though many mutualisms involve multiple interaction partners. We studied 10 communities in which one to three species of oil-collecting Rediviva bees pollinate the long-spurred, oil-producing flowers of Diascia "floribunda" to examine how pollinator diversity affects covariation of functional traits across sites and trait matching within sites. Floral spur length was significantly correlated with weighted grand mean foreleg length of the local bee community but not with foreleg length of individual bee species. The closeness of trait matching varied among populations and was inversely related to pollinator community diversity. For all bee species, trait matching was closest at sites characterized by exclusive pairwise interactions. Reduced trait matching associated with increased community diversity for individual pollinator species but close matching at the community level supports the importance of community context for shaping interacting traits of flowers and pollinators.

Flora of Ferruginous Outcrops Under Climate Change: A Study in the Cangas of Carajás (Eastern Amazon).

Climate change has impacted biodiversity, affecting species and altering their geographical distribution. Besides understanding the impact in the species, it has been advocated that answering if different traits will be differently impacted could allow refined predictions of how climate change will jeopardize biodiversity. Our aim was to evaluate if climate change will potentially impact plant species differently, considering their traits. We evaluated 608 plant species that occur in the naturally open areas of ferruginous outcrops (namely, cangas) in the National Forest of Carajás (Eastern Amazon). Firstly, we estimated the effects of climate change on each species using species distribution modeling, and analyzed this impact in the set containing all species. Secondly, we classified plant species considering the following traits: (i) pollination syndromes (melittophily, phalaenophily, psychophily, cantharophily, entomophily, ornithophily, chiropterophily, anemophily); (ii) habit (tree, shrub, herb, liana, parasite); and (iii) the main habitat of occurrence (open areas and forests). Thirdly, we investigated if the effects of climate change could be significantly more intense considering all the different traits quoted. Our results showed that most plant species will potentially face reduction of suitable habitats under future climate and the scenarios showed that 42% of them may not find suitable areas in the cangas of Carajás. We found no significant difference within each analyzed trait, considering the potential impact of climate change. The most climatically suitable areas (i.e., areas with high probability of species occurrence in the future) are those in the southwest of the study area. These areas can be considered as priority areas for species protection against climate change.

Using Matching Traits to Study the Impacts of Land-Use Intensification on Plant-Pollinator Interactions in European Grasslands: A Review.

Permanent grasslands are suitable habitats for many plant and animal species, among which are pollinating insects that provide a wide range of ecosystem services. A global crisis in pollination ecosystem service has been highlighted in recent decades, partly the result of land-use intensification. At the grassland scale, however, the underlying mechanisms of land-use intensification that affect plant-pollinator interactions and pollination remain understudied. In this review, we first synthesise the literature to provide new insights into the relationships between land-use intensification and pollination by using matching community and interaction traits. We then identify knowledge gaps and summarise how land-use intensification of grassland influences floral traits that may in turn be associated with modifications to pollinator matching traits. Last, we summarise how these modifications may affect pollination function on permanent grasslands. Overall, land-use intensification may lead to a shift in flower colour, a decrease in mean nectar tube depth and a decrease in reward production and pollen quality at the community level. This, in turn, may generate a decrease in pollinator mouthparts length and body size, that may favour pollinators that require a low amount of floral reward. We found no study citing the effect of land-use intensification on volatile organic compounds emitted by flowers despite the importance of these molecules in pollinator community composition. Overall, our review highlighted major knowledge gaps about the effects of land-use intensification on plant-pollinator interactions, and suggests that land-use intensification could favour plants with generalised floral traits that adversely affect pollination.

Context-Dependent Effect of Dietary Phytochemicals on Honey Bees Exposed to a Pesticide, Thiamethoxam.

Honey bees continue to face challenges relating to the degradation of natural flowering habitats that limit their access to diverse floral resources. While it is known that nectar and pollen provide macronutrients, flowers also contain secondary metabolites (phytochemicals) that impart benefits including increased longevity, improved gut microbiome abundance, and pathogen tolerance. Our study aims to understand the role of phytochemicals in pesticide tolerance when worker bees were fed with sublethal doses (1 ppb and 10 ppb) of thiamethoxam (TMX), a neonicotinoid, in 20% (w/v) sugar solution supplemented with 25 ppm of phytochemicals-caffeine, kaempferol, gallic acid, or p-coumaric acid, previously shown to have beneficial impacts on bee health. The effect of phytochemical supplementation during pesticide exposure was context-dependent. With 1 ppb TMX, phytochemical supplementation increased longevity but at 10 ppb TMX, longevity was reduced suggesting a negative synergistic effect. Phytochemicals mixed with 1 ppb TMX increased mortality in bees of the forager-age group but with 10 ppb TMX, mortality of the inhive-age group increased, implying the possibility of accumulation effect in lower sublethal doses. Given that the phytochemical composition of pollen and nectar varies between plant species, we suggest that the negative impacts of agrochemicals on honey bees could vary based on the phytochemicals in pollen and nectar of that crop, and hence the effects may vary across crops. Analyzing the phytochemical composition for individual crops may be a necessary first step prior to determining the appropriate dosage of agrochemicals so that harm to bees Apis mellifera L. is minimized while crop pests are effectively controlled.

Effects of temperature and photoperiod on the seasonal timing of Western honey bee colonies and an early spring flowering plant.

Temperature and photoperiod are important Zeitgebers for plants and pollinators to synchronize growth and reproduction with suitable environmental conditions and their mutualistic interaction partners. Global warming can disturb this temporal synchronization since interacting species may respond differently to new combinations of photoperiod and temperature under future climates, but experimental studies on the potential phenological responses of plants and pollinators are lacking. We simulated current and future combinations of temperature and photoperiod to assess effects on the overwintering and spring phenology of an early flowering plant species (Crocus sieberi) and the Western honey bee (Apis mellifera). We could show that increased mean temperatures in winter and early spring advanced the flowering phenology of C. sieberi and intensified brood rearing activity of A. mellifera but did not advance their brood rearing activity. Flowering phenology of C. sieberi also relied on photoperiod, while brood rearing activity of A. mellifera did not. The results confirm that increases in temperature can induce changes in phenological responses and suggest that photoperiod can also play a critical role in these responses, with currently unknown consequences for real-world ecosystems in a warming climate.

Covering and shading by neighbouring plants diminish pollinator visits to and reproductive success of a forest edge specialist dwarf species.

The pollination and reproductive success of flowering plants can be negatively influenced in various ways by neighbouring heterospecific plants, such as resource competition and reproductive interference. We hypothesized that covering together with shading by neighbouring plants may reduce pollinator visits to and reproductive success of plants by reducing floral attractiveness and pollinator activity and by interrupting flower access, respectively. To test this hypothesis, we examined whether shaded and covered flowers suffered from pollinator limitation and low reproductive success in a population of the dwarf herb Lithospermum zollingeri, which co-exists with woody and herbaceous plants in anthropogenically maintained forest edge meadows. Here, shaded and covered flowers were defined as those beneath the shade of the woods and those whose front portion was covered by any vegetative part of neighbouring plants, respectively. The shaded and covered flowers were visited by significantly fewer pollinators than sunlit and open flowers in the field. However, three major pollinator species responded differently to shading and covering. Significant pollen limitation reduced seed set in covered flowers, and shaded flowers produced fewer seeds. Pollen removal from the anthers was not influenced by shading or covering. Our study demonstrates the negative effects of covering on pollinator visits and seed production. It also elucidates the negative effects of shading on reproductive success in L. zollingeri, which depends on managed semi-natural conditions. Land management abandonment, which has increased shaded and covered conditions in artificial forest edge meadows and open forest floors, might promote a rapid reduction in the populations of such dwarf plants.

Warming and elevated CO2 induces changes in the reproductive dynamics of a tropical plant species.

Tropical plant species are vulnerable to climate change and global warming. Since flowering is a critical factor for plant reproduction and seed-set, warming and elevated atmospheric carbon dioxide concentrations (eCO2) are crucial climate change factors that can affect plant reproductive dynamics and flowering related events in the tropics. Using a combined free-air CO2 enrichment and a free-air temperature-controlled enhancement system, we investigate how warming (+2 °C above ambient, eT) and elevated [CO2] (~600 ppm, eCO2) affect the phenological pattern, plant-insect interactions, and outcrossing rates in the tropical legume forage species Stylosanthes capitata Vogel (Fabaceae). In comparison to the control, a significantly greater number of flowers (NF) per plot (+62%) were observed in eT. Furthermore, in warmed plots flowers began opening approximately 1 h earlier (~09:05), with a canopy temperature of ~23 °C, than the control (~09:59) and eCO2 (~09:55) treatments. Flower closure occurred about 3 h later in eT (~11:57) and control (~13:13), with a canopy temperature of ~27 °C. These changes in flower phenology increased the availability of floral resources and attractiveness for pollinators such as Apis mellifera L. and visitors such as Paratrigona lineata L., with significant interactions between eT treatments and insect visitation per hour/day, especially between 09:00-10:40. In comparison to the control, the additive effects of combined eCO2 + eT enhanced the NF by 137%, while the number of A. mellifera floral visits per plot/week increased by 83% during the period of greatest flower production. Although we found no significant effect of treatments on mating system parameters, the overall mean multilocus outcrossing rate (tm = 0.53 ± 0.03) did confirm that S. capitata has a mixed mating system. The effects of elevated [CO2] and warming on plant-pollinator relationships observed here may have important implications for seed production of tropical forage species in future climate scenarios.

Asymmetries of reproductive isolation are reflected in directionalities of hybridization: integrative evidence on the complexity of species boundaries.

The complex nature of species boundaries has been a central topic in evolutionary biology ever since Darwin. Despite numerous separate studies on reproductive isolation and hybridization, their relationship remains underinvestigated. Are the strengths and asymmetries of reproductive barriers reflected in the extent and directionalities of interspecific genetic exchange? We combined field, experimental, and molecular data to quantify strengths and asymmetries of sympatric reproductive barriers and hybridization between florally heteromorphic primroses. We also assessed whether generalist pollinators discriminate between different floral cues and contribute to reproductive isolation, a long-debated topic. Sympatric reproductive isolation is high but incomplete, and most phenotypic intermediates are genetic F1 hybrids, whereas backcrosses are rare, revealing low interspecific gene flow. Species integrity rests on multiple barriers, but ethological isolation is among the strongest, demonstrating that even generalist pollinators crucially contribute to the maintenance of species boundaries. Furthermore, reproductive barriers are weaker for Primula veris and short-styled plants, results corroborated by molecular data. Thus, in florally heteromorphic systems, both species- and morph-dependent asymmetries affect permeability of species boundaries. Our study illustrates how the interactions between complex floral syndromes and pollinators shape species boundaries in unique, previously undescribed ways.

Using ecological context to interpret spatiotemporal variation in natural selection.

Spatiotemporal variation in natural selection is expected, but difficult to estimate. Pollinator-mediated selection on floral traits provides a good system for understanding and linking variation in selection to differences in ecological context. We studied pollinator-mediated selection in five populations of Dalechampia scandens (Euphorbiaceae) in Costa Rica and Mexico. Using a nonlinear path-analytical approach, we assessed several functional components of selection, and linked variation in pollinator-mediated selection across time and space to variation in pollinator assemblages. After correcting for estimation error, we detected moderate variation in net selection on two out of four blossom traits. Both the opportunity for selection and the mean strength of selection decreased with increasing reliability of cross-pollination. Selection for pollinator attraction was consistently positive and stronger on advertisement than reward traits. Selection on traits affecting pollen transfer from the pollinator to the stigmas was strong only when cross-pollination was unreliable and there was a mismatch between pollinator and blossom size. These results illustrate how consideration of trait function and ecological context can facilitate both the detection and the causal understanding of spatiotemporal variation in natural selection.